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Digital sensor for high precision pressure calibration
15th March 2007  Primary pressure standards such as dead weight testers offer the highest level of pressure measurement precision and accuracy for most pressure calibration laboratories, but they are mostly manually operated and the results have to be typed into the computer or handwritten into calibration reports.
If you are calibrating a large number of pressure instruments everyday then you will need a more efficient method for pressure calibration which does not compromise on pressure reading accuracy. Depending on your budget you could invest in a pressure controller calibrator that is computer controlled which will automatically set and read the pressure for you and record the results. If you are calibrating pressure sensors you can also add a high accuracy digital multi-meter to record the output signal fully automating pressure calibration. Pressure sensor manufacturers will add a temperature controlled oven to an automated pressure calibration system so that they can test the performance of the pressure transducer for compensation and characterisation purposes over the operating temperature range. However most pressure calibration laboratories will only calibrate at room temperature and will not include an oven. This is an expensive approach but if you have the budget it is the ultimate in high volume pressure calibration. If you want to calibrate pressures on a more modest budget then you should look to compromise on the sophistication of the pressure control. This is not critical to the pressure reading accuracy although you need a method of applying pressure in one direction with minimal risk of pressure over-shoot. This will ensure no pressure hysteresis is introduced into an increasing or decreasing pressure set-points. A simple hand pump or pressure regulator can be used for this but the finer the control the easier it will be to set calibration pressures in one direction. Depending on the pressure calibration accuracy you want to achieve you will need a pressure sensor that has a precision that is better than the required calibrator accuracy. This is because the pressure sensor will need to be regularly calibrated and the accuracy of the sensor will be derived from consideration of the accuracy of the transfer pressure standard used and the precision of the pressure sensor. How often you need to calibrate your reference standard pressure sensor will depend on the long term stability of the sensor technology used inside the pressure sensor. Although a pressure sensor with very good long term stability may seem expensive in the short term, the cost of ownership is going to be much lower because you will not need to calibrate it so often. To maximise the pressure reading accuracy avoid using an analogue output signal which will introduce extra uncertainties into measurements because the signal will be processed four times before it is recorded by the computer: The sensor element analogue signal is converted to digital and then digitally characterised before being converted back to analogue to provide an output signal from the sensor. Then a digital voltmeter is used to measure the analogue output signal which in turn converts to a digital signal. If you adopt this approach you will introduce the uncertainty of three analogue conversions into the overall calibrator accuracy. You will eliminate two out of the three analogue conversions if you use a pressure sensor with a RS232 or RS485 serial interface instead fully exploiting the pressure sensors accuracy. The 6100 ultra high accuracy digital pressure transducer offers a precision of 0.003% full scale and a long term stability of 0.01% FS every 180 days. This high precision pressure sensor achieves an accuracy of 0.01% full scale over a temperature 15 to 45 Degrees Celsius which includes linearity, hysteresis, repeatability and thermal errors. Any pressure range can be specified for the 6100 high precision digital pressure sensor between 25mb and 400 bar gauge or 350 mbar and 400bar absolute. The output signal of the 6100 digital pressure reference standard can be factory set to a RS485 or RS232 digital interface and if necessary there is the option for an analogue voltage output of 0-1Vdc, 0-5Vdc or 0-10Vdc. You can choose from an exhaustive list of pressure units for the pressure reading output of the 6100 laboratory pressure transducer which include bar, psi, inHg, mH2O, mmHg, cmHg, inH2O, ftH2O, inSW, ftSW, mSW, Atm, mbar, mmH2O, cmH2O, Torr, mTorr, hPa, MPa, kPa, Pa, D/cm², g/cm² and Kg/cm².
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Site Last Updated: 13/05/08
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