A pressure transducer (variously known as a pressure sensor or a pressure transmitter) is a device which measures pressure and then converts into an analog signal. Converting it into this signal allows it to either be read by an end user or to be fed into a larger device so that it can perform a different function.
There are a range of different kinds of pressure transducers but the most commonly used is known as a strain gauge pressure transducer. These are able to measure pressure from the movement of strain gauges which are attached to the diaphragm of the transducer. When pressure is applied to the diaphragm it flexes, this puts strain on the gauges. This strain produces electrical resistance which can be interpreted by a Wheatstone bridge.
How do pressure transducers work?
The uses for pressure transducers can vary quite significantly. On a basic level a pressure transducer can have a binary function, with an on/off switch to simply measure whether pressure is present or not. Alternatively, they can be quite complicated, understanding changes in pressure across a device or taking a measurement against ambient air pressure.
The functions of transducers are wide ranging and they are often found in everyday devices. Speed cameras, for example, use pressure transducers to understand whether or not to take a photograph. Transducers also have uses in barometers and tyre pressure applications.
What kind of pressure transducer do you need?
When deciding what kind of pressure transducer you need for a function, it’s first important to understand the kinds of pressure that can be measured. The three most important kinds of pressure sensor are absolute, gauge and differential. Absolute pressure sensors measure in reference to a vacuum, gauge pressure sensors measure in reference to ambient atmospheric pressure and differential pressure sensors measure the different between two pressures.
Perhaps the most important aspect of choosing the right pressure transducer for your needs comes from the range it needs to have. In effect you are trying to find a happy medium between a range that is low enough to provide extremely accurate readings, and a range that is high enough to avoid overpressure. Overpressure damage can be a serious problem that can cause the transducer to function incorrectly.
Many pressure transducers, therefore, have built-in overpressure protection. There are two different kinds of serious overpressure that can be expected; pressure spikes or consistent overpressure for a long duration. You need to consider ways of protecting the transducer against these types of overpressure in different ways.
When you expect spikes in overpressure it makes sense to install a device known as a snubber. These are able to stop spikes from occurring but do have the drawback of making the transducer less responsive. Alternatively, if you expect periods of sustained overpressure you can make a pressure relief valve a part of the design. This immediately relieves the strain on the transducer, but has the disadvantage that the device won’t be functional while the valve is open.
Whenever you are thinking of installing a pressure transducer you have to make sure that it meets the corrosion requirements for whichever task it is looking to perform. It’s also important to be aware whether explosive vapours will be present where the transducer is located. If so, the transducer must be protected from these vapours.
If the transducer needs to operate in very high or low temperatures it will also need special consideration. If it is to work at a high temperature it will need to be cooled. This can be achieved with water or through electrical cooling. In freezing conditions, the transducer can be fitted with thermal insulation as well as resistance heating.
The bottom line
Before choosing a pressure transducer you need to think very carefully about the way it needs to function and the likely pitfalls that will occur during its operation. If you can foresee the potential problems, you’ll be in a great position to put protection in place to deal with them.
Article provided by Mike James, working together with Sussex-based application engineering firm App Eng, who were consulted over this post.