In sociology a can quickly create the positive feedback of a. The above photo is of the. See also.Positive feedback is a process that occurs in a in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation.That is, A produces more of B which in turn produces more of A. In contrast, a system in which the results of a change act to reduce or counteract it has.

• Positive Feedback Homeostasis
• Homeostasis Negative Feedback
• Homeostasis Effector

Both concepts play an important role in science and engineering, including biology, chemistry, and.Mathematically, positive feedback is defined as a positive around a closed loop of cause and effect.That is, positive feedback is the input, in the sense that it adds to make the input larger.Positive feedback tends to cause. When the loop gain is positive and above 1, there will typically be, increasing, or other divergences from. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system into a new stable state. Positive feedback may be controlled by signals in the system being, or, or it can be cancelled or reduced by adding negative feedback.Positive feedback is used in to force voltages away from intermediate voltages into '0' and '1' states.

When microprocessor 72 receives a signal from a pressure sensor such as a pressure transducer 74 in plenum 66, it compares the signal with the upper and lower set points. If the signal is below the lower set point, microprocessor 72 activates pump 54 which continues in operation until microprocessor receives a signal from pressure transducer 74.

On the other hand, is a type of positive feedback that can destroy. Positive feedback in can increase the rate of reactions, and in some cases can lead to. Positive feedback in mechanical design causes, or 'over-centre', mechanisms to snap into position, for example in. Out of control, it can cause.

Positive feedback in economic systems can cause. A familiar example of positive feedback is the loud squealing or howling sound produced by in: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.

Contents.Overview Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. For example, when part of an electronic output signal returns to the input, and is in phase with it, the system is increased. The feedback from the outcome to the originating process can be direct, or it can be via other state variables.

Such systems can give rich qualitative behaviors, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results. Positive feedback reinforces and negative feedback moderates the original process. Positive and negative in this sense refer to loop gains greater than or less than zero, and do not imply any as to the desirability of the outcomes or effects. A key feature of positive feedback is thus that small disturbances get bigger. When a change occurs in a system, positive feedback causes further change, in the same direction.Basic. In a circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.In the real world, positive feedback loops typically do not cause ever-increasing growth, but are modified by limiting effects of some sort.

According to:'Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That’s why there are so few of them.

Usually a negative loop will kick in sooner or later.' Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback. When the gain of the feedback loop is above 1, then the output moves away from the input: if it is above the input, then it moves towards the nearest positive limit, while if it is below the input then it moves towards the nearest negative limit.Once it reaches the limit, it will be stable. However, if the input goes past the limitthen the feedback will change sign – and the output will move in the opposite direction until it hits the opposite limit. The system therefore shows behaviour.Terminology The terms positive and negative were first applied to feedback before. The idea of positive feedback was already current in the 1920s with the introduction of the.described regeneration in a set of electronic amplifiers as a case where the 'feed-back' action is positive in contrast to negative feed-back action, which they mention only in passing. 's classic 1934 paper first details the use of negative feedback in electronic amplifiers.

According to Black:'Positive feed-back increases the gain of the amplifier, negative feed-back reduces it.' According to confusion in the terms arose shortly after this:'.Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier’s gain.

In contrast, Nyquist and Bode, when they built on Black’s work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition.' ( p121) Examples and applications In electronics.

A vintage style regenerative radio receiver. Due to the controlled use of positive feedback, sufficient amplification can be derived from a single or valve (centre).were invented and patented in 1914 for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single amplifier can multiply its by 1,000 or more. Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50.

The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the design, with many more amplification stages, but much more stable operation and no positive feedback.The oscillation that can break out in a regenerative radio circuit is used in.

By the use of or a (commonly ), the signal that is amplified by the positive feedback remains linear. There are several designs for such, including the, and the. They all use positive feedback to create oscillations.Many electronic circuits, especially amplifiers, incorporate. This reduces their gain, but improves their linearity, and, and stabilises all of these parameters, including the closed-loop gain.

These parameters also become less dependent on the details of the amplifying device itself, and more dependent on the feedback components, which are less likely to vary with manufacturing tolerance, age and temperature. The difference between positive and negative feedback for signals is one of: if the signal is fed back out of phase, the feedback is negative and if it is in phase the feedback is positive. One problem for amplifier designers who use negative feedback is that some of the components of the circuit will introduce in the feedback path.

