How To Choose A Differential Pressure Sensor
This article discusses the factors to consider when choosing a differential pressure sensor. These include the differential pressure range and the maximum system pressure that the device can handle. A differential pressure transducer detects the difference between two or more pressures that are required as input for the feedback system. One should be careful when determining the appropriate specifications for this device because the magnitude of the differential pressure is smaller than the actual pressure that it is subjected to. Thus, there is a danger of subjecting the transducer to too much pressure and this may damage the accuracy of a differential pressure transmitter. This may even permanently spoil the precision of the differential pressure sensor especially for the relatively small differential pressures.
Another potential problem with the failure to make the proper specifications for a differential pressure sensor is the possibility that the pressure applied by the processes may vary at both ends of the device. This may result in unnecessary stress to its mechanical structure and if it has not been designed to handle this kind of pressure, this may cause a shift in the analog output signal. This is possible because the sensing diaphragm is linked to the neighboring parts and can be easily influenced by any unevenness in stress. This unequally distributed stress can cause the sensing diaphragm to shift slightly from its original position.
There are devices that are oil filled and have slim isolation membranes on both ends of the sensing diaphragm. If the pressures applied against the isolation membranes are not the same, the oil in the cells will either expand or contract and the sensing diaphragm will be affected. To prevent this from occurring, the design of the isolation membranes can be modified to permit changes in the oil volume with minimal effect on the sensing diaphragm. On the other hand, dry sensors like Keller pressure sensors are not prone to this kind of error caused by the thermal expansion of the oil. However, they are still not totally free from thermal errors because the mechanical structure and the sensing technology will still be affected by thermal changes.
Differential pressure sensor manufacturers have come up with various ways to counteract the above effects to reduce error. Some devices have mechanical diaphragm stops and these are suitable for those situations when there is a high probability for overloading one side of the sensing diaphragm. And for those conditions when the differential pressure is relatively high in contrast to the static line pressure, devices that have two absolute pressure transducers may be preferred. For those situations when there is a significant variation in the system line pressure, you may want to take into account a DP transmitter that has an absolute pressure sensor that is utilized to compensate for the changes in line pressure. Meanwhile, if accuracy is very important, you will need to examine the product data sheet carefully with regards to thermal zero errors. Many accuracy specifications only indicate the precision at room temperature and you will need to find the errors due to temperature variation in other parts of the data sheet.
