Home

DP Flow Transmitter Calibration: A Step-by-Step Guide

Differential pressure flow meters are widely used in the industry for the measurement of flow in a closed pipe.

This kind of flowmeter consists of two parts:

• The primary element
• The differential pressure transmitter

The DP flow meter will calculate the flow rate by measuring the differential pressure across a primary element.

Various types of primary elements can be used to obtain differential pressure. Some of the most common primary elements are orifice plates, (averaging) pitot tubes, venturi tubes, and flow nozzles.

They all operate on the same principle that the flow through the primary element is proportional to the differential pressure across the element.

Although DP type flow transmitters are known for their high performance and reliability, they will need a recalibration from time to time.

For the purpose of the calibration procedure that we will describe further on, we will consider a DP transmitter with an orifice plate as the primary element.

To be clear, we are not going to calibrate the entire flow measurement, but only the DP flow transmitter. To calibrate the complete flow measurement, the primary element and the DP flow transmitter would have to be demounted and then installed on a flow test bench to be compared with a reference flow meter. This is something that is typically done in a calibration lab.

Relationship between Flow Rate and Differential Pressure

The following equation describes the relationship between flow rate and differential pressure:

where Q is the volumetric flow rate, k is a constant, DP is the differential pressure, and the square root function represents the relationship between velocity and pressure.

From this equation, we can see that an increase in differential pressure will result in an increase in flow rate. However, the relationship between flow rate and differential pressure is not linear but square-rooted, as you can see from the diagram below.

A 25% increase in differential pressure will result in a 50% increase in flow rate.

Low-flow Cut-off

Let’s have another look at the DP vs Flow curve above. Check in particular how the curve behaves in the region around the origin.

In this region, the slightest change in differential pressure causes the flow to change rapidly. A little noise on the DP signal gets multiplied by a factor of 10 or more on the flow signal.

This makes the flow signal very unstable and unreliable. Therefore, vendors will define a low-flow cut-off for the flow meter. That means that the output signal will be set to 4mA once the flow drops below 10%.

The low-flow cut-off is only present in DP transmitters with internal square root extraction. If the square rooting is done by the PLC/DCS, then the DP transmitter has a linear output and the low-flow cut-off should be programmed in the PLC/DCS.

A low-flow cut-off at a flow rate of 10% corresponds to an output of 5.6 mA. So, between 4 and 5.6 mA there is no useful measurement possible because of too much inaccuracy.

All of that will have consequences on the calibration of our DP flow transmitter. For a typical five-point calibration of a transmitter with linear output, the test points would be 0, 25, 50, 75, and 100% of the flow rate. But, if the DP flow transmitter performs the square root calculation and a Low-Flow Cut-Off is considered, it has no sense to have a test point below 10% of the flow rate. So, the test points should be 10, 25, 50, 75, and 100% of the flow rate.

What are the tools needed to calibrate a DP type flow transmitter?

Besides the DP flow transmitter itself, we will need the below-mentioned tools to perform a calibration:

• A calibrated pressure calibrator capable of indicating pressure in mmH2O, because here the DP transmitter for measuring flow has a range in mmH2O.
• A calibrated pressure gauge of the appropriate range.
• A 24Vdc power supply (if the pressure transmitter is not connected to the system and needs an external power supply)
• A digital Multimeter
• A HART communicator

Some important things to consider before starting the calibration of a DP-type flow transmitter

• Check all the tools mentioned above. Check their calibration date’s validity and ensure they are charged one day before planning the job.
• Obtain a work permit with all risk assessments performed.
• Make sure you have the datasheet of the DP flowmeter when performing the calibration task.
• Go through the C&E (Cause and Effect Matrix) and bypass all the interlocks present for the DP-type flow transmitter. Follow the proper bypass procedure and fill out all necessary interlock bypass requests.
• Inform all involved process operators at the start of the calibration job.

The DP type flow transmitter calibration procedure

When all the preparatory work has been completed, we can now start calibrating the DP flow transmitter.