If there is a frequency (usually a high frequency) where the phase shift reaches 180°, then the designer must ensure that the amplifier gain at that frequency is very low (usually by ). If the (the product of the amplifier gain and the extent of the positive feedback) at any frequency is greater than one, then the amplifier will oscillate at that frequency. Such oscillations are sometimes called. An amplifier that is stable in one set of conditions can break into parasitic oscillation in another. This may be due to changes in temperature, supply voltage, adjustment of front-panel controls, or even the proximity of a person or other conductive item.Amplifiers may oscillate gently in ways that are hard to detect without an, or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs.

Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note. The effect of using a Schmitt trigger (B) instead of a comparator (A)Many common circuits employ positive feedback. While normal simple boolean usually rely simply on gain to push digital signal voltages away from intermediate values to the values that are meant to represent '0' and '1', but many more complex gates use feedback.

When an input voltage is expected to vary in an way, but sharp thresholds are required for later digital processing, the circuit uses positive feedback to ensure that if the input voltage creeps gently above the threshold, the output is forced smartly and rapidly from one logic state to the other. One of the corollaries of the Schmitt trigger's use of positive feedback is that, should the input voltage move gently down again past the same threshold, the positive feedback will hold the output in the same state with no change. This effect is called: the input voltage has to drop past a different, lower threshold to 'un-latch' the output and reset it to its original digital value. By reducing the extent of the positive feedback, the hysteresis-width can be reduced, but it can not entirely be eradicated. The Schmitt trigger is, to some extent, a circuit.

Illustration of an R-S ('reset-set') flip-flop made from two digital gates with positive feedback. Red and black mean logical '1' and '0', respectively.An electronic, or 'latch', or 'bistable ', is a circuit that due to high positive feedback is not stable in a balanced or intermediate state. Such a bistable circuit is the basis of one of electronic. The flip-flop uses a pair of amplifiers, transistors, or logic gates connected to each other so that positive feedback maintains the state of the circuit in one of two unbalanced stable states after the input signal has been removed, until a suitable alternative signal is applied to change the state. Computer (RAM) can be made in this way, with one latching circuit for each bit of memory.occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question.

If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design. A phonograph turntable is prone to acoustic feedback.and systems can demonstrate positive feedback. If a picks up the amplified sound output of in the same circuit, then howling and screeching sounds of (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and by the characteristics of the audio system and the room.Audio and live music (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a or ) and an audio output (for example, a loudly-amplified ). In this example, a signal received by the microphone is and passed out of the loudspeaker.

The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small the sound is readily recognized as a loud squeal or screech.Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a. Use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, players in bands using loud and effects have intentionally created guitar feedback to create a desirable musical effect.

'I Feel Fine' by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, 'I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way.'

The principles of audio feedback were first discovered by Danish scientist. Microphones are not the only transducers subject to this effect. Can do the same, usually in the low frequency range below about 100 Hz, manifesting as a low rumble.

Was an innovator in the intentional use of guitar feedback in his to create unique sound effects. He helped develop the controlled and musical use of audio feedback in playing, and later was a famous proponent of the technique. Video Similarly, if a is pointed at a screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback.

This video feedback effect was used in the opening sequences to the series of the television program.Switches In, including based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state. In biology. Positive feedback is the amplification of a body’s response to a stimulus.

For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called (3).

Oxytocin is then carried via the bloodstream to the (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth. In physiology A number of examples of positive feedback systems may be found in. One example is the onset of in, known as the. When a contraction occurs, the hormone causes a nerve stimulus, which stimulates the to produce more oxytocin, which increases uterine contractions.

This results in contractions increasing in. ( pp924–925). Another example is the process of. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.

( pp392–394). also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the gland to produce more to produce more milk. ( p926). A spike in during the of the menstrual cycle causes. ( p907). The generation of is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels, resulting in a change in the membrane potential, which in turn causes more opening of channels, and so on.

So a slight initial leakage results in an explosion of sodium leakage which creates the nerve. ( p59). In of the heart, an increase in intracellular calcium ions to the cardiac myocyte is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum which transport calcium out into the cytosol in a positive feedback physiological response.In most cases, such feedback loops culminate in counter-signals being released that suppress or break the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot.

Lactation stops when the baby no longer nurses. In gene regulation Positive feedback is a well studied phenomenon in gene regulation, where it is most often associated with. Positive feedback occurs when a gene activates itself directly or indirectly via a double negative feedback loop. Genetic engineers have constructed and tested simple positive feedback networks in bacteria to demonstrate the concept of bistability.

