Can someone explain the difference between the two and the advantages of one over the other?

I have one source for information, but would like other opinions and facts.

Thanks,

Danny

Danny,
Compression mode sensors are cheaper to buy but come at a cost in disadvantages. They experience longer setteling times, are affected by temperature swings, and do not standup to the abuse of walk around collection. Great for permanent mount applications.
Shear mode sensors cost more upfront but save time in the long run. Faster settling time, not as affected by temp., and much better for walk around.

Hope this helps,
Nicholas A. Martin

I don't think the major suppliers of industrial accelerometers even make compression mode accelerometers any more. Cost may have been an advantage at one point, but no longer.

As Nicholas says, compression-type accelerometers are extremely sensitive to temperature *changes*. B&K, with the original patent on their "Delta-shear" accelerometers had a great demo. If you look at the output of a compression accelerometer and just blow on it, the output level bounces from one extreme to the other for several seconds after you stop blowing!

In machinery monitoring, this will result in problems when moving between hot and cold mounting locations, or where an accelerometer might have steam hitting it one second and cooling air the next. The result is amplifier overload and ski-sloping. Compression-mode accelerometers are also subject to base-strain when the mounting surface isn't absolutely flat.

I did an interesting project a number of years ago monitoring a container crane at the Port of Tacoma. A lot of points were monitored, requiring all of our shear mode accelerometers plus some compression mode units. Reviewing the data, the compression mode data looked like an event marker channel. Every time one of these accelerometers moved from the sun to the shade or vice-versa, there was a step in the accelerometer output! Not so good for trying to monitor vibration transients....but pretty handy to verify spped of transit (too bad that's not what I was after!).

Their are a lot of Wilcoxon compression-mode accelerometers in the world which probably cost $300 - $400. Sadly, their performance is surpassed by many of the low-cost models available today for around $85.

Jon
Spintelligent Labs

Thanks for the comments Nick and Jon.

This pretty much confirms my source and adds a few things.

Based on what you guys have told me, they may be perfect for the application I have in mind-if the price is right.

Thanks again,

Danny