What to consider when selecting a differential pressure sensor

17th August 2008
Differential pressure measurement can be one of the most complex and confusing applications when it comes to choosing the right pressure sensor for the job. Not only must you consider the differential pressure range required, but also whether the sensor is designed to withstand the system pressure which is often much higher than the differential pressures being measured.

Since you are often measuring a differential pressure that is lower than the system line pressure there maybe a high risk of accidental over-pressure which can put a differential pressure transmitter out of calibration or out of service permanently particularly at very low differential pressures.

Some processes have a varying line pressure which means that the stress on the mechanical structure of each side of the differential pressure transducer is going to change. If the differential pressure cell or dp cell has not been mechanically designed to balance these stresses equally on both sides, a change in line pressure will result in a zero shift offset in the output signal. This is because the sensing diaphragm is coupled to the surrounding components and will be sensitive to any imbalance in stress causing a slight diaphragm deflection.

Some differential pressure transmitters use oil filled capsules with thin isolation membranes on both sides of the sensing diaphragm. As the temperature increases or decreases the oil will expand and contract and this will be sensed by the diaphragm if the oil filling is not equally balanced. The isolation membranes if convoluted will reduce this effect by allowing the oil fill volume to increase and decrease with minimal stress on the sensing diaphragm.

So called dry cell differential pressure transmitters do not have oil filled cells and thus do not exhibit thermal errors associated with oil expansion. However, like all types of electro-mechanical pressure sensors they are not completely immune to thermal errors, since the sensing technology and the mechanical construction will still react to changes in temperature.

To combat these issues with differential pressure measurement, some manufacturers have incorporated features to protect differential pressure sensors and improve on their performance.

If the application has the potential for high over-load differential pressures on one side then look for a differential pressure transducer which incorporates mechanical diaphragm stops or if the differential pressure is relative high compared to the static line pressure consider a differential pressure transmitter that uses two absolute pressure sensors instead.

If the system line pressure varies considerably and you are looking for a high accuracy differential pressure measurement then consider a dp transmitter that includes an absolute pressure transducer which is used to compensate for line pressure drift.

If the operating temperature varies over a wide range, check the thermal error quoted on the product data sheet particularly the thermal zero errors. When checking the accuracy of a pressure sensor on the product data sheet of any kind it is important to understand what the accuracy statement includes, since many only include the accuracy at room temperature and you have to look elsewhere on the data sheet to find the errors associated with a temperature change.

Related Products

Differential pressure sensors and instrumentation

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