1. Arrange all the tools as shown in the above figure.
2. The DP-type power transmitter can be powered through the field cables connecting the DP transmitter to the system, or by using an external power supply shown in the figure.
3. If the differential pressure flow transmitter is in the field, then isolate the DP transmitter. Flush the differential pressure transmitter in a closed drain if it comes into contact with a dangerous process medium like a hazardous chemical.
4. Connect the power supply, the multimeter, and the DP flow transmitter in a series configuration. Make sure to connect the multimeter leads to the mA terminal and the common terminal.
5. Now connect the DP transmitter to the pressure calibrator using the appropriate tubing. Here we can use PU tubing because the pressure to be applied to the DP flow transmitter is in the mmH2O range.
6. Connect a precision pressure gauge of appropriate range to the pressure-generating device if for any reason this device would not display the generated pressure.
7. Prior to starting the calibration, it is necessary to determine whether the square root extraction takes place in the DP transmitter or in the PLC/DCS. The output value (mA) of the DP transmitter will depend on where this square root extraction occurs. So, check the datasheet or the configuration parameters of the DP transmitter to see if the transmitter does the square root extraction. A third option is to simulate 25% of the DP range with the pressure calibrator and look at the output (mA) of the DP transmitter:
   • If the output is about 12mA, the sqrt extraction is performed by the transmitter.
   • If the output is about 8mA, the sqrt extraction is performed by the PLC/DCS.
8. Let’s take an example to see what we can expect. Imagine a DP flow meter with the following ranges:
   * Flow range: 0 – 1000 m³/h
   * DP range: 0 – 5000 mmH₂O
   We want to do a five-point calibration of the transmitter‘s mA output.
   Now there are 2 possibilities: the square root is done in the transmitter, or it’s done in the PLC/DCS.
   Let’s have a look at both possibilities.
   • If the transmitter performs the square root extraction, we have a mA output representing the flow rate.
     So since we want to calibrate the transmitter’s output, we will have to distribute our test points evenly over the output range. Considering what we said earlier about the inaccuracy at the low flow range, we will test for 10%, 25%, 50%, 75%, and 100% of the flow range.
     We will arrange these values in the table below and add the corresponding DP-values and mA-signals.
     

Test points	Flow (%)	Flow (m3/h)	DP (mmH2O)	Output (mA)
TP1	10 %	100	50	5,6
TP2	25 %	250	312.5	8
TP3	50 %	500	1250	12
TP4	75 %	750	2812.5	16
TP5	100 %	1000	5000	20

The values in the DP (mmH2O) and Output (mA) columns have been calculated with the following formulas:

Flow value = flow rate at the test point

Flow span = 1000

DP span = 5000

• • If the PLC/DCS performs the square root extraction, we have a mA output representing the measured DP.
    In this case, if we want the test points evenly distributed over the output, we will test for 0%, 25%, 50%, 75%, and 100% of the DP range.
    We will arrange these values in the table below and add the corresponding Flow values and mA-signals.

Test points	Flow (%)	DP (mmH2O)	Flow (m3/h)	Output (mA)
TP1	0 %	0	0	4
TP2	25 %	1250	500	8
TP3	50 %	2500	707,1	12
TP4	75 %	3750	866	16
TP5	100 %	5000	1000	20

The values in the Flow (m3/h) and Output (mA) columns have been calculated with the following formulas:

DP value = DP at the test point

DP span = 5000

Flow span = 1000

9. Now that we know where the square root extraction is done and we have calculated our test points, we can start the validation procedure. We will do an ‘As Found’ test on all test points.
   This test is done for two reasons:
   • If this test proves that the instrument is still within the required accuracy for the application then we do not have to recalibrate it.
   • With this test, we will record the history of the device performance data. This will be helpful to decide if we need to shorten or prolong the calibration frequency.

• We will use the following table for our ‘As Found’ test:

Test Points	Applied		Transmitter output		Error %reading
	DP (mmH2O)	Flow (m3/h)	DP (mmH2O)	Flow (m3/h)
50
312,5
1250
2812,5
5000
2812,5
1250
312,5
50

• I have filled in our test points based on a dp transmitter with square root extraction.
• You can see that our test points are going in ascending order from minimum to maximum value and then in descending order back to minimum. This is important because the results can be different for the same test point when approached from below or from above. So, be very careful when approaching a test point not to overshoot it because if you have to come down again it will not give you the same result.

