Differential pressure sensors are commonly used in test benches, wind tunnels, leak detection systems, and other applications. This article outlines six key characteristics and considerations to consider when selecting sensors for differential pressure, critical pressure applications.
I. Orientation Effect: Incorrect installation, vibration, or even system maintenance can cause changes in the orientation of the sensor, which is known as the orientation effect.
Orientation effects have always been a problem for other types of sensing technologies. Even correctly installed sensors can experience edge gravity effects because rotating the sensor 180 degrees changes it from positive to negative gravity, resulting in a 2G force change. In this case, the sensor cannot distinguish between the force exerted by gravity and the force exerted through the pressure port. Therefore, the sensor combines the gravity-weighted effect with the port pressure and sends incorrect signals.
For sensors filled with silicone oil or other insulating media, the directional effect during sensor rotation becomes more pronounced. The weight of the diaphragm and the filling liquid both affect the sensor. Consequently, the sensor may fail to measure the true pressure and send erroneous values influenced by orientation variations.
Fortunately, some sensors use very thin, lightweight capacitive diaphragms and are not filled with liquid, which minimizes the effects of gravity on them. Therefore, these sensors can be mounted in any orientation with the assurance that they will provide highly reliable measurements. This is especially useful for installations where space is extremely limited.
For optimal installation, the sensor should be installed in the same orientation as it was calibrated. For example, if it was factory-calibrated with the pressure port facing downwards in a vertical position, it is recommended to install it in the field in the same manner to minimize orientation effects. If this is not possible, the drift of the minimum zero-point offset can be compensated for by manually adjusting the sensor's zero-point offset or by using the safety calibration key.
Second, low-frequency vibrations from nearby motors or fans can also affect the accurate positioning of sensors.
For example, the liquid in an oil-filled sensor may pick up low-frequency vibrations and apply an inertial load to the diaphragm, which could be mistaken for a change in process pressure. To avoid this vibration effect, end users need to install the sensor in a quiet, remote location.
Similarly, if the reference port is open to the atmosphere, it needs to be connected to an area free from vibration, noise, and wind. For wind tunnels, due to the installation of a pitot tube, both pressure ports can be connected to remotely mounted sensors via hoses or semi-hose, preventing air turbulence noise or mechanical vibration from being transmitted to the sensors. Engineers can consider using capacitive sensors designed to minimize directional and vibration issues; these sensors use a stretched stainless steel diaphragm and are not filled with liquid. The only gravitational effect is the weight of the diaphragm, which, while not negligible, has a very small impact and can be easily compensated for in the field.
III. Overpressure Protection Overpressure protection and reverse pressure protection have always been the most important issues for leak detection system manufacturers.
These systems seek low leakage rates in differential pressure and high static pressure applications. Leak detection manufacturers are constantly looking to measure increasingly lower leakage rates. Since leakage rates are directly proportional to differential pressure, these manufacturers want to be able to measure increasingly smaller differential pressures. To achieve this, static test pressures need to be increased significantly.
Unfortunately, sensors experiencing unexpected overpressure under differential pressure and very high static pressure conditions may require recalibration and, more likely, become ineffective. A large leak in the measured system will also lead to the same result. The latest generation of sensors must address these issues. Therefore, sensors have become more robust. They can withstand relatively higher overpressure ranges, both positive (process) and negative (reference). This is a very important new feature.
Previously, sensors only had overpressure protection in the forward direction, but leakage in the reverse direction could cause reverse overpressure. Sensors with adequate protection in both directions are suitable for applications where unexpected overpressure or large leakage may occur. If this happens, the sensor will continue to operate normally. The sensor can withstand unexpected overpressure at its rated withstand pressure (e.g., 150 PSI) and then return to normal. If the withstand pressure is exceeded, the diaphragm may permanently deform, causing zero-point drift. If the pressure at any port exceeds the rupture pressure (e.g., 300 PSI), it will damage the sensor chamber, leading to weld failure, seal leakage, or diaphragm or housing rupture. OME and application engineers must understand the sensor's withstand and rupture pressure limits. Furthermore, they must understand that their own systems may have unexpected venting, or that components of the test equipment may not be properly sealed, both of which could damage the sensor. Therefore, they should use small, robust stainless steel sensors with adequate protection in both directions to withstand these unexpected conditions.
Figure 1: Performance Limits of Pressure Sensors
IV. In addition to overpressure, changes in pipeline pressure also need to be considered, especially in leak detection applications where static pipeline pressure is high.
Pipeline pressure is the absolute pressure applied to the sensor port. However, variations in static pipeline pressure can cause slight stress deformation in the sensor's shape. These stresses, in turn, alter the sensor's calibration response, affecting its zero point and range.
The latest generation of sensors employs a design that significantly reduces the strain on the sensing element caused by static pressure. Look for sensors with a rated low-pressure effect, such as 2%FS/100PSIG. Fortunately, errors caused by line pressure can be corrected by recalibrating the device or re-zeroing. This can be done manually with a potentiometer, or a simple and safe adjustment can be made using advanced models with a small calibration button (equipped with a digital display mounted on the sensor). The calibration button functions include resetting the zero point and range, as well as restoring factory settings.
V. Response Time Response time is another important factor, especially for pressure control and wind tunnel applications.
The response time of a sensor (the time interval between a sensor responding to an applied pressure and generating an output signal) is primarily determined by the technology and electronics used in the sensor's sensing element. Diaphragms using capacitive sensing technology typically respond very quickly. They detect and measure pressure by sensing changes in voltage across a capacitor, one plate of which is a diaphragm capable of reflecting even slight changes in applied pressure.
The resulting capacitance change is detected by the sensor's electronics, which, after linearization, thermal compensation, and modulation, output a proportionally high-level signal. The requirement for a fast response time depends on the application. For example, in wind tunnel applications measuring dynamic airflow velocity changes, the sensor's signal output must change with wind speed, thus requiring a fast response time. For most test bench, leak detection, and wind tunnel applications, a response time of 10–80 mm is generally acceptable.
For routine processing and monitoring applications where response time is less critical, response times are typically several seconds, not milliseconds. Understanding the response time requirements of pressure sensors is crucial when designing a system; faster isn't always better. If a sensor responds too quickly, it may react to brief, unfiltered bursts of unwanted system noise or turbulent pressure fluctuations. In such cases, filtering the output signal can attenuate these unwanted disturbances.
Other considerations include selecting sensors with high stability, capable of maintaining performance characteristics over a relatively long period, especially range stability. Typically, stability should be less than ±0.15%FS/Yr. This rating is given in the sensor's specification sheet, which can also be consulted to check if the sensor is CE and RoHS certified. The CE mark indicates that the sensor complies with EU customer safety, health, and environmental requirements. RoHS certification specifies the maximum permissible levels of six hazardous substances in the sensor: lead, mercury, cadmium, hexavalent chromium, PBB, and PBDE.