A classic example of positive feedback is the in E. Positive feedback plays an integral role in cellular differentiation, development, and cancer progression, and therefore, positive feedback in gene regulation can have significant physiological consequences. Random motions in coupled with positive feedback can trigger interesting effects, such as create population of phenotypically different cells from the same parent cell. This happens because noise can become amplified by positive feedback. Positive feedback can also occur in other forms of, such as enzyme kinetics or metabolic pathways.

In evolutionary biology Positive feedback loops have been used to describe aspects of the dynamics of change in biological. For example, beginning at the macro level, (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems. At the human level, (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence.

(2004) discussed several other examples of positive feedback loops in evolution. The analogy of provide further examples of positive feedback in biological systems. During the Phanerozoic the shows a steady but not monotonic increase from near zero to several thousands of genera.It has been shown that changes in through the correlate much better with hyperbolic model (widely used in and ) than with and models (traditionally used in and extensively applied to as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a arising from resource limitation. Hyperbolic model implies a second-order positive feedback.

The hyperbolic pattern of the has been demonstrated (see below) to arise from a second-order positive feedback between the population size and the rate of. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.

Immune system A, or hypercytokinemia is a potentially fatal immune reaction consisting of a positive feedback loop between and, with highly elevated levels of various cytokines. In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop. Cell death is a -mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as. The very core of the apoptotic process is the auto-activation of caspases, which may be modeled via a positive-feedback loop.

This positive feedback exerts an auto-activation of the by means of intermediate caspases. When isolated from the rest of apoptotic pathway, this positive-feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.

When this core process is complemented with inhibitors and enhancers of caspases effects, this process presents bistability, thereby modeling the alive and dying states of a cell. In psychology Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on. Winner termed this positive feedback loop as a 'rage to master.'

Vandervert (2009a, 2009b) proposed that the can be explained in terms of a positive feedback loop between the output of thinking/performing in, which then is fed to the where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory. Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for evolution in working memory.In economics Markets with social influence Product recommendations and information about past purchases have been shown to influence consumers choices significantly whether it is for music, movie, book, technological, and other type of products. Social influence often induces a rich-get-richer phenomenon where popular products tend to become even more popular. Market dynamics According to the theory of advanced by, price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction. Systemic risk is the risk that an amplification or leverage or positive feedback process presents to a system.

This is usually unknown, and under certain conditions this process can amplify exponentially and rapidly lead to destructive or chaotic behavior. A is a good example of a positive-feedback system: funds from new investors are used to pay out unusually high returns, which in turn attract more new investors, causing rapid growth toward collapse. Has also studied and written on positive feedback in the economy (e.g.

Brian Arthur, 1990). Proposed a theory that certain credit expansion practices could make a market economy into 'a deviation amplifying system' that could suddenly collapse, sometimes called a '.Simple systems that clearly separate the inputs from the outputs are not prone to. This risk is more likely as the complexity of the system increases, because it becomes more difficult to see or analyze all the possible combinations of variables in the system even under careful stress testing conditions. The more efficient a complex system is, the more likely it is to be prone to systemic risks, because it takes only a small amount of deviation to disrupt the system.

Therefore, well-designed complex systems generally have built-in features to avoid this condition, such as a small amount of friction, or resistance, or inertia, or time delay to decouple the outputs from the inputs within the system. These factors amount to an inefficiency, but they are necessary to avoid instabilities.The incident was blamed on the practice of (HFT), although whether HFT really increases systemic risk remains controversial. Human population growth. Main article:Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures which are more susceptible to.Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent of the human population in the past, instead of a simpler.It is proposed that the growth rate is accelerating because of second-order positive feedback between population and technology.

( p133–160) Technological growth increases the carrying capacity of land for people, which leads to a growing population, and this in turn drives further technological growth. ( p146) Prejudice, social institutions and poverty described a of increasing inequalities, and poverty, which is known as '. In meteorology intensifies through positive feedback. A lack of rain decreases soil moisture, which kills plants and/or causes them to release less water through. Both factors limit, the process by which water vapor is added to the atmosphere from the surface, and add dry dust to the atmosphere, which absorbs water.

