We had the pleasure of sitting down with out Technical Director, John Shelton to discuss his background and his thoughts on vibration instrumentation past and present.
It depends on the application, they are both vibration transducers. Acceleration and velocity are both vibration just different descriptors of vibrations. So in that sense they are both good transducers. Accelerometers tend to be more fragile in the sense that if you drop them on a concrete floor, as they are based on a crystal sensing element it is possible to crack the crystal and not know about it.
Geophones have always been used in rugged environments such as construction and mining. They are fundamentally limited in the sense that they are moving coil devices so they have a resonance of around 3 to 4 Hz depending on the type. That is a problematic frequency if you are trying to measure very low frequency vibration for things such as building damage. The geophone has to be conditioned so that is less of a problem. Geophones are not very good at very high frequencies as they are velocity transduces. So if you want to measure at high frequency vibration, an accelerometer is best.
An accelerometer at low frequencies is very good assuming the signal is conditioned correctly. These days most accelerometers have built in conditioning using the IEPE powering that is familiar on most instrumentation now.
So which one is best? Well if you are going to be measuring velocity for building vibration, then both are fine but it makes sense to measure velocity with a velocity transducer. So a well designed geophone will have better sensitivity. If you are measuring medical devices, cat scanners etc a geophone will be best, as you can get down to VC curves. Whereas with an accelerometer you might not be able to get down that low.
If I wanted to measure human vibration, which is characterised in terms of weighted acceleration. It makes sense to you an accelerometer.
With future developments, I don’t anticipate much development happening in geophones as they are a very well established technology that has been around for years. For example, the geophone we are using in the SV 803 is a well-known type with well characterised performance, very stable and linear.
Whereas accelerometers, I think we are moving towards using MEMS type accelerometers, these are currently used in seat pads and vibration dosimeters, As you are dealing with high levels of vibrations. The great thing about MEMS transducers is the price.
This blog forms a series in which we sat down with our Technical Director, John Shelton to discuss how he got into instrumentation and his thoughts on vibration monitoring, past, present and future.
