How can i make loop check for pressure transmitter and temperature transmitter, and what tools needed? http://answers.yahoo.com/question/index?qid=20100205043159A AmPuUV When checking an industrial control loop you need to be able to supply the correct calibrated signal at standard points. (4-20ma , 0 to 5V or 0 to 10V) You an use DVM ,Loop calibrators and / or a HART interface. HART is a digital communicator that lets you "talk" to the transmitter to configure it and place it into diagnostics mode 4-20ma current loops are the most common. Many transmitters now provide functonality to send what ever signal is dialed in on the faceplate. HART can als provide access to this functionality Otherwise you have to open the loop and replace the transmitter with a calibrated standard. You qualify the receiver with standard points 4ma 12ma & 20ma that correspond to 0% 50% & 100% scale and ensure that the receiver is correctly displaying these points. Reconnect Reconnect the transmitter tr ansmitter into the loop then apply a calibrated physical standard. In the case of a pressure transmitter this is a pressure applied to the transmitter input that is campared to a qualified gauge. You then check that the output signal is correct for the applied value of pressure. Temperature is a lttle harder to do as you either have to use a temperatrure standard which is cumbersome to use in teh field OR a signal simulator that replaces the sense elements signal Such as a thermocouple simulator which generates a known voltage to match the seebeck voltage of a given type at standard points.
2 Response . Several questions come to mind. Is this an electronic or pneumatic system? That determines exactly what devices you need to do the check. Are these transmitters connected to a controller or just to an i ndicator or computer system? With a control loop l oop you would generally check the feedback from the controller to the control device in a similar manner as you do for the transmitter In any case the process is similar. You apply a known pressure or temperature to the transmitter and check the readout at the point where the signal is being sent. You would normally check the bottom end of the transmitters range, the top end and then the midpoint. rd
3 response
If I understand your question correctly you want to create a system which constantly checks/monitors pressure and transmitter, yes? If so then you will need to create a circuit which consists of: 1. The actual sensor or a transducer (in its appropriate configuration...i.e. standalone or in a bridge ckt) ---you could use a RTD, thermocouple, etc or a pressure plate, pressure transducer, etc. 2. You'll need a signal condition circuit 3. An OP amp in either basic amplifier configuration or industrial amplifier (3 OP-AMP setup) 4. If you are goign to relay the information via LCD display... the 2 things you'll need are a microprocessor and a LCD or 7segment display (for which the m.processor will have to interface the m.processor with the device) This is very generic, it depends on the types of sensors or transducers you use. A typical example of a such a circuit looks like: http://i.cmpnet.com/planetanalog/2009/10…
Understanding How a Temperature Transmitter Works
A temperature transmitter combines a temperature sensor – RTD or Thermocouple and a transmitter in the same instrument. The sensor measures the temperature while the transmitter amplifies and transmits the signal to the monitoring system or a control room. A temperature transmitter measures temperature and converts it into a current signal of 4 – 20mA that is proportional to the temperature measured How the Transmitter Works Temperature transmitters are widely used in modern instrumentation systems because they allow long cable runs to be used in the instrumentation and control system. They also perform some signal conditioning function.
As shown in the schematic above, a two wire temperature transmitter accepts a thermocouple or three wire Pt100 input and converts the ''temperature'' output into a 4-20mA current signal. The transmitter usually requires a 24VDC supply which is connected in series with the two wire interface or provided by the host instrument. The amplified temperature signal can be transmitted via a long cable run if required. This is a key advantage especially when dealing with large site installations. What about the signal conditioning function of the transmitter? Signal conditioning This is the process of modifying the raw input signal in one or more ways to facilitate communication and measurement. The transmitter is a simple form of signal conditioner but signal conditioners usually provide linearization scaling facilities and other functions. Signal conditioners are particularly useful when different parameters are measured in a process (e.g. Pt100 and thermocouple outputs, flow rates, pressure and force). The output from all of the appropriate sensors or transducers can then be rationalised into a common interface such as 4-20mA or 1-5V. Owing to advancement in technology, most transmitters are becoming intelligent end devices largely because they are micro-processor based. Most of these devices are now regarded as “smart”devices that can be programmed to achieve a desired result in the plant. These programmable and so called “smart” transmitters effectively combine transmission and signal conditioning functions. In addition to manipulating the input-output function, a variety of transmission modes can be selected. Isolation of input to output further enhances their scope of applications; for example a multi-sensor installation with individual transmitters can be used without danger of earth loops establishing spurious potentials. Programming is performed via a PC using software normally supplied or via a plug-in module.
