From time to time, a customer will call us and say, “I just held my new pressure transducer in my hand and the readings are out of spec!“
We have heard this scenario a few times before. You just received your brand new pressure transducer and you do a quick bench test to make sure everything is in order before you install it in the field. After wiring everything up, you notice that when you are holding the unit by the process connection/diaphragm, the output on your meter jumps a little. We understand that this can be a little alarming and even frustrating. Your brand new pressure transducer is busted, right?
Actually, except for rare exceptions, it's not.
To start, it has to do with heat. When the heat transfers from your hand to the pressure transducer diaphragm, it causes the metal to expand slightly. This heat transfer interrupts the raw function of how a pressure transducer works; the strain/flex of a diaphragm. Changing temperatures at the diaphragm is expected though. This is why most pressure transducers have built-in temperature compensation so that the readings remain stable.
Two reasons. First, the heat transfer from your hand to the diaphragm is not at all like you would usually see in an industrial process. The heat transfer is not distributed evenly across the face of the diaphragm. The outside of the diaphragm is heated before the inside causing distortions at the Wheatstone bridge resulting in an erratic signal.
Second, it depends on the temperature averaging the transducer has. In most industrial processes, temperature changes are not instantaneous but gradual. So if the temperature jumps 20 degrees Fahrenheit, it will take a few moments for the transducer to catch up and for the signal to stabilize.
As you know, pressure transducers come in a wide variety of combinations. Not only do they differ in output and process connection (among many other things), they will often differ in the pressure cell technology itself. For example, here at APG we use 3 primary types of pressure cell technologies: MEMS, Foil, and Silicon strain-gauge.
Each pressure cell type deals with heat transfer differently. With a MEMS cell, the cell is actually separated from the metal diaphragm by a layer of oil. So when your hand touches the process connection, the heat is transferred in a much more even manner, so you don’t see the signal go erratic.
On the other hand, a Silicon cell is fused directly to the metal diaphragm. There is no layer of oil to even out the heat transfer. So you are much more likely to see this type of behavior with a silicon type sensor; especially on a sensor with a flush mount process connection.
Most of our pressure transducers are built in configurations where this type of behavior is extremely unlikely to exist. However, in some cases when there are special requirements (such as a flush mount unit that requires a silicon cell) this type of behavior can be expected.
If you have any questions about picking the right transducer for your application, please give us a call or send us an e-mail. We are always happy to help.