Less water vapor means both low temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation. Lastly, without clouds, there cannot be rain, and the loop is complete. In climatology. See also:Climate 'forcings' may push a climate system in the direction of warming or cooling, for example, increased atmospheric concentrations of cause warming at the surface. Forcings are external to the climate system and feedbacks are internal processes of the system. Some feedback mechanisms act in relative isolation to the rest of the climate system while others are tightly coupled.

Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes. The main positive feedback in is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming. The main negative feedback comes from the, the amount of heat radiated from the Earth into space is proportional to the fourth power of the temperature of Earth's surface and atmosphere.Other examples of positive feedback subsystems in climatology include:. A warmer atmosphere will melt ice and this changes the which further warms the atmosphere. Methane hydrates can be unstable so that a warming ocean could release more, which is also a greenhouse gas., occurring naturally in, contains carbon. When peat dries it, and may additionally burn. Peat also releases.

Global warming affects the cloud distribution. Clouds at higher altitudes result in more infrared radiation reflected back at the Earth's surface than immediately reflected back into space.The (IPCC) states that 'Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.' In sociology A is a social positive feedback loop between beliefs and behavior: if enough people believe that something is true, their behavior can make it true, and observations of their behavior may in turn increase belief. A classic example is a.Another sociological example of positive feedback is the. When more people are encouraged to join a network this increases the reach of the network therefore the network expands ever more quickly. A is an example of the network effect in which to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including.

In many cases population size is the limiting factor to the feedback effect.In chemistry If a chemical reaction causes, and the reaction itself at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, can occur and very quickly lead to a chemical.In Conservation Many wildlife are hunted for their parts which can be quite valuable. The closer to extinction that targeted species become, the higher the price there is on their parts. This is an example of positive feedback. See also.: in social and financial systems, a complex of events that reinforces itself through a feedback loop. Positive: a situation in where a consequence increases the frequency of a behaviour. Praise of performance: a term often applied in the context of, although this usage is disputed.

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Reprinted with permission of Eutech InstrumentsMonitoring the quality of water is one of many important aspects in environmental control. With the increasing growth in human population, water for human consumption has become scarce and increasingly polluted. There are many parameters that define the quality of water but the most important of these is probably pH. Electrical InterferencepH is a deceptively simple measurement. However, there are many factors that need to be taken into account for a reliable reading. The most important characteristic of pH electrodes is it's very high impedance, of the order of 109 ohms. This is compounded by noisy factory environments and by long distances between the electrode and the controller.A typical pH measuring device would be normally configured to operate in the single ended mode, also known as the asymmetrical mode.

This means that the reference electrode would be connected to the ground potential of the amplifier. This configuration works very well as long as the environment is electronically noise-free. This is not the situation in an industrial environment. It is very commonly seen that the readings on a pH controller suddenly fluctuates, even to over-range or under-range condition.

Homeostasis Negative Feedback

This situation arises, when for example, the mixing motor is switched on. An old leaky motor might inject some electrical interference of 1 to 2 volts into the liquid whose pH is monitored. This noise being a common signal, is picked up by both the pH and the reference electrodes. Since in the asymmetrical mode, the reference electrode is grounded, the electrical noise is present only on the pH electrode. This noise would be amplified along with the pH signal and thus the fluctuating readings.

If the electrical noise was from a DC source, typically like those in an electroplating tank, the problem would not be fluctuating readings mostly stable but incorrect values. A simple way to solve this problem would be to reconfigure the input to floating differential mode. In other words, not to ground the reference electrode. Therefore, the electrical disturbance will be present equally on both the pH and reference electrodes. It therefore becomes a common mode signal and hence can be rejected very easily by the operational amplifier.

However, this brings in the necessity to use an additional grounding electrode commonly referred to as potential matching pin. Relay HuntingHunting of the relays around the set point is a very common problem faced in the industry. This may even lead to the breakdown of expensive dosing pumps and solenoids at times.

Let us look at what causes this problem. Let us assume that in atypical case, a low set point of 6 pH has been set.

It would mean that when the pH drops to a value below 6, the caustic dosing pump starts. The addition of caustic solution would start to increase the pH. When the pH reaches 6, the pump would stop. The mixing pump would still continue operating and hence the pH would drop back below 6.

This would start the cycle all over again and so on. This results in the hunting of the relay around the set point.A simple way to overcome this problem would be allowing the pump to continue to dose even beyond the set point, say until 6.5 pH in the above example.