Read more: http://instrumenttoolbox.blogspot.com/2012/03/understanding-howtemperature.html#ixzz2G7sphfW9 How to Calibrate DP Pressure Transmitters: 8 Effective Tips that Works Calibration of a DP pressure transmitter involves a process by which the output of the transmitter is adjusted to properly represent a known pressure input. Calibration is one of the most frequently performed maintenance operations on pressure transmitters. If well performed, the transmitter’s performance improves otherwise its performance could deteriorate with grave consequences. A pressure input is used to provide zero and span adjustments to the transmitter in the calibration process. Consult my previous post: How to Calibrate Your DP Transmitter for a detailed guide on how to calibrate a DP pressure transmitter.
Owing to the fact that a plant could go berserk, if one or two critical pressure transmitters are wrongly calibrated, it is important the calibration process and procedure be done properly. The following tips are general guides that you should have at the back of your mind when calibrating a DP pressure transmitter: Tip 1: Read and understand the calibration procedure in the manufacturers’ instruction manual. The calibration procedures in the manual should be followed carefully to ensure a proper calibration. Tip 2: The use of proper calibration equipment is crucial. The pressure source and any readout device in use must be of greater accuracy than the instrument being calibrated. Some experts in calibration have posited that as a general rule, the pressure source and readout device should be at the minimum four times more accurate than the device being calibrated. High accuracy measurements cannot be obtained when the calibration is done with low-accuracy equipment. It should be a regular practice to check the accuracy of calibration equipment against a higher standard on a regular basis to maintain the accuracy of the calibration equipment.
Tip 3: When doing calibration, leaks are a potential source of error. Eliminate all leaks in the calibration system. Use TEFLON tape on all pressure connections. Tip 4: Trapped liquids in the pressure transmitter are also a potential source of error. Drain all liquids from the transmitter and impulse piping before starting calibrating. Tip 5: Linearity adjustments are crucial in any calibration process involving transmitters. Linearity adjustments should only be made at one point. All other points should be used to check the adjustments only. Tip 6: Most DP pressure transmitters come with an electronic damping pot for curbing erratic output. Therefore, damping should only be set after the pressure transmitter is placed in service. Tip 7: Temperature is a critical parameter in transmitter calibration. Transmitter performance is affected by changes in ambient temperature. To minimize the effect of temperature change, calibration should be done at the expected ambient temperature. If temperature is expected to fluctuate, it will be good practice to calibrate between the extremes.
Tip 8: Transmitter performance is also affected by changes in static pressure. We can reduce these effects if we calibrate at the line pressure. If this is not practicable then the pressure transmitter should be put in service after calibration and rezeroed after the transmitter has reached the operating pressure.
Read more: http://instrumenttoolbox.blogspot.com/search/label/DP%20transmitters#ixzz2G9J J8gDw
Temperature transmitters
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Function and application
A temperature transmitter is a sensing device that senses and transmits the output of a sensed temperature. In a process control system, process control transmitters are used to measure the process parameters and variables. Temperature transmitters are widely used for controlling industrial processes. They can sense a temperature and transmits the output to a remote location.
A process temperature transmitter offers an output that is related to the actual temperature of the sensed surface. The output obtained from the temperature transmitter can either be communicated to a VDU or to another device so that the process can be controlled and monitored.
How does a temperature transmitter work?
A temperature transmitter introduces a current into the temperature sensor and the resulting voltage drop across the sensor is used for measuring the resistance. This voltage is then converted into a digital form with the help of an analogue to digital converter and offered to a microprocessor.
Once the digital output is fed to the microprocessor, it converts the calculated voltage into a digital value of the temperature. The temperature transmitter comprises a housing and a temperature probe that is attached to the housing. For monitoring a process temperature, the transmitter often includes a sensor, such as a thermocouple or a resistance temperature device (RTD).
An RTD changes the resistance with the change in temperature. This temperature can be measured by calculating the resistance of an RTD. Similarly, a thermocouple also offers a voltage output with a change in temperature.
The Difference between Analog and Digital Electronics All of electronics can be divided into two broad categories: analog and digital. One of the most common examples of the difference between analog and digital devices is a clock. On the analog clock, the time is represented by hands that spin around a dial and point to a location on the dial that represents the approximate time. On a digital clock, a numeric display indicates the exact time.
