Pressure sensor diaphragms come in a variety of sizes and materials. And they’re attached to a variety of sensing elements and technologies. These variables all affect how sensitive a pressure transducer is to damage to the diaphragm.
It is somewhat common for piezoresistive pressure sensors to be mishandled and damaged. That is because the sensor diaphragm is quite thin, and is the most vulnerable part of the sensor. It must remain free of dents, scratches, punctures, and any other damage to work as designed and as expected.
A good rule of thumb is the pressure range itself. Generally speaking, the lower the pressure range is, the more susceptible the diaphragm to damage. This is due to the thickness of the diaphragm. A 10,000 psi sensor will have a relatively thick diaphragm, whereas a 5 psi sensor’s will be very thin.
How Damage Occurs
Your sensor’s diaphragm can be damaged in a number of ways, from scratching the surface, to denting and puncturing. This happens when pressure is not evenly applied to the diaphragm. Sharp points should be avoided.
Damage can also come from dropping the sensor, even only a few inches, or carrying it carelessly and bumping it into another object. The chances of damage are much less with a diaphragm that is well protected by the sensor housing. However, a flush mount sensor is exposed, and should be handled with extreme caution until it is properly mounted.
Anything that deforms the diaphragm, even if it is not visible, will cause performance issues.
However, not all performance issues are life ending for the sensor. Sometimes, the bonding layer between the diaphragm and the sensing elements is simply disturbed, and must be allowed to settle again.
The bonding layer is cured and aged during the manufacturing process. Once finished, it is checked for performance characteristics that confirm it is ready to ship. If the diaphragm is bumped, it can temporarily reverse this process. Allowing the sensor time to “recover” may fix the output drift.
Different Sensing Elements Affect Susceptibility
The sensing element attached to the diaphragm affects how thick it can be. For example, we use MEMs, Foil, and Silicon gages on our pressure transducers. Each type of sensing element requires a different thickness of the diaphragm, and determines how the diaphragm changes relative to the pressure range.
If a diaphragm is “oil-filled”, having a volume of oil between the diaphragm and the sensing element, it will be quite thin and easily damaged. Foil gages use a slightly thicker diaphragm and handle stress a little better. Silicon gages can use an even thicker diaphragm (still very thin), and therefore are a touch more durable. For example, you’ll get a better over-pressure rating from a pressure sensor that uses a silicon gage.
MEMs gages do have oil between them and the diaphragm. There are a few tricks that reduce the susceptibility of the diaphragm – such as a smaller surface area to limit flexing. This change can reduce the number of incidents quite a bit. There are also ways to keep the volume of oil to a bare minimum. This helps with thermal stability.
Despite the issues, there are still those who manufacture pressure transducers with oil-filled diaphragms that are very large and have a lot of oil. These are notorious for being especially susceptible to damage. In fact, they can be thermally unstable as the oil expands and contracts during temperature changes.
In the end, it is the thickness of the diaphragm that offers protection against damage. But nothing protects a sensor better than smart handling practices.
The key to avoiding damage to your pressure sensor during handling is to protect the diaphragm. Do not drop or bump the sensor. Be careful when touching the diaphragm that you apply even pressure. Never push on the diaphragm, and keep pointed objects away.
You should also be aware of the type of sensor you have. If it has an oil-filled diaphragm, avoid touching it altogether. Under any circumstance, realize that it is a sensitive instrument designed to respond to the slightest change in pressure. By very nature, piezoresistive pressure sensors are susceptible to damage. However, handling the sensor carefully is quite easy as long as you make a conscious effort.