A regular calibration of your measuring equipment gives you security in the production process or the maintenance of the equipment. However, this only applies if the calibration has been carried out correctly. We have listed 5 tips that you should consider for your pressure calibration.
A calibration is only meaningful if the measurement uncertainty of the calibration is included in the conformity statement. The greatest uncertainty of measurement comes from the reference instrument. The reference instrument should be many times more accurate than the pressure gauge which is to be calibrated. In practice, a ratio of 1:4 has proven to be practical. For critical calibrations, the uncertainty of the calibration can be determined according to GUM (Guide to the Expression of Uncertainty Measurement). In this process, uncertainty factors are determined using statistical methods and included in the measurement uncertainty consideration.
In calibrations of pressure instruments, the circuit between the reference instrument and the test item is closed. In closed gas systems, the gas temperature affects the volume. With rapidly increasing pressures, the gas temperature may also increase, resulting in an increase in volume and thus an increase in pressure. In contrast, when the gas temperature decreases, the pressure decreases. This can result in a falsification of the measurement results. Therefore, the pressure steps should be kept small and the pressure should be kept stable at one value for a while before recording the measurement results.
A common mistake made when calibrating pressure gauges is not considering the hysteresis error. If you increase the nominal pressure from 0 to 100% when measuring a pressure sensor, the output signal will always be less than the real value. If the pressure is subsequently reduced from 100% to 0%, the value displayed will always be slightly too high. The maximum difference between the upward and downward characteristic curve is then indicated as a hysteresis error. The hysteresis corresponds to the elastic properties of the sensor element and its design. The inertia of the sensors causes them to "lag" the actual nominal pressure. By repeating several up and down series during the calibration of a test device, the hysteresis error can be determined.
During a calibration, the calibration cycles should be repeated several times to determine the repeatability error of the test equipment. Poor repeatability occurs if the test equipment being calibrated gives significantly different results at each different calibration cycle. If a piece of test equipment is calibrated in only one cycle, only a snapshot can be recorded. By repeating the cycles and forming the average value, the expected measured value of the test equipment is approximated.
There are also several points to consider when documenting calibration results on a certificate or calibration certificate. A calibration certificate should contain at least the following information:
• Address of the performing calibration laboratory
• Address of the customer
• Designation of the measuring equipment
• Date of calibration and date of issue of the calibration certificate
• Ambient conditions
• Method used
• Reference equipment used and its traceability
• Modification of the measuring equipment in the course of repairs/adjustments
• Uncertainty of measurement
• Statement of conformity
• Inspector and second instance (supervisor)