Analog refers to circuits in which quantities such as voltage or current vary at a continuous rate. When you turn the dial of a potentiometer, for example, you change the resistance by a continuously varying rate. The resistance of the potentiometer can be any value between the minimum and maximum allowed by the pot. If you create a voltage divider by placing a fixed resistor in series with a potentiometer, the voltage at the point between the fixed resistor and the potentiometer increases or decreases smoothly as you turn the knob on the potentiometer. In digital electronics, quantities are counted rather than measured. There’s an important distinction between counting and measuring. When you count something, you get an exact result. When you measure something, you get an approximate result. Consider a cake recipe that calls for 2 cups of flour, 1 cup of milk, and 2 eggs. To get 2 cups of flour, you scoop some flour into a 1-cup measuring cup, pour the flour into the bowl, and then do it again. To get a cup of milk, you pour milk into a liquid measuring cup until the top of the milk lines up with the 1-cup line printed on the measuring cup and then pour the milk into the mixing bowl. To get 2 eggs, you count out 2 eggs, crack them open, and add them to the mixing bowl. The measurements for flour and milk in this recipe are approximate. A teaspoon too much or too little won’t
affect the outcome. But the eggs are precisely counted: exactly 2. Not 3, not 1, not 11/2, but 2. You can’t have a teaspoon too many or too few eggs. There will b e exactly 2 eggs, because you count them. So which is more accurate — analog or digital? In one sense, digital circuits are more accurate because they count with complete precision. You can precisely count the number of jelly beans in a jar, for example. But if you weigh the ja r by putting it on an analog scale, your reading may b e a bit imprecise because you can’t
always judge the exact position of the needle. Say that the needle on the scale is about halfway between 4 pounds and 5 pounds. Does the jar weigh 4.5 pounds or 4.6 pounds? You can’t tell for sure, so you settle for
approximately 4.5 pounds. On the other hand, digital circuits are inherently limited in their precision because they must count in fi xed units. Most digital thermometers, for example, have only one digit to the right of the decimal point. Thus, they can indicate a temperature of 98.6 or 98.7 but can’t indicate 98.65.
Here are a few othe r thoughts to ponder concerning the differences between digital and analog systems:
Saying that a system is digital isn’t the same as saying that it’s binary. Binary is a particular type of digital
system in which the counting is all d one with the binary number system. Nearly all digital systems are also binary systems, but the two words aren’t interchangeable.
Many systems are a combination of binary and analog systems. In a system that combines binary and analog values, special circuitry is required to convert from analog to digital, or vice versa. An input voltage (analog) might be converted to a sequence of pulses, one for each volt; then the pulses can be counted to determine the voltage.
Difference Signals
between
Analog
and
Digital
A signal is simply an indication of movement of something. It can be current, light, music or even the heat. Being quite crucial to science, signals have been widely divided into two classes in order to ease out their study. Analog and digital are types of signals that follow different mannerisms in propagating any signal. Signals that follow analog methodology propagate in such a manner that they can be studied at any instant of time. On the other hand, digital signals can be defined at discrete values which are a whole number. For instance, consider a sound signal that can be measured in time interval between 0 second to 10 seconds. When it comes to measuring an analog signal, it can be measure at 0 seconds, 0.5 seconds, 0.7 seconds, 1.5 seconds and such values which can be in decimal. The numbers after decimal can extend to any limit of specificity. When the same signal is studied in a form of a digital signal, it can be only measured at discrete levels such as 1, 2, 3, 4 and such values which can only be defined at non decimal numbers.
Analog and Digital Signals Other characteristic that creates significant difference between analog and digital signals is the amplitude limitation. Analog signals are independent from the bounds of amplitude. These signals can have as much as amplitude. The amplitude depends on how intensive the input signal is. Usually, louder or brighter the input signal is, higher is the voltage. In digital signals, amplitude has only two states i.e. ON and OFF. At ON position, the signal has high amplitude meaning it contains information while at the OFF state, the signal has low amplitude which usually means that no significant information is being conveyed through the signal. The signals that are transmitted in analog signals
have no defined form or shape but digital signals have a defined shape such as a square or a rectangle. Both the signal transmission methodologies have their pros and cons and are used in almost every electrical and electronic application as per the specification of the device.
Basics of Analog and Digital Signal Analog signals are the usual simple signals which can be observed in a normal human voice or the wind in the ambient conditions. Analog signals can be literally called a wave. These signals are what the nature has given humans. The way the water hits the rocks, the manner in which sand moves in the ocean, the essence which defines how bright the sunlight is and several natural factors are one or the other form of analog signals.In commercial applications, analog signal is found in audio and video applications. Being specific into audio, analog signals are what that is released from the speaker or the headphones. Analog signals are used in transmission when the data is required to be sent at smaller distances. For transmission of analog signals, both the transmitter and the receiver are located in the vicinity of each other so that information can be send easily or the transmitter should have repeaters that can repower data and transmit it to the next destination.