10. Start the validation procedure with zero pressure on the pressure calibrator and increase the pressure until you reach the pressure value of the first test point. Note down the reading from the pressure calibrator in the second column ‘Applied DP’ and the reading from the transmitter in the fifth column ‘Transmitter output Flow’.
    Similarly, apply the pressures of the other test points in the order they appear in the table. Note down the readings from the pressure calibrator and the transmitter in the appropriate columns.
    Every time you reach a new test point, the applied pressure value should remain stable for 30 seconds before reading the output value from the transmitter. If the pressure drops during these 30 seconds, then there is a leakage in the tubing or an issue with the pressure calibrating device. Identify the root cause and attend to it.
11. As the square root extraction is done by the transmitter, its output represents the flow rate. To calculate the error on the output, we will have to compare the measured flow rate with the applied flow rate. As we can only apply a DP, we will have to calculate the applied flow with the following formula:

• Do this calculation for every test point and fill in the flow rates in the third column ‘Applied Flow’.

12. Now calculate the error % from the Flow values we got with the next formula:

13. If the calculated errors are all well within the required accuracy for the application, then the DP type flow transmitter is ok and should not be recalibrated. If the error obtained from the calculations is more than the required accuracy for the application then a calibration of the DP flow transmitter is needed.
14. A recalibration of a dp flow transmitter with internal square root extraction consists of a lower trim at the 10% flow point and a span trim at the 100% flow point. We can not do a zero trim for a transmitter with square root extraction because the zero point is too unstable. Any fluctuation in the input signal, no matter how small, is multiplied tenfold at the output.
    First, apply 1% DP on the input of the transmitter and check if this corresponds to 10% flow at the output. If this is not ok then start a lower trim at the 1% input / 10% output level (not zero) using the HART communicator.
15. Now check the 100% flow point by applying the 100% DP pressure to the input of the transmitter. If the reading does not indicate 100% flow, perform a span trim using the HART calibrator.
16. When the lower trim and span trim is completed, the transmitter is calibrated. To check whether everything went well and whether the transmitter will measure correctly, we will now do an ‘As Left’ verification.
    Repeat the procedure from step 9 to the end. Set up the ‘As Left’ table with the same test points as those from the ‘As Found’ table.
    If the calculated errors from the ‘As Left’ table fall within the required accuracy for the application, the calibration is successful.
17. All that remains is to return the transmitter to service. Inform the operator that the DP flow transmitter calibration job is finished and remove any interlock bypasses you placed at the start.

Why a differential pressure flow transmitter needs to be calibrated

All instruments need to be recalibrated over time. A DP flow meter makes no exception to that rule. A slight progressive accuracy degradation also called drift is the reason for these recalibrations. Drift is caused by the aging of the material and the wear and tear on mechanical and electronic components.

The sensor of the transmitter can be considered as a mechanical component. During its lifetime, this sensor will be subjected to constant pressure fluctuations, causing it to be repeatedly tensioned and released. This can eventually lead to small deformations causing small inaccuracies. The same can happen to the diaphragm that separates the sensor from the process, the body of the pressure transmitter, …

The transmitter’s electronics also age under the influence of temperature fluctuations, humidity, mechanical shock, or vibration… which changes the processing of signals over time.

To compensate for all these small changes, we occasionally need to recalibrate the transmitter.

Factors affecting the DP flow transmitter calibration frequency

• Process functionality and importance of the flow measurement
  When the differential pressure flow meter is part of a flow control loop, it will be more important than if it is only used for indicating flow on the control panel. More important measurements should be calibrated more often.
• Operating conditions
  If the DP transmitter is frequently required to operate close to its operating limits, the deviation of the output signal will be greater over time.
• The quality of the instrument
  Differential pressure transmitters fabricated with the latest technology, using high-standard components will drift much less than lower-quality instruments. Therefore their calibration interval will be longer. The best ones can reach a calibration interval of 5 to 10 years.
• Regulations and standards
  Because regulations and standards don’t know what type of instrument you are using and what your application is, they do not give you the exact calibration frequency for the instrument, but they do give you a measurement accuracy that you need to achieve. Based on that information, you then have to determine for yourself which calibration interval will be required.
• Manufacturers recommendations
  If you bought an instrument from a reputable manufacturer, the manual should normally state the calibration interval. Of course, this is only a recommendation from the manufacturer who does not know what application you are using the instrument for. It gives you the ideal calibration interval for using the instrument under normal operating conditions. If you have no experience with your application, this is a good value to start with.

How often should you perform a DP flow transmitter calibration?

When you install a DP flow meter on a new application, you want to have an idea of when you will need to recalibrate it. Well, there is a way to determine the calibration interval by calculation.