In such a situation, when dosing pump stops, the pH might drop to probably 6.2 pH which is still above the set point and hence hunting is prevented. This extra band that has been introduced is known as the hysteresis band. In modern day controllers, independent and adjustable hysteresis bands are available for the high and low set points. Limit and Proportional ControlThe function of a pH controller would be mainly to monitor the pH and activate the pumps if the pH value goes out of the set points, and dose the respective chemicals which would bring the pH back within limits.

The manner in which this corrective action takes can be in two ways, namely limit control and proportional control.Limit control is a coarse method since it keeps the relays on permanently if the pH is out of limits. The dosing of the chemical would not be regulated based on the deviation of the pH from the set point but at a steady and fixed rate. This would causeovershoot and undershoot of the process and hence the control will not be smooth.In applications where fine control is required like those in food or pharmaceutical applications which usually operate within a narrow band, a limit control would not be acceptable. The best option would be to switch to 'Proportional Control'. This, as the name suggests, would offer a control action which is proportional to the deviation of the pH value from the set point. In other words, the further the pH value from the set point, the longer the dosing.

As the pH approaches the set point, the dosing reduces and finally stops when the pH reaches the set point.There are two methods of applying 'proportional control', Pulse Length and Pulse Frequency. In the pulse length mode of operation, the total time of the pulse can be fixed by the user typically anywhere between point 0.5 and 20 seconds. The 'ON' time of the pulse would vary depending on the deviation of the pH from the set point.

The further away the pH, the longer the 'ON' time and hence, the higher the dosing. As the pH approaches the set point, the 'ON' time and hence dosing keep reducing.In the Pulse Frequency mode of operation, the frequency can be set by the user typically anywhere between 60 and 120 pulses per minute. The frequency of the pulse would vary depending on the deviation of the pH from the set point. The further away from the pH from the set point, higher the frequency, hence higher dosing. As the pH approaches the set point, the frequency and the dosing reduces. In-Line CalibrationIn most of the industrial applications, the pH controller would be calibrated at the beginning of the process. As the process progresses, bleed samples would be taken and analyzed separately in the laboratory.

At times, it is noticed that there is a mismatch between lab results and the readings on the controller. This might have come about due to the soiling of pH electrode being used in an aggressive environment which might also be a long process running a few days in some cases. In order to correct this problem, the electrode may have to be taken out of the process tank which means halting the process. This may not be acceptable in most cases. A simple way to overcome this problem would be to provide a facility of a one-point calibration which can be done on-line. This does not affect the original slope of the calibration but offsets the line. Alpha-pH1000 controller allows one-point on-line calibration to be done without having to disassemble the electrode housing.

Current Transmission—4/20 mAIn most industries these days, it is essential to have a hard copy of the parameter that is monitored over the entire time frame of the process. The easiest and most economical way of achieving this would be to connect up the controller to a chart recorder. It is therefore necessary for the controller to have a 4/20 mA transmission capability. Most present day controllers come with this facility built in. However, there are only a few which offer many features in this mode.Conventionally, in the controllers, 4 mA would correspond to 0 pH and 20 mA to 14 pH.

When such a device is connected to a 1/4 DIN panel mounted chart recorder, the pH value can be constantly monitored. If the process requires a stringent control and is also operating within a narrow band of set points, say 1 pH; the recording on the chart paper will not be well resolved.

This is due to the fact that in the conventional controller the range of 14 pH is distributed over 16 mA. It can therefore be seen for a 1 pH variation, the current varies only by 1.15 mA approximately. This on the chart paper would be a movement across a width of less than 1/2 cm for the recorder mentioned above It is clear from this example that recording would not be well resolved and hence would not be of much use.An easy and economical solution to the problem would be to have a controller with 'zoom' facility for the current output. This in simple terms would mean that the user should have the facility to fix the pH values he wants to the 4 and 20 mA output. Let us consider a process operating within 1 pH band say 6 and 7 pH. If it was possible to se the controller to deliver 4 mA at 6 pH and 20 mA at 7 pH, it can be seen that we now have the entire 16 mA over just 1 pH band.

This has now given us a 'zoom' of almost 14 times. It would be an added advantage if this 'zoom' band could be set anywhere on the entire pH scale. PH Measurement in Liquids with Hydrofluoric AcidIn certain processes Hydrofluoric (HF) is bound to be present. Typical cases would be like those in glass industries. PH measurements in such situations poses a problem.

