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    When I started out in hydraulics as a greenhorn assistant, the only pressure gauge I was familiar with was the most common 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 needed no more to carry around a "gauge for a range", and was able to measure accurately 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 highest and lowest pressure peaks, and, to top it up, had a very neat black "professional" touch, which definitely seemed like the last straw to snap the analogue camel's back...

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

    The most obvious advantage of digital gauges is their accuracy and large pressure range. In my tool box I have a digital gauge measuring from one bar negative (vacuum) to 660 bar, with a 0.5% full scale precision (declared by manufacturer, I tested it and it's more accurate, actually...). Being digital devices, such gauges normally register maximum and minimum pressure values, and are capable of displaying results in different units. Most modern gauges have back-lit displays, which can come quite handy. Some advanced models incorporate wireless technology, which is also a very nice feature. Next very important advantage of digitals over analogues is their ability to "capture" pressure spikes, which are virtually undetectable with liquid filled gauges. 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, that gives you an extra professional look.

    Principal advantage of analogue needle-type pressure gauge over digital is the needle display, which is far easier to read, especially from distance, compared to digits, which you actually have to read and interpret. This is an enormous advantage when you have to take 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 quality, when you need to measure a pressure subject to high frequency spikes, the needle will show you the resulting average reading, while some digital gauges, especially the ones with low scanning rates, will provide unstable readings, which may be confusing and lead to wrong conclusions. And, the last but not the least (for me it is the most important advantage of analogue gauges), is the fact that needle-displays make it very easy to monitor dynamic pressure behavior (how fast the pressure changes). Digits must be read and interpreted, while the needle's speed rate is instantly captured by our brain. Take a look at the video. 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 destroked, an orifice is mounted in the tank line. Note also how, as the system pressure rises (hear the roar!), the pressure cut-off system kicks in, lowering the servo pressure. On these gauges you could easily see this slight needle drop even from distance. Of course, there are more advanced models of digital gauges that include analogue scales, which are nice looking but utterly useless.

     The most depressing disadvantage of digital gauges is their high cost. Also, as any battery dependent gadget, the battery will die on you when you least expect it. Oh yeah, and if you overheat it, or leave it in direct sunlight, the LSD will go black on you, making you  wonder if you had just busted a 500 dollar piece of diagnostic equipment. Don't worry, it'll be ok when it cools down (in most of  cases...)

    Another good thing about analogue gauges is the glycerin inside. You can use it for many peaceful projects, like, say, nitroglycerin production. A friend of mine mixes the glycerin (from the gauges, yes, he happens to work in a hydraulics workshop) with ethanol and uses the mixture on tires of his car to give them glossy stylish look.

     So, in most cases, for simple accurate readings, or when spike-hunting, go digital. But when you need to monitor several pressures at the same time, or when you need to evaluate dynamic response of a system, a set of analogue gauges is definitely the best choice.

P.S.
    Both digital and analogue gauges tend to loose accuracy when dropped from the fifth floor on a concrete floor.
P.P.S.
    As a matter of fact, the glycerin from pressure gauges is no good for nitroglycerine synthesis - too much water in it...
    The pump goes full stroke to one side, then to the other. Then it goes full stroke again and the system pressure rises (by closing the test rig needle valve), when it hits the set pressure (around 440 bar for this  particular pump), the pressure cut-off system kicks in, causing the servo pressure to drop.