I’ll show you how to do it.

First, you need to decide on the accuracy required for your application, and by required accuracy I don’t mean the reference accuracy of the DP transmitter but the total performance of the installed measurement.

Errors that occur through ambient temperature changes and high static pressure on the differential pressure transmitter should therefore be taken into account.

Generally, for pressure transmitters, the total installed performance will be between 0,5 and 2,0% of calibrated span. Here is a short list of applications with their required installed performance to help you make your choice:

● Plant safety and efficiency control	0,5%
● Regulatory control	1,0%
● Supervisory control	1,5%
● Monitoring	2,0%

Once you have determined the installed performance, I want you to calculate the total probable error with this formula:

To populate the equation you will need to dive into the published specifications of the differential pressure transmitter.

The reference accuracy will be given in % of span. Find that number and put it in the formula.

The ambient temperature effect can be given with a formula where a part of it is expressed in the unit ‘% URL’ and another part in the unit ‘% span’. When this is the case, you should convert the unit ‘% URL’ to the unit ‘% span’ as follows:

The static pressure effect also called the line pressure effect, could be given as a % of reading. Because we have to convert everything to % of span, we will simply consider the pressure reading at full span. This way, we can just keep the percentage value and replace the unit ‘% of reading’ with ‘% of span’.

Now, populate the above formula with the 3 results we have found and calculate the Total Probable Error.

The next thing to do is to search for the stability specification of the output. This will be given as a % of URL for a number of years, meaning that the instrument will lose accuracy at a certain % of URL over the stated number of years.

You should now convert the stability specification to engineering units, like e.g. inH2O, by replacing URL with its pressure value. This gives the stability in engineering units over a number of years. From this, you can calculate the stability per month instead of over a number of years.

Finally, you have to convert the required accuracy for your application and the total probable error to engineering units by replacing the word span with the value of the span.

By using the following formula with the engineering units, you will find the calibration interval in months.

Related topics

Temperature calibration – How to do it right

Calibration test points calculator

How to do a pressure calibration

Advertisement

How to do a pressure calibration

Get a better grip on your process with more accurate pressure measurements

Pressure calibration is something that needs to be done from time to time to keep your process running smoothly.

Anyone who is professionally involved with instrumentation will have experienced that measurement signals can deviate over time.

To avoid this deviation, pressure transmitters must be calibrated regularly.

This article will explain how to perform such a calibration.

What is pressure calibration?

Every pressure transmitter senses the pressure and generates an output signal per the design of the pressure sensor’s electronics.

Unfortunately, this pressure output signal will not be accurate forever.

After a certain time, the response of the pressure transmitter will change. This deterioration goes so slow it cannot be felt or visualized as a trend in many cases. Consequently, the process will get affected due to incorrect pressure values.

To overcome this problem, regular preventive maintenance of the pressure transmitter is required, or when any issue is suspected in the pressure transmitter. One of these preventive maintenance actions is the calibration of the pressure transmitter.

During the pressure calibration procedure, actual pressure is applied to the pressure transmitter using a very precise and well-known pressure source. If a problem is found, appropriate action is taken to correct the transmitter output.

After proper calibration of the pressure transmitter, the pressure transmitter electronics will provide an accurate output corresponding to the pressure detected by the pressure sensor.

Things to consider before calibrating a pressure transmitter

Before you start calibrating a pressure transmitter, some basic, but very important things need to be taken care of. It is better to prepare a checklist to prevent problems.

Here are the four things to consider:

1. Check the Interlocks associated with the pressure transmitter. If any Interlocks are there, then bypassing the interlock with a proper interlock bypass procedure is a must.
   
2. Check whether the pressure transmitter is used for controlling a control valve. If a control valve is controlled by the pressure transmitter in auto mode, switch the control loop to manual mode. During calibration, the pressure values will vary and if the control valve is in auto mode, it will start to move. This unwanted movement of the control valve will adversely affect the process.
   
3. Also, check whether the pressure value is used by other parts of the plant. If the pressure values of the pressure transmitter under calibration are used by other parts of the plant, inform the relevant operators to avoid problems.
   
4. Some critical pressure transmitters could be configured in a text messaging system. That system will send a text message to predefined phone numbers when there is a high or low-pressure reading. So, inform those people if such a possibility exists to avoid panic situations.
   

