Pressure Transducer Response Time/Overload/Vibration/Pipeline

Nowadays, the performance of pressure transducers has increased to the level of solutions available for demanding applications. This article describes the six characteristics and conditions that must be considered when selecting a pressure transducer for measuring differential pressure and critical pressure applications.

• Response time of the pressure transducer

Response time is an important factor affecting the use of pressure transducers, especially for pressure control and wind tunnel systems. The response time of a pressure transducer is the time interval from when the transducer responds to an applied pressure to when generate an output signal. It is primarily determined by the technical and electronic components employed by the sensing elements of the pressure transducer. Diaphragms using diffusion silicon technology typically respond faster. The pressure of measured medium causes the diaphragm to generate a micro-displacement proportional to medium pressure, which changes the resistance value of the transducer. This change in resistance is detected by the electronic components of the pressure transducer. The electronic components are linearized, thermally compensated, and modulated to output a proportional high level signal.
The need for fast response times depends on the application. For example, in a wind tunnel application that measures the change in velocity of a dynamic airflow, the signal output of the pressure transducer must vary with wind speed, thus requiring fast response times. For most test benches, leak detection and wind tunnel applications, a response time of 10-80 milliseconds is usually acceptable. For conventional processing and monitoring applications where response time is less important, the response time is typically a few seconds instead of a few milliseconds. When designing a system, it is important to understand the response time requirements of the pressure transducer, not always as fast as possible. If the pressure transducer response is too fast, sometimes the transducer will respond to brief, unfiltered and unwanted system noise or turbulent pressure fluctuations. In this case, filtering the output signal can attenuate these unnecessary interferences.

• Overload protection of the pressure transducer

Overload protection and reverse pressure protection are always the most important issues for manufacturers of leak detection systems. These systems seek for small leak rates in differential pressure and high static pressure applications. Leak detection manufacturers always want to measure increasingly lower leak rates. Since the leak rate is directly proportional to the differential pressure, these manufacturers want to be able to measure smaller and smaller pressure differences. To achieve this goal, static test pressure needs to be increased to a higher level. Unfortunately, under micro-pressure difference and very high static pressures, pressure transducers that experience unexpected overloads may require recalibration and are more likely to become ineffective. If a large leak occurs in the measured system, the same result will occurs.

• Overload protection of the pressure transducer

Overload protection and reverse pressure protection are always the most important issues for manufacturers of leak detection systems. These systems seek for small leak rates in differential pressure and high static pressure applications. Leak detection manufacturers always want to measure increasingly lower leak rates. Since the leak rate is directly proportional to the differential pressure, these manufacturers want to be able to measure smaller and smaller pressure differences. To achieve this goal, static test pressure needs to be increased to a higher level. Unfortunately, under micro-pressure difference and very high static pressures, pressure transducers that experience unexpected overloads may require recalibration and are more likely to become ineffective. If a large leak occurs in the measured system, the same result will occurs.
The latest generation of pressure transducers must address these issues. Therefore, the pressure transducer becomes more robust. They can withstand relatively higher overload, either positive (process) or negative (reference). This is a very important new feature. Previously, pressure transducers only had overload protection in the positive direction. However, a reverse leak will result in a reverse overload to the pressure transducer. Pressure transducers that are fully protected in both directions are suitable for applications where accidental overload or large leaks may occur. If it happens, the pressure transducer will continue to work normally.
The pressure transducer can withstand unexpected overloads other than its rated withstand pressure (such as 150 PSI) and then return to normal. If the withstand pressure is exceeded, the diaphragm will be permanently deformed, causing a zero drift. If the pressure at any of the ports exceeds the burst pressure (such as 300 PSI), the pressure transducer's measurement chamber will be destroyed, causing weld failure, seal leakage, or rupture of the diaphragm or casing. The user must be aware of the pressure transducer's withstand pressure and burst pressure limits. In addition, the user must understand that the system may be accidentally ventilated or that the components of the test equipment are not sealed, which will damage the pressure transducer. Therefore, users should avoid these unexpected situations by using a stainless steel pressure transducer that is small, sturdy and has sufficient overload protection in both directions.

• Vibration of the pressure transducer

Experience has shown that low frequency vibrations from motors or fans near the pressure transducer can also affect the transducer. For example, liquid in an oil-filled pressure transducer may pick up low-frequency vibrations and apply an inertial load to the diaphragm, which is mistaken for process pressure changes. To avoid this vibration effect, the end user need to install the pressure transducer in a quiet area at the far end. Similarly, if the reference port is in communication with the atmosphere, the pressure transducer needs to be connected to an area where there is no vibration noise and wind. For wind tunnels, because of the installation of a pitot tube both pressure ports can be connected to remotely mounted pressure transducers via hoses or half hoses. It prevents air disturbance noise or mechanical vibration from being transmitted to the pressure transducer.

• Effect to pipeline pressure of the pressure transducer

In addition to overload, changes in pipeline pressure need to be considered, especially in leak detection applications with high static pipeline pressure. Pipeline pressure is the absolute pressure applied to the pressure transducer port. However, some changes in static pipeline pressure will cause slight stress distortion in the shape of the pressure transducer. These stresses in turn will change the response and calibration of the pressure transducer and affect its zero and measurement range.
Fortunately, the measurement error caused by pipeline pressure can be corrected by recalibration or re-zeroing for the pressure transducer. It can be done manually with a potentiometer, or it can be easily and safely adjusted using an advanced model with a small calibration button (equipped with a digital display mounted on the transducer). Calibration key functions include zero point, range reset, and factory reset.

• Other considerations

In addition to the above points need to be noted, it is recommended to select a pressure transducer with excellent long-term stability. It will enable good performance over a relatively long period of time, especially the stability of the range. Generally, the long-term stability of the pressure transducer should be less than 0.2% FS/year. This rating is given in the specification sheet of the pressure transducer.
Finally, look for a supplier of pressure transducers that supply a variety of configurations for pressure transducer. It will enable end users to work with a manufacturer to obtain a pressure transducers with specific accuracy, output signals and ranges for a variety of different applications.