Digital signals are a processed form of analog signals, purely synthetic, which has been created for electronics and communications purposes. These signals are a requirement of transmitting information in a simple ON and OFF format. Digital signals are made using converters and signal levelers that convert an analog signal to a digital one. Creation of digital signals is an expensive task and is only carried out when information has to be sending to longer distance or the data is quite significant that it cannot be lost. In the field of communications, digital signals are used in cellular communications, internet networking. The high data exchange modes such as 3G, Bluetooth etc. are a result of digital communications. In the field of electronics, digital signals are used in creating new codes for computer and mobile circuitry. Devices such as multiplexers, digital gates
and other combinational and sequential circuit designs are some of the wide known applications of digital signal utilization.
Conversion from Analog to Digital and Vice Versa Analog to digital converters (ADC) are used to convert analog signal to a digital signal. The technical word for creating a digital signal out of an analog signal is called quantization. The major principle that is used inquantizationis the rate with which signal is converted to digital levels. Analog signals are sampled at short intervals of time in order to be created into digital. The rate by which they are sampled is called sampling rate and for a proper conversion, it is mandatory for the sampling rate to be at least twice as fast as the rate with which the signal repeats itself (frequency). When the sampling rate is more than the frequency, it is easy to digitize the signal and least amount of information is lost in the process. The condition when the sampling rate is unable to fulfill the aforementioned criteria is called Aliasing. In this condition, the digital signal created is unable to generate the full amount of information that the original analog signal carried. Thus, sampling rate has to be kept constantly boosted up so that the data is kept intact while the signal is converted from analog to digital.
Similar to analog-digital converters (ADC), digital to analog converters are used in order to convert the digital signals back to their analog signal format. These converters are also dependent on the sampling rate which the ADC converters use. They can create a true analog signal only when the ADC converters are made to work at sampling rates at least equal to the double of the original signal frequency.
Comparison between analog and digital Difference between analog and digital signals can be made on several grounds out of which economic and technological ones are the most talked about. Under the technological part, Digital signals take a clear cut majority. Transmission of digital signals is fixed in terms of amplitude; the signals have a more balanced structure as well as data remains protected through these signals. Analog signals, on the other hand, don’t have any defined level of uniformity and controllability in terms of amplitude. As a result, these signals are more susceptible to distortion. This significantly reduces the signal’s chances for being transmitted for long distances. Moreover, these signals can be easily intervened and disturbed. The transmitter and receiver circuits also become more complex due to unpredictability of the signal which in turn increases the complications in the circuitry of these signals.
In terms of economic aspects, analog signals are one of the most economic type of signals in the case when there signal is just to be transmitted over smaller distances. In television broadcast, analog systems are quite used. Also in FM and AM types of radio transmissions, such systems are used. Digital signal generating circuits are usually sophisticated in terms of electronic components. These systems are expensive and are used in long distance communication. Some luxurious home appliances brands also exploit digital signal technique for better outputs.
In some systems, both the types of signals are used. In some amplitude dependent systems which just require data to be sent over large distances, signal is first converted to digital in the transmitter and is then converted to analog in the receiver. Such technique is used in signal repeaters which receive and retransmit the signal
Build a Smart Analog Process-Instrument Transmitter with Low-Power Converters and a Microcontroller By Albert O’Grady & Jim Ryan An analog transmitter is a field-mounted device that senses a physical parameter such as pressure or temperature and generates a current proportional to the measured variable in the standard r ange, 4 to 20 mA. Providing the output as a current i n a twisted-pair loop has many advantages: the measurement signal is insensitive to noise and is not affected by changes in loop resistance; transmitters meeting the standard are interchangeable; and the power required to energize the transmitter circuits can be derived from a remotely supplied loop voltage. Figure 1 shows a conventional transmitter circuit, consisting of a power supply, a current-manipulating transmitter, and a receiving controller.
Figure 1. Analog Transmitter.
Transmitter design has responded to the requirements of users for improved performance and versatility, plus reduced cost and maintenance. A second- generation “smart analog transmitter” has a microprocessor (and data conversion), to provide remote memory and computing power (Figure 2). It can condition the signal remotely before converting it to current and transmitting it back to the controller. For example, it can normalize gain and offsets, linearize sensors having known nonlinearities (such as RTDs and thermocouples) by converting to digital, processing with arithmetic algorithms in the µ P, converting back to analog and transmitting on the loop as a standard current. This reduces the control room’s signal processing burden, a big advantage if a large number of signals must be dealt with.
Figure 2. Smart Transmitter.
The third generation, “smart-and-intelligent” transmitters, add digital communications, which share the same twisted-pair line with the traditional 4-20-mA “dc” signal (Figure 3). The communication channel allows both analog and digital versions of the measured variable to be transmitted over the twisted pair, as well as control signals and diagnostic data relevant to the transmitter, such as calibration coefficients, device ID, and data relevant to fault diagnosis. Transmitter faults can be diagnosed remotely very useful for transmitters in hazardous locations.