Tools and tackles needed for the pressure transmitter calibration job

1. A digital multimeter capable of measuring DC voltage and milliampere.
2. A pressure gauge with a range that is 1.5 times greater than that of the pressure transmitter.
3. A pressure-generating device such as a pressure pump, pressure calibrator, or dead weight tester, depending on the range of the pressure transmitter.
4. A HART communicator
5. Both 1/4″ and 1/2″ temporary tubes.
6. Various fittings of different sizes for connecting the temporary tube to the pressure transmitter.

Make sure the digital multimeter, HART communicator, and pressure-generating device are calibrated. The digital multimeter and the pressure-generating device must have a greater accuracy (minimum 4 times) than the accuracy required by the pressure transmitter. The pressure gauge must be calibrated with the main calibration device which is 4 times more accurate than the pressure transmitter.

Pressure transmitter calibration procedure with pressure calibrator

For this calibration, we will use a hand pump to provide the test pressure.

Both pneumatic and hydraulic hand pumps are available. The choice between the two depends on the pressure you want to test at:

• Pneumatic hand pumps can be used up to about 600psi (41bar)
• Hydraulic hand pumps operate at pressures up to approximately 10000psi (690bar)

Below mentioned steps can be followed to do the calibration of a pressure transmitter:

1. Obtain a proper permit to work and do a proper risk assessment.
2. Bypass all interlocks with a proper interlock bypass procedure.
3. Put the control valve or ON-OFF valve in manual mode if any control valve or ON-OFF valve is being controlled by the pressure transmitter to be calibrated.
4. Inform all the operators (also at other parts of the plant) who are using the pressure transmitter signal and disable the alert system, if any exists.
5. Ask the operation department to isolate the root valve and put a proper isolation tag on the root valve for identification purposes.
6. Now the pressure transmitter needs to be drained for removing the material between the root valve and the pressure transmitter. If the pressure transmitter is used for measuring the pressure of a fluid that is toxic, flammable, or dangerous to humans or the environment, then do an arrangement for a closed drain. If the pressure transmitter is used for a safe fluid like regular water or air, then we can directly open the pressure transmitter to the atmosphere.
7. After the material between the root valve and the pressure transmitter is removed, open the pressure transmitter to the atmosphere. We can remove the vent plugs from the vent ports behind the valve manifold. In this condition, the pressure transmitter should show 0 kg/cm2 or 0 psi depending on the unit configured. This is the case for a gauge pressure transmitter. An absolute pressure transmitter will show 1 kg/cm2 or 14.696 psi depending upon the type of unit configured. This step helps to check the pressure transmitter’s response at atmospheric pressure conditions.
8. Now we need to check the response of the pressure transmitter with actual pressure. So, connect the hand pump to the pressure transmitter. Use proper fittings that do not cause damage to the pressure transmitter and do not leak while pressurizing the pressure transmitter. Configure the pressure range of the pressure generator to the same measurement range as the pressure transmitter.
9. Also, connect a pressure gauge with the correct range (usually 1.5 times that of the pressure transmitter) to the hand pump and the pressure transmitter. This is done to double-check the pressure exerted by the hand pump.
10. Connect a digital multimeter to the 24 VDC supply loop of the pressure transmitter if the pressure transmitter is operating at 4 mA to 20 mA. If the Foundation Fieldbus protocol is used, you do not need to connect a digital multimeter.
11. Connect the HART communicator to the pressure transmitter.
