This article will cover common challenges faced when calibrating differential pressure (DP) sensors, before exploring different methods of calibration.
Figure 1. Pressure chart
The Affect of Temperature
The effect of the external temperature can be the biggest problem when it comes to calibrating DP sensors. When a
DP sensor is being used to take readings at pressures low in the full-scale (FS) range, small temperature fluctuations can introduce large errors.
Temperature fluctuations can result in a lack of precision for both the sensor being tested (the reference gauge) and the calibration standard (the pump).
Producing Constant Pressure
Pressure must be stable and therefore produced by a stable source in order to take a useful calibration measurement. DP sensors have a high sensitivity so the generated pressure must be extremely stable with a resolution which is capable of producing pressures at exact points.
The generation of a stable pressure, which is accurately controlled, can be difficult as most pressure pumps use non-returning (check) valves to provide stability. Non-returning valves can degrade and begin to leak over time, meaning it becomes difficult to create pressures stable enough for the calibration of DP sensors.
Variations in Atmospheric Pressure
Many DP calibrations are recommended to take place with the reference port (or low port) open to the external environment. However, for the calibration to be accurate the external atmosphere would have to be at a constant pressure and this is unlikely. Changes in external pressure can result in a bad calibration.
Methods of Calibration
1. Using the ADT761-LLP or ADT761-D for Automated Calibration
To carry out this test, automated testing using the ADT761 is ran, which means the pressure will be automatically generated and stabilized followed by a reading of the
DP gauge. Depending on the device being tested the gauge may need exercising a few times to full scale.
3-5 different calibration points will be taken in upwards and downwards directions which will then be used for automatic hysteresis calculation and determination of the pass or fail.
This method is advantageous as it can be fully automated, including calculations and interpretations, and is not significantly impacted by external changes in pressure or temperature. However, the system is the most expensive to run.
Figure 2. ADT681 DP gauge and ADT901 pump
2. Using an ADT901 Pump, ADT681 DP Reference Gauge with the DUT's Reference Port Open to Atmosphere
To carry out this test method the vent valve must be open to the ADT901 and the device and reference gauges zeroed. The ADT901 vent valve should then be closed and, once stabilized, readings at 3-5 calibration points (upwards and downwards) should be taken to calculate the hysteresis.
This is a very economical method that is easy to follow. However, changes in the external environment must be calculated throughout the calibration and this can result in inaccurate measurements. As a result, this is the least reliable method of DP sensor calibration.
Figure 3. ADT681 DP gauge and ADT912 pump
3. Using an ADT901 Pump, ADT681 DP Reference Gauge with the DUT's Reference Ports Connected Together
The start of this calibration method varies depending on the device being tested. If the device is analog (i.e. doesn’t require zeroing) then the reference ports should be opened to zero the gauges after which they should be reconnected. If the device is digital then it should be connected to the pump and then zeroed.
The ADT901 vent valve should then be closed and, once stabilized, readings at 3-5 calibration points (upwards and downwards) should be taken to calculate the hysteresis.
This method is also inexpensive but considers changes in the external atmosphere better than in method 2, so each measurement point is more accurate. However, it is more complicated to run and temperature changes can have a greater negative impact as the system is sealed and interconnected.
Figure 4. ADT761 calibrator with DUT
Further Information of the Equipment
Accurate and stable measurement for
DP sensor calibration is possible using the Additel 761 Automated Pressure Calibrator as it is highly precise and uses next-generation sensor technology. The calibrator uses its own integrated pump for gas and electricity free pressure generation and uses two temperature compensated sensors to ensure temperature compensated readings. The sensor calibration process of transducers, switches, transmitters, and gauges can be carried out automatically, including with report generation.
Calibrating DP sensors can be even more difficult if carried out in an uncontrolled environment. The Additel 901 and 912 pumps are equipped with a thermally isolated chamber which means temperature can be controlled more tightly, in addition, their screw presses mean that the pressure can be precisely adjusted with no check valve – a common cause of instability. For these reasons, the Additel 901 and 912 are perfect for DP sensor calibrations.
Calibration tools in the ADT672 DP and ADT681 DP series allow differential pressures at a variety of pressure ranges. The ADT672 can measure pressure, as well as voltage, current, pressure switching, and a transmitter; whereas the ADT681 measures only pressure.
This information has been sourced, reviewed and adapted from materials provided by Chamois Metrology Ltd.
For more information on this source, please visit
Chamois Metrology Ltd.