In other words calibration of the transmitter is required to make the transmitter’s percent input equal to the transmitter’s percent output. This is accomplished by adjusting
screws located and clearly marked as ZERO and SPAN on the analog transmitter’s outer casing. The ZERO and SPAN screws may also be referred to as the ZERO and RANGE adjustment screws for some manufacturers of DP transmitters.
Whatever the model/manufacturer of your DP transmitter, it can be easily calibrated according to the manufacturers specific instruction on how to calibrate it. For every calibration you need to do, consult your manufacturer’s specific instruction for calibrating the specific DP transmitter.
Ensure all the materials needed for the calibration are within reach e.g meters, pressure source, pressure gauge, Digital multimeter, power supply module(24V) etc
Record and put down the following(can easily be sourced from transmitter nameplate): (a) Transmitter make and model (b) Transmitter calibration range (c) Transmitter span (d) Transmitter MWP(Maximum Working Pressure)
Connect all the equipment needed for the calibration exercise in the appropriate manner. To ensure you don’t make any mistake, you should draw a connection diagram for all equipment involved paying particular attention to polarity of transmitter and power source! Then connect them according to your connection diagram. A typical DP cell transmitter calibration diagram is shown below:
For your application, this could be modified slightly. For example if the pressure source is a hand pump, you can easily control the pressure applied to the DP cell. However, if you are doing field calibration that requires the use of the actual process pressure, you will need a pressure regulator in conjunction with a pneumatic calibrator to help you control the pressure applied to the DP cell.
Most transmitter calibration done is a five point calibration. That is for 0%, 25%, 50%, 75% and 100% of input span or range (in this case pressure input into the DP transmitter). This should correspond exactly to 0%, 25%, 50%, 75% and 100% of the transmitter output span (4- 20mA). The graph below illustrates the correlation between input and output values.
Readings are taken for both increasing and decreasing input values and the corresponding transmitter output values are recorded
In most calibration done, you will be doing either a bench(shop) calibration – A bench calibration is a procedure where the device is calibrated at a calibration bench using calibration devices to simulate the process, – or a field calibration where the actual process is used. Whether you are doing a bench calibration or a field calibration, the low port of the DP transmitter cell is vented to the atmosphere(as shown in the connection diagram above) and the high port of the DP transmitter connected to a pressure source e.g a hand pump or any other suitable pressure source in a bench calibration or the actual process pressure through a pressure regulator and a pneumatic calibrator in a field calibration. So once your equipment is well setup, power it up and pressurize the high port of your DP transmitter. Record the current reading in (m A) which will be your first data point. Continue pressurizing the transmitter and recording your readings for the five points (0%, 25%, 50%, 75% and 100% of input pressure). All the reading obtained will be the as found readings. If you calibrate the DP transmitter before first testing and recording the as found data, the history of the device performance data will be lost
The potentiometer has a maximum of 20 turns between minimum and maximum resistance therefore turning the ZERO or SPAN screws clockwise or counter-clockwise for 20 turns will cause the potentiometer to be at either maximum or minimum. Please note that for the analog DP transmitter, the ZERO and SPAN adjustments are interactive. That is, adjusting one has an effect on the other. Specifically, changes made to the span adjustment almost always alter the DP transmitter’s ZERO point. This back and forth adjustment of the ZERO and SPAN is what makes the DP transmitter calibration sometimes tedious.
Turn the ZERO and SPAN screws both 20 turns clockwise. Next turn both screws10 turns counter clockwise to approximately adjust the potentiometer to the mid resistance point (50%).
Adjust the ZERO screw on the transmitter while observing the current meter to cause the indication to be 4m A, which is the transmitters LRV output. This may not be exactly 4mA but depending on your plant’s acceptable margin of error, you should get a value very close to 4mA.
Next pressurize the high side of the DP transmitter to cause the pressure applied to the high side to increase to the 100 percent value (URV) of the calibration range.
Once you are satisfied with the level of accuracy of the calibration result, you are done with the calibration of the DP transmitter otherwise you will have to continue fine tuning the calibration process until a reasonable accuracy is achieved.
You must understand that for accurate calibration of any instrument including your DP transmitter, the manufacturer’s manual is the ultimate guide towards achieving a very good calibration. The steps outlined above may differ from your manufacturer’s procedure however, they are a good guide.
For More Information on DP Transmitters, Check out these Resources: