Insane Hydraulics

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Digital vs Analogue Pressure Gauges for Hydraulic Troubleshooting

When I started out in hydraulics as a greenhorn assistant, the only pressure gauge I was familiar with was the liquid filled Bourdon tube type. I remember carrying that tool box full of gauges everywhere...

Later on, when I laid my hands on a brand new Parker Service Junior digital pressure gauge for the first time, I thought I'd found the holy grail solution for all of my pressure measuring needs. Indeed, the fact that I no longer needed to carry around a "gauge for a range", and was able to measure both 1 bar and 450 bar with the same instrument, was a very strong motive to put away the old gauge box for good. The digital gadget was even capable of measuring negative pressures and registering pressure dips and spikes, and, to top it up, had a very neat black "professional" touch, which definitely felt like the last straw to snap the analogue camel's back...

Some time and many an assistance call later, however, I had to wipe the dust off of my old gauge box and put it back to service once again. At present time, whenever I go out in the field, I always bring a set of analogue gauges along with the digital ones, and every now and then I get asked why the hell do I carry around this bunch of "ancient clocks"? What is it, that a "stone age" needle can do, that the silicone age liquid-crystal-computerized-micro-processor-controlled-ultra-modern apparatus can't? Isn't a single universal and compact gadget better then a whole lot of them steam-gauges? Well, in this article I'll try to answer this question, and will top it up with a cool bonus example.

The most obvious advantage of digital gauges is their accuracy and the extended pressure range. I have a digital gauge in my tool box that can measure pressures ranging from one bar negative (vacuum) all the way up to 600 bar, and although the manufacturer declares a 0.5% full scale precision for this model, I did run some tests against a calibrated precision gauge once, and was surprised to discover that it was more accurate than that, especially mid-range.

Being digital devices, such gauges are capable of displaying results in different units and have other useful extras like back-lit displays or even data-logging and data-transmitting. Next very important advantage of digitals over analogues is their ability to "capture" pressure spikes that are virtually undetectable with mechanical measuring systems. The spike-catching ability will depend on the gauge's scanning rate (time between two readings). There are gauges with scanning rates from 500 to 1 ms, the lower the rate, the better is the spike-catching ability. Another great thing about digital gauges is their "psychological" impact on clients who are not familiar with this type of equipment (c'mon, do you really think someone would entrust such fancy tool to anyone but an undeniable expert?)

The main advantage of analogue needle-type pressure gauge over digital is the needle display, which is far easier to read, especially from a distance, compared to digits, which you actually have to read and interpret. This is an enormous advantage when you have to monitor several pressure readings at the same time, because you do not have to look straight at the gauge to see the position of the needle. Furthermore, the spike dampening quality of liquid filled gauges is an excellent feature when you need to measure pressures subjected to high frequency spikes - the needle will show you the resulting average reading, while some digital gauges, especially the ones with low refresh rates, will provide unstable readings, which may be confusing and lead to wrong conclusions.

Last but not least - needle-displays make it very easy to monitor dynamic pressure behavior (i.e. how fast the pressure is changing). Digits must be read and interpreted, while the needle's speed rate is perceived instantly. Take a look at the video (this was shot way back - so please, don't mind the quality of the image). Here you can see servo pressures of a closed loop Rexroth A4VG pump. Notice how easily you can read both gauges at the same time. To an experienced technician the dynamic behavior of the pressure changes will tell quite a lot. Note that when the pump's control is activated, you can actually catch the moment when the servo piston hits the mechanical stop. Judging by the way the pressure rises in the tank side servo piston when the pump is de-stroked, an orifice is mounted in the tank line. Notice also how, as the system pressure rises, the pressure cut-off system kicks in, lowering the servo pressure. With these gauges you could easily see this slight needle drop even from far away. Of course, there are more advanced models of digital gauges that include analogue bar graphs, which are nice looking but often utterly useless.