Can you trust an analog pressure gauge?

[tomwalz]

Can you trust an analog (dial, needle) pressure gauge, even one that is oil filled?

I went to a seminar this week on improving cutting in sawmills and one of the speakers strongly suggested that you switch from analog gauges to digital gauges for air pressure. He said he’d seen failure rates as high as 50% on the analog gauges.

He brought up two particular failure modes. One was that the needle was just pinned on and that the needle could shift with enough bouncing. In addition he said that these gauges were only really accurate in the middle of their range so they were not at all satisfactory if you are trying to keep 10 pounds of pressure with a 0 to 100 PSI gauge.

In his experience he has seen a production lines where five out of 10 gauges were bad. He talked about a troubleshooting situation where he replaced 10 gauges and five days later three of them were bad.

This guy is a mechanical engineer from a well - respected company and he said his company has done a great deal of testing and gauges has not been able to find a satisfactory analog gauge.

His examples were all in sawmills, specifically in use in log handling equipment. There is a great deal of vibration in everything while the line is running.

My question is whether these analog pressure gauges are suitable for use in any application?

Thanks,
Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[IRstuff]

I would not disagree with him; the stereotype in the movies is almost always someone tapping the glass on the gauge to try and jiggle the needle. It seems to me that the two examples cited are possibly poorly designed or poorly construct gauges. Certainly, electrical analog gauges work tolerably well, the needles in VU meter can routinely bang against the upper stop with nary a pin. But, I concede that there's some difference between electrical and pressure. And, I guess there aren't too many systems that still have the electrical gauges; it's a bit sad to not see the VU needles bouncing around on some lively music.

I think that for general use, i.e., for non-critical accuracy applications, it's OK. We use pressure gauges for cryogenic gases, but we don't necessarily care if the reading is 20% off.

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[arunmrao]

Most of us grew up believing the analog gauge and several times preferred the analog over the digital during initial days. Slowly, our mindset has changed and we have begun to accept digital gauges.

 

[Compositepro]

It's true that the cheapest pressure gages are very poorly made. But this has nothing to do analog versus digital. The cheap gages are Bourdon type with a stamped rack and pinion gear that is very easy to damage or misalign simply by bending the case slightly. These should not be used in any application where the reading is important. Diaphragm and bellows gages are generally far more robust, but anything can be made badly. The same can be said for pressure regulators. It is incredible how poorly the cheap little utility regulators work. It is also incredible how precise and robust an instrument grade regulator can be.
Industrial quality gages are not cheap. You will not find them at the Home Depot or even most industrial supply stores unless these stores sell to chemical plants. Chemical plants are loaded with analog pressure gages and these gages are quite reliable. Tapping them is still a good practice, though. The tapping simply verifies that the linkages in the gage are not stuck or disconnected, in which case the gage is very likely to be reading correctly.

 

[EdStainless]

We use a lot of mechanical pressure gages,
1. only between 20% and 80% of full scale
2. check calibrations (removed from system) regularly (depending on service, usually 3 months)
3. they are $400 gages, not $7.
The only draw back to digital is the zero drift. We often find zero off by 10psi.
But if they give you a reading they are working, if they break you get nothing, not a wrong value.

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[redpicker]

Any time you use a broad brush, you are bound to paint over things that don't need painting.

There are clear advantages to digital gauges, but as has been pointed out, many of these have nothing to do with being analog or digital, but, rather, of gauge conctruction quality. If fact, a digital pressure gauge is a specialized form of an analog gauge, the only difference is the output signal is digitized prior to display. It is still using an analog relationship between the pressure and the output.

In the seminar speaker's experience, the cautionary recommendation against is probably appropiate for applicaitons for which the speker has experience. To say this applies to all applications is a pretty broad extrapolation. This is like saying digital calipers should always be used over dial calipers (or even vernier calipers). While in many applicaitons, digital calipers do well, but I have seen cheap digital calipers I would not trust and, while a vernier caliper can be difficult to learn how to use, there are applicaitons where they would be preferred.

rp

 

[Tmoose]

If the displayed parameter pressure/voltage/etc is subject to continual fluctuation some kind of rounding option is often required to make any sense of it. An analog display will handle it better, although not necessarily accurately.

I think a purely digital tachometer would be of little use for anything but top speed runs.

http://www.youtube.com/watch?v=-6j3glUtecY

 

[Ron]

If you are doing precision work, then use precision gauges; whether digital or analog. Precision analog gauges are generally accurate. Pressure transducers and electrical response can vary as can the mechanical response of analog gauges. Use them properly and most likely either will suffice.

 

[moltenmetal]

Digital transmitters of any kind, pressure included, have a "believability bias" because their output is a number, usually with more significant figures than the precision of the measurement instrument.

The precision of an analog dial measurement is visually apparent merely by looking at the gauge- particularly when the measurement is fluctuating.

If you buy decent bourdon-tube gauges, install snubbers and/or oil fill them when they're in vibrating service etc., they can be reasonably accurate and last a long time. But they are definitely susceptible to damage from vibration, overpressure, or especially when people use the case rather than a wrench to install them.

Good quality industrial pressure transmitters should hold their zero value stable for a long time. The cheap piezoresistive ones that are often used in "digital gauges" are another matter entirely- they vary greatly in quality, durability etc. and there are probably a hundred brands on the market.

Mechanical differential pressure gauges on the other hand are not my favourite. And the zero of most industrial differential pressure transmitters shifts significantly with changing common-mode pressure. Yokogawa and ABB now have technology in their DP transmitters which can null out that effect very well, and also give you the gauge pressure at the + or - leg as a 2nd output over HART etc. Rosemount hasn't caught up with that yet.