Types of pressure calibration & methods
Types of pressure gauge
Pressure measuring instruments are used in a variety of applications and industries. In order to ensure and maintain the proper performance of pressure instruments, initial and regular calibration is advised. However, with there being several types of pressure instruments, there are three types which are mainly calibrated in laboratories:
- Manometers
These manometers are complete mechanical measuring instruments that indicate units of pressure (bar, Pa, psi, etc.). Depending on the application they may consist either an analogue or digital indication.
- Pressure transducers
They convert the measured pressure into an analogue electrical signal that is proportional to the applied pressure. Depending on the model and the application, the analogue output signal can be voltage, current or frequency. Pressure transducers need external continuous power supply.
- Pressure transmitters
They are units consisting of a pressure transducer and a module for conditioning and amplifying the transducer signal. Depending on the type and application, the output information of a pressure transmitter can be voltage (5V, 10V, etc), current (0-20mA, 4-20mA, etc), frequency or a digital format such as RS 232. Pressure transmitters also need external continuous power supply.
Pressure calibration equipment
The most common instruments used by calibration laboratories for pressure calibration are dead weight testers and pressure calibrators.
Dead weight testers
Traceable weights apply pressure on a fluid for the purpose of checking the accuracy of pressure measuring instruments. They use a piston cylinder on which a load is placed to make an equilibrium with an applied pressure underneath the piston. Dead weight testers can operate either hydraulically or pneumatically. These testers are considered to be highly accurate and they are characterised as primary standards.
Pressure calibrators
The most frequently used instruments for pressure calibration. They are digital instruments and they can be handheld or benchtop. They can contain internal or external pressure pump, internal or external pressure transducers and they can also be intrinsically safe for use in potential explosive environments. Pressure calibrators can contain electrical or temperature modules in order to be able to measure and/or generate voltage, current and temperature. These instruments are really easy to use and are very convenient especially for on-site pressure calibrations.
Method of calibration
Before beginning any calibration procedure, all other metrological requirements must be met. The instrument must be left in the laboratory to stabilize for sufficient time into the environmental conditions. The condition of the pressure gauge must be checked to ensure that there is no obvious damage or malfunction. Special care must be given to the manufacturer’s specifications about mounting position, torque or any other special requirements.
In the example below, we will assume that our reference standard is a pressure calibrator with internal pump. The pressure gauge must be connected to the output of the pressure calibrator. Attention must be given to the connections’ tightness in order to avoid leakages which will affect the measurement result.
Initial checks
Before proceeding to the calibration procedure, an initial check must be performed, in order to determine the pressure gauge’s condition:
- Operate the instrument and bring it at least twice to its upper pressure limit and keep the pressure for at least one minute.
- During the first pressure rise, check out the indication obtained for the conformity with the specifications.
- Read the indications of the gauge at 0%, 50% and 100% of its measurement span.
Pressure calibration procedure
The most usual calibration procedure is the Basic procedure described in Euramet/CG-17/v.01 “Guidelines on the calibration of electromechanical manometers”:
Calibration is performed once at six pressure points (spread over the measuring span of the gauge) in increasing and decreasing pressures. Repeatability is estimated from three repeated measurements at one pressure point (preferably at 50% of full scale).
At each pressure point at least the following data shall be recorded:
- The pressure indicated by the reference instrument
- The indication of the instrument under test
- The values of the influence quantities (temperature, atmospheric pressure)
- The identification parameters of the instrument under test
- The identification of the instruments included in the measuring system
Pressure calibration report
The calibration report must present the results in a way that they can be easily evaluated by the end user. Besides the mean value of the increasing and decreasing pressure measurements and the deviation of the mean value from the reference value, the repeatability error and the hysteresis error must be presented as well.
Hysteresis relates to the mechanical hysteresis of the sensing diaphragm where the reading of the pressure gauge varies depending on whether the pressure has been increasing or decreasing prior to the measurement. The hysteresis error is defined as the deviation in value between the increasing and decreasing pressure value measured at the same step point.
Pressure gauge uncertainties
Where the calibration concerns an analogue pressure measuring instrument, there are several sources of uncertainty. Some of them are presented below:
- Uncertainty of the reference instrument in the conditions of use (calibration certificate uncertainty, long term stability, environmental conditions, etc)
- Uncertainty due to repeatability
- Uncertainty due to the finite resolution of the instrument under test
- Uncertainty due to hysteresis of the instrument under test
- Uncertainty due to estimation of the head correction between the instrument under test and the reference instrument
- Personal bias in reading analogue instruments
Following the principles of the EA-4/02 “Expression of the Uncertainty of measurement in Calibration” we can evaluate the uncertainty for each measurement value.
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