Since conventional pH electrodes are themselves made of glass, especially the 'bulb', in a short span of time depending on the concentration of the HF, the electrode dies. Most often the solution to this problem would be using electrodes made of high HF resistance glass. This, as one may see does not totally eliminate the problem. Logically, the best solution would be to use a material that is not attacked by HF. Antimony electrode, which is basically an ORP electrode can be used here. The solution is not as simple as it sounds. This is due to the fact that the property of the glass electrode is very different from that of the Antimony electrode.

Therefore these electrodes cannot just be swapped, one in place of the other.The alpha-pH1000 controller has the facility to switch from one to another and in that process change internally all the settings associated with the type of electrode, either glass or Antimony. This method, though not the best, is still a good and economical way to measure pH in HF environment. Alarm FunctionMost of the mid-range controllers these days come with a separate relay for the 'alarm' function.

This relay generally activates if the measured parameter is out of either of the set points. What this means is that in the event there is a control action taking place, the alarm relay is also active and hence the alarming device be it a siren or a flashing light. As a result of this, generally there is not much attention given when the lights starts to flash or the siren starts to hoot. Invariably there is a reset switch wired up externally, which will be operated to stop the noise.

We see therefore in such situations, the alarm device is looked at as a nuisance rather than a facility. How could we better utilize this feature.Let us look at a practical situation. Assume that the low set point is fixed at 6 pH. When the pH drops to a value below 6, the set point relay gets activated and hence the chemical dosing starts. Under normal circumstances, the alarm relay also would have got activated and as long as the alarm condition exists, the siren keeps hooting.

If by experience one would know how long it would take for the dosing action to correct the situation, say 2 minutes in this example, it would be great advantage if one could set a delay time of 2 minutes for the alarm relay to function. What this means in reality is that once the set point is exceeded, only the dosing pump starts and the alarm relay circuit starts the counter. If in 2minutes the problem did not get corrected, which may be due to airlock in the pump or empty chemical tank, then the alarm relay activates and therefore the siren or the flashing light This would be more meaningful. Prevention of Chemical WastageIn certain industries it becomes essential to monitor two parameters simultaneously and carry out corrective action based on one parameter first followed by the other. PH and ORP measurements would be a good example in 'electroplating' and 'swimming pool'. The pH is first adjusted to a specific level and then the ORP control is done. Thepractical problem one would face here is that the pH and ORP are independent.

While the pH is being corrected, the ORP gets affected and vice versa. If it were possible to HOLD the ORP controller while the pH is being corrected and let it start the control only when the pH is within the acceptable limits, then that would result in a large saving of the chemicals. If this control process could be carried out automatically, it would be an added advantage.

ORP MeasurementsORP or Redox is the oxidation-reduction potential usually measured in millivolts. This is not a specific ion measurement since all the ions present in the liquid will contribute to the ORP potential. It can be seen therefore that in areas like wastewater treatment, the ORP in actual millivolt would not make much meaning. Instead if it was possible for the controller to measure the millivolt and display it as a relative percentage, then the transition from one state to another can be more easily seen. For this to be possible, the controller should be capable of operating in what is known as the ORP% mode, which is available in the alpha-pH1000 controller. The user can calibrate the unit with two liquids whose relative toxic levels are known and then use it to measure and control the ORP% value of the water being processed. Manual Temperature CompensationThere are many instances where a temperature probe is not being used.

This typically would be when there is no large variation in temperature of the process. It is still necessary to apply a compensation for correcting for the effect of temperature if the process temperature is going to be anything other than 25°C. Most of the controllers these days have the facility of Manual Temperature Compensation (MTC).

The drawback however in most of them is that there is only one setting for the MTC. If for example, the MTC is set to 40°C and measurement is being carried out, the compensation corresponding to 40°C would be applied. If now the user decides to carry out a calibration, care should be taken to reset the MTC to 25°C. If this step is forgotten, then calibration would be wrong. This error may not be very high in case of pH measurements. However in Conductivity and Resistivity measurements, assuming that the temperature coefficient of the liquid is around 2% per°C, the error for a 15°C variation could be as high as 30%.

Homeostasis Effector

This would in no way be acceptable. A simple solution to this problem is by providing two settings for the MTC namely one for Process temperature and another for the Calibration. This way the controller can apply the correct compensation based on the mode, 'measurement' or 'calibration', in which it is operating.