12. Check the measuring range of the transmitter and calculate 5 calibration test points. These calibration points will correspond to 0%, 25%, 50%, 75%, and 100% of the range. You can find the range by looking into the configuration settings of the transmitter or finding it on the datasheet. The 5 test points can easily be calculated with this calibration test points calculator.
13. Use the hand pump to start applying pressure to the transmitter. First, apply a 0% value. Check the value on the hand pump, the pressure transmitter, and the double-check gauge. All the values should be equal to the 0% value. Now confirm the value indicated on the operator’s HMI or the system. If any discrepancy is found, then troubleshoot it. In many cases, the issue is a value mismatch between the field and the HMI. There could be many reasons for this difference. Some reasons are listed below:
    • The range of the pressure transmitter is different from the range configured in the system. For example, the range of the transmitter is 0 to 100, while the range in the system is 0 to 50. So when the field device shows 50, the system shows only 25. Changing the range to the correct one will help solve this problem.
    • Instead of 4 to 20 mA, a range of 0 to 20 mA or some other range is sometimes configured in the field. But in the system, the range is 4 to 20 mA. So due to the difference in the mA range, the value will also be different. Or on the system side, the mA range is incorrect according to the datasheet. Changing the incorrect mA range to the correct mA range can solve this problem.
    • The transmitter is having a pressure-sensing problem. So, calibration is needed.
    • If an intermediate system is used to transfer the data, the range may also be misconfigured in that system. Often data is passed from the ESD control system to the DCS. So due to incorrect range configuration on either side, the value will be different.
14. Apply 25%, 50%, 75%, and 100% pressure values to the pressure transmitter. Wait a few seconds after applying the value so that the system becomes stable at that point. Check for leaks in between, if the readings do not match, or if the pressure is unstable. Record all the respective measured values in the calibration sheet.
15. Similarly, start from 100% by applying descending values in 25% increments and record all measured values.
16. Calculate the error% with the formula :
    (measured value – reference value) / reference value *100
17. If the error% is within the limits mentioned in the datasheet, then the pressure transmitter is ok.
18. If the error% is greater than the limit, then zero and span calibration is needed.
19. For a zero calibration, use the hand pump to apply the 0% of range value. Go to the calibration menu in the HART communicator and press the “Zero Trim” button. Wait for a few seconds till a success message is displayed.
20. Use the hand pump to apply the 100% of range value. Again, go to the calibration menu in the HART communicator and press the “Span Trim” button. Wait for a few seconds till a success message is displayed.
    (Some old transmitters have a zero and span setting screw. So, set zero and span from the screws one by one after applying the respective pressures)
21. After the pressure transmitter has been adjusted, it is necessary to reverify it. So, use the hand pump again to apply 0%, 25%, 50%, 75%, and 100% of range values. Note down all the values as you did previously. Also, check the values in decreasing order in steps of 25%.
22. Again, calculate the error%. The error% should be within the maximum allowable limit. If the error% is still too high, then do the calibration of the pressure transmitter one more time. Follow all the steps from step 18 onwards one more time.
23. If the error% still exceeds the limits, the pressure transmitter must be replaced.
24. If the calibration was successful, apply a calibration sticker to the pressure transmitter.
25. Restore all the bypassed interlocks after informing the process operators. If the loop is still in manual mode, ask them to put the loop back in auto mode. Inform other plants about the completion of the calibration job. Also, restore the messaging system if changes have been made or notify all involved persons of the completion of the pressure calibration.

