Insane Hydraulics
Bold and audacious blog about fluid power
When I started in hydraulics as a greenhorn assistant, the only pressure gauge I was familiar with was the liquid-filled Bourdon tube one. I still remember carrying that toolbox full of gauges everywhere I went...
Later on, however, when I laid my hands on a brand new Parker ServiceJunior digital pressure gauge for the first time, I thought to myself: "Wow! I found the "holy grail" 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 5 bar and 500 bar with the same instrument, was a very strong reason to put the old gauge box away for good. The digital gadget was even capable of registering pressure dips and spikes, and, to top it off, had a very neat "professional" touch to it (or, at least, it seemed that way to me back then) which was the last straw to snap the "analog camel's back".
Some time and many an assistance call later, however, I saw myself wiping the dust off of my old gauge box and putting it back to service. At present time, whenever I go out in the field, I always bring a set of analog 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 instrument better than a whole lot of them steam-gauges? Well, I'll try to answer these questions in this article.
The most obvious advantage of digital gauges is their accuracy and the extended pressure range. I have a digital gauge in my toolbox that can measure pressures ranging from one bar all the way up to 600 bar, and although the manufacturer declares a 0.5% full-scale precision, I ran some tests and was surprised to discover that it was actually way 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. The next very important advantage of digitals over analogs 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 the spike-catching ability.
The main advantage of an analog needle-type pressure gauge over digital is the needle display, which is far easier to read from a distance, compared to digits, which you have to actually read and interpret. This is an enormous advantage when you have to monitor several pressure readings at the same time because you do not need to be looking straight at the gauge to see the position of its needle.
The second advantage would be the spike-dampening quality of liquid-filled gauges - which 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 lower refresh rates, will provide unstable readings, which can be confusing and even lead to wrong conclusions.
And last but not least - needle displays make it very easy to monitor dynamic pressure behavior (i.e. how fast a pressure is changing). Digits must be read and interpreted, while the needle's speed rate is perceived instantly.
Let me explain with an example. I have an old video on YouTube that shows two analog pressure gauges side by side, reading the servo pressures of a closed-loop pump (Rexroth A4VG). For some unknown reason, YouTube reduced the max. available resolution of the video to 240p (probably because it's very old), and I even thought about bringing it down altogether, however, it occurred to me that this is still a perfect example of how reduced visibility does not affect your ability to read an analog gauge! (The only information you need is the range of the gauges, which is 60 bar).
So, check the video out, and 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 catch the moment when the servo piston hits the mechanical stop.
You can also tell, judging by the way the pressure rises in the tank side servo piston when the pump is de-stroked, that there's an orifice in the control's 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.
There are more advanced models of digital gauges that include analog bar graphs with relatively high refresh rates that kind of give you that "analog gauge vibe", and they are nice-looking and all... but still utterly useless when you need to be "grasping" them from the corner of your eye (at least in comparison with a black needle against a white background).
How often do I use the analog gauges then? I would say that very rarely. Especially after I got my wireless pressure measuring system up and running. However - very rarely is not never. Up to this day, I still come across situations where the best pressure measuring tool is an analog pressure gauge, and I am happy to be carrying a box full of these with me wherever I go.