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How to select hydraulic pressure gauge ranges
Feb 05,2021
Specifying a standard hydraulic pressure gauge range makes life so much easier.

We thought we understood it but even now still struggle every time somebody tells us the range specified is not available.

Our Micropac pumps are used on pressures between a few bar and 700 bar (or 10,000psi). If you are pressure, proof or burst testing using Micropac pumps, a hydraulic pressure gauge is a must. If you are calibrating, you obviously need to know the pressure. Even if you are charging or simply “pumping”, it would be quite reckless to carry on oblivious of the risk of damage or bursting something. Yes, a hydraulic pressure gauge or indicator is quite basic to hydraulics. 

Pick your range.
Your first requirement is to specify a range in psi (pounds per square inch) or in Bar. One bar is 14.504 pounds per square inch. You may also find metric gauges “graduated” in kilo Pascal (or thousand Pascal), where Pascal (Pa) is the SI Metric unit of pressure measurement. 100 kilo Pascal is equivalent to 1 bar.

In the old days, if you were buying US gauges from somebody like Ashcroft they would be imperial and gauges from a European manufacturer like Wika or Badotherm would be based on bar. Imperial or metric gauges would very probably have a dual scale in the corresponding units. 

Imperial gauges would typically be in 1000 psi jumps, normally missing out 4,000, 5000, 8000 and 9000. Or at least that is what the catalogue says.

Homing in on European gauges. Metric gauges are invariably based on the DIN EN 837 standard which sets out

Pressure ranges in bar

Based on 6: 0 to 0.6 bar, 0 to 6 bar, 0 to 60 bar, 0 to 600 bar 

Based on 10: 0 to 1 bar, 0 to 10 bar, 0 to 100 bar, 0 to 1000 bar.

Based on 16: 0 to 1.6 bar, 0 to 16 bar, 0 to 160 bar, 0 to 1600 bar

Based on 25: 0 to 2.5 bar, 0 to 25 bar, 0 to 250 bar

Based on 40: 0 to 4 bar, 0 to 40 bar, 0 to 400 bar.

Some are really useful. For our pumps, 100 bar, 200 bar and 400 bar are a nice fit with our pump working pressures, subject to our observation later in our post about extended use at full deflection.

The right pressure gauge selection for the application.
We are fretting about finding the right hydraulic pressure gauge range for the job, but what are we worrying about?

Firstly, we would say using enough of the scale to make it worthwhile. It would seem imprecise to fit a 700 bar gauge on kit that is only working to 50 bar, although subject to what accuracy of measurement you want, the gauge may be doing its job. In passing, we have frequently fitted low and high range gauges in the same system with a gauge over-pressuration valve from Schneider. Talk to Ashford Instrumentation on these.

Secondly, if you are using pretty well the whole range and say running a 100 bar gauge to 100 bar with the operator limiting the pressure, it is inevitable that they will over pressurise the unit at some point. If you are using one of our Micropac MP pressure test sets, looking at the test piece plus keeping an eye on the gauge, you could over-pressurise. For this reason, a relief valve is invariably fitted to limit the pump pressure.

Thirdly, try not to use gauges right up to full range (or “full scale deflection” normally abbreviated to f.s.d.) A 100 bar gauge repeatedly used to 100 bar risks fatiguing the bourdon tube, so we were always told. Two thirds or three quarters of the range used might be better practice. This said, using a Micropac hand pump to full range on a hydraulic pressure gauge will be a lot less punishing than a power pack with rapid pulsations on the tube and gauge mechanism. That would be quite destructive for the bourdon tube.

101 different types to choose from once you know the range.
And where do you start on understanding all the variations on hydraulic pressure gauges and properly specifying what you want? You could write page after page. Connections, size, gauge diameter, connection position, accuracy class, calibration, materials, damping, case type and so it goes on. Talk to a specialist like our friends at Ashford Instrumentation. Our only tip after all this time is that buying a cheap and cheerful hydraulic pressure gauge is a false economy if it fails and the warranty cost to your business is substantial. We just would not risk it.

Calibrated or for indication only?
This is very basic for gauges and any measurement kit. If it isn’t calibrated relative to a National Standard, it could be miles out. If you can live with that risk, mark it “for indication only.”

Easy connection.
Standard connection sizes in the UK are BSP male, sealing on the nose using a copper or alternative sealing washer. G1/8 is common for air gauges and G1/4, G3/8 or G1/2 on hydraulic pressure gauges. Check carefully on what connections are available for what size and range. A G1/4 on a 100mm gauge will be special and even on 100mm, some pressures will not be available in both G3/8 or G1/2. Adaptors are available, but can be clunky. American hydraulic pressure gauges will invariably be NPT which is a taper thread. In Europe you will find BSP “Gas” connections and metric threads.

If you need to take the gauge off for recalibration, you need easy. On a lower connection pressure gauge, you also need to be able to easily orientate the face to be able to see it. We look for easy removal, simple orientation and an auxiliary outlet as well. We made our own 316 stainless gauge banjo because we couldn’t source one. It is slick.

Easy to find “kits”.
A hydraulic pressure gauge kit is great for the right application. You have a pressure gauge, a test hose and couplings in a neat carry case from somebody like Hydrotechnik or Stauff. That can be just the job on a large site or where you need to work between a number of test points or connectors, checking the pressure on each.

Why not electronic?
Why haven’t we short circuited this whole discussion by saying “use an electronic hydraulic pressure gauge?” Bourdon tube “old style” hydraulic pressure gauges are still massively entrenched. Why? Is this strange in a tech world? Low tech still triumphs over a device with a battery and will continue to do so in our world. Yes, electronic measurement is massive in control and instrumentation where pressure readings are just one parameter to be monitored in a complex system. For low tech and inexpensive monitoring of pressure with a reading visible all the time, we think traditional hydraulic pressure gauges still have a lot of life in them.