Differential Pressure with a Tiny Sensor

Process control requires system operators to monitor and control the condition and movement of liquids and gases. Several instruments are available for this, allowing measurement and monitoring of variables, and these fall under the categories of pressure, temperature, level, and flow. Among the pressure-gage category, differential-pressure gages receive the widest recognition for being the largest specialty type – useful in filtration, flow, and level measurements.

While standard pressure gages measure pressure at a single point in a system, differential pressure gages measure pressures at two points and display the difference on a single dial. This makes it easy for the operator to know at a glance, which of the two points is at a higher pressure, and by how much. Use of differential pressure gages greatly reduces operator error, protecting expensive equipment. They reduce operator training and maintenance time, thereby improving process efficiency.

For instance, differential pressure gages are popularly applied in filtration. In this process, a filter separates unwanted contaminants or particles from a gas or liquid system. However, with the progress of the process, the filter becomes increasingly clogged, leading to a drop in efficiency and pressure at the outlet.

It would seem enough to use a single standard pressure gage at the outlet to monitor the health of the filter and assess the time for its inspection and replacement. However, the situation is complicated, as most processes do not maintain a steady working pressure. Several factors are responsible for this, such as compressor or pump on-off cycles or valve open-close cycles, causing wide pressure fluctuations in most processes. For many systems, operators expect such fluctuations of pressure as normal, within limits.

Using two standard pressure gages, one at the input and the other at the output, introduces two additional problems for the operator. First, this compounds the accuracy errors resulting from the two gages as against error from one gage. Second, the operator needs training in reading the two gages, then subtracting the readings, and finally, interpreting the result. History shows many operators do not truly understand the importance of the calculation.

Installing one differential pressure gage using the same taps at the filter inlet and outlet solves all the problems listed above. The accuracy goes up as the rate of error drops. Additionally, the operator does not have to rely on mathematics to understand and interpret the reading – most differential pressure gage dials feature a red arc to indicate the clogging of the filter.

The SDP3x differential pressure sensor from Sensirion is a tiny device. Its dimensions are only 5x8x5 mm, making it one of the smallest of its kind, but with countless new possibilities of applications. It is well suited for use in portable medical devices as well as in consumer electronics.

Users can choose between an analog signal output and a digital one from two versions of the fully calibrated and temperature-compensated differential pressure sensor. The digital sensor, the SDP31, comes with an I2C interface, while the analog sensor, the SDP36, offers an analog output signal. The sensors have a sampling rate of 2 KHz with a resolution of 16-bits, and a measurement range of +/-500 Pa with a span accuracy of 3% of the reading.