Pressure transmitter calibration procedure with dead weight tester

A dead weight tester is generally used in calibration laboratories. Dead weight testers have 5 to 10 times higher accuracy than a pressure calibrator or a pressure pump.

The accuracy to be obtained from a dead weight tester is about 0,006% of reading.

With a dead weight tester, discs with a very accurate weight are stacked on top of a piston that is filled with fluid. The piston converts the mass of the discs into an extremely precise fluid pressure.

Different types of fluids are available. Some dead weight testers work with air, others with oil or water. The choice of fluid depends on the test pressure that the dead weight tester must be able to supply.

Broadly speaking, the choice comes down to this:

• air: up to 14MPa (140bar)
• water: up to 70MPa (700bar)
• oil: up to 140MPa (1400bar)

Special hydraulic dead weight testers can even produce pressures up to 400MPa (4000bar) or more.

The steps below can be followed to perform the calibration of a pressure transmitter using a dead weight tester.

Attention: If the pressure transmitter is used to measure oxygen gas or any other chemical which can react adversely with the oil of the dead weight tester, then do not use the dead weight tester.

1. Set up the dead weight tester, pressure transmitter, 24Vdc power supply, HART communicator, and multimeter as in the picture above.
2. Before connecting the power supply directly to the pressure transmitter, check the output voltage and set it according to the requirements of the pressure transmitter. Use a 250 Ohm resistor in series in the case of a HART protocol pressure transmitter and a Foundation Fieldbus (FF) power module in the case of an FF-type pressure transmitter. Now, turn ON the power.
3. Check the oil level inside the dead weight tester.
4. Put the appropriate weight on the weight platform according to the dead weight tester’s chart showing weight versus pressure.
5. Use the hand pump to apply pressure until the weight on the weight platform begins to float between the red and blue lines.
6. Check the value on the pressure transmitter and note it down.
7. Check all values at 25% intervals, starting from 0% to 100%, and vice versa from 100% to 0%. This leads to a 5-points calibration. You can calculate the pressure value of these 5 test points by using our calibration test points calculator.
8. Calculate the error% as we did in the case of a pressure transmitter calibration using a pressure calibrator.
9. If the maximum value of the error% is more than the maximum allowable error%, perform a zero and span calibration.
10. Repeat step 7 and recalculate the error.
11. If the maximum error% is still greater than the allowable error, try a zero and span trim one more time. If the error remains too high, replace the pressure transmitter.
12. After calibration, gently clean the sensor unit of the pressure transmitter with a suitable cloth to remove the oil on the sensor side.

How to choose which calibration method to use?

The method of performing a pressure calibration is highly dependent on the desired accuracy of the pressure transmitter.

Hand pumps have a maximum accuracy of 0,1% of reading, dead weight testers go as far as 0,006% of reading.

Based on the fact that calibration equipment must be at least 4 times more accurate than the device to be calibrated, we arrive at the following conclusion:

• with a hand pump, pressure transmitters can be calibrated that must be accurate to a maximum of 0.5%
• with a dead weight tester, we can calibrate pressure transmitters to an accuracy of 0.025%

Pressure transmitters used in critical services or used for invoicing must have very high accuracy and should be calibrated with a dead weight tester.

If only a few of these high-accuracy applications need to be calibrated each year, it may be better to have this performed by an external test lab.

If the company has many of these demanding applications, it may be more cost-effective to purchase a dead weight tester and perform the tests in-house.

Keep in mind that for some applications it is required that the test lab is ISO/IEC 17025 accredited.

These high accuracies are not always necessary to control a process. Usually, the repeatability of the measurement is much more important. Many pressure transmitters for process controls only need an accuracy of 0.5% or 1% and can therefore be calibrated with a hand pump.

In addition to the two categories already mentioned, there is a third category of pressure transmitters where accuracy does not play a major role. These are the pressure transmitters that only serve as a visual indication of the process pressure. Just to have an idea of the pressure in the pipe or vessel. They only need an accuracy between 1% and 5%. In this case, it may not even be necessary to calibrate them.

Why does a pressure transmitter need to be calibrated?

A pressure transmitter has a diaphragm or pressure sensor assembly to measure the pressure and an electronic circuit to convert the sensor signal to an output signal.

A membrane or pressure sensor assembly is nothing more than a mechanical device, subject to mechanical stress, abrasion, chemicals, or degradation of the fill fluid. So, it is obvious that all this will have an impact on the electrical properties of the sensor, e.g. a change in capacitance or resistance, whatever measuring principle is used.

Also, the transmitter is installed in the field and is powered on for 24×7 hours. The electrical current constantly flowing through the electronic components will slightly alter the characteristics of these components over time. Temperature variations, moisture, and vibration will also add to the electronics’ aging. The output signal strength can vary depending on how the electronic components react to this aging effect.

Under the influence of the above effects, the accuracy of a pressure transmitter will slowly deteriorate. This slow process of deterioration is called drift. There are two types of drift:

• Zero drift: here the zero point will shift and the entire calibration curve moves up or down with it.
• Span drift: here the shift is in the slope of the curve, and the zero point remains in place.

Zero drift is more common than Span drift.

So, over a period of time, the response of the transmitter will change because of the aging effects on the diaphragm as well as the electronic circuits. The measurement starts losing reliability, the process is less under control and safety becomes less guaranteed.

Hence it becomes necessary to validate the response of the transmitter. If during validation, any deviations are found, then calibration of the transmitter is necessary to avoid problems or measurement inaccuracies. A calibrated pressure transmitter can help increase process efficiency as well as prevent unwanted shutdowns and fatalities.

How often should you calibrate a pressure transmitter?

The calibration frequency of a pressure transmitter is determined by the process criticality. In general, pressure transmitters are calibrated with an interval of 3 to 5 years. In case of malfunction or incorrect operation, the pressure transmitter must be calibrated.

Related topics

Temperature calibration – How to do it right

Calibration test points calculator

DP Flow Transmitter Calibration: A Step-by-Step Guide

RTDs can also be checked by tolerance testing. Here’s an easy tool to do it:

• Tolerance testing tool