I have used Parker ServiceJunior digital pressure gauges in all of my hydraulic ventures for many years. The ServiceJunior gauge line offers several pressure ranges, but since I often troubleshoot closed-loop systems, and therefore routinely need to read pressures above 400 bar, I have always picked the SCJN-600-01 model for my "daily driver". This digital pressure gauge has a 600 bar range and advertised accuracy of 0.5% full scale, and is the cheapest and the most readily available model Parker can offer.
You can get two more models in this range, namely the 600-02 and the 600-03, with the respective accuracy of 0.25% and 0.1% full scale, but these versions cost twice as much and their availability, at least in our corner of the world, is limited due to large delivery terms, and therefore the -01 models are the ones that we usually use, recommend and sell.
These pressure gauges are ubiquitous. Around here we refer to them as SensoControl ServiceJunior because we get them from Parker, but you will find these gauges branded as Webtec (HPM-110), Stauff (SPG-DIGI), Hydracheck (HC-06-DIGI), Minipress (same reference as Parker), and I believe Wika PG500 and Sika E2 to be the same gauge as well. It seems there's an OEM factory (probably somewhere in China) that churns out these gauges by hundreds and then re-brands them according to a buyer's choice.
When you troubleshoot and adjust hydraulic systems, you need to measure low pressures as often as you need to measure high pressures. By low pressures, I mean pressures in the range of tens of bar (for pilot lines) and ones of bar (for return lines and case pressures). And of course, since my pressure measuring tool of choice has such a high range, and such a "cheap" accuracy, I have always wondered how reliably it could be used to measure pressure in the low bar range.
From specs alone, a 600 bar pressure gauge with 0.5% full-scale accuracy delivers a ±3 bar precision, which is acceptable for measuring pressures above 50-100 bar, but I would say that is questionable for measuring pressures below 20 bar, and definitely not acceptable for measuring anything below 5-10 bar, so, "by the books" my pressure gauge of choice should really suck for this purpose, But is it really so? Well - that's exactly what I set out to find out.
I ran some tests on our test bench and quickly realized that using a normal test bench for testing pressure gauges is a loud and not practical exercise in heating hydraulic oil. I definitely needed something more simple and more stable for the task, something like a hand pump. Ideally - a dead weight pressure calibration stand, or maybe one of those hand pumps with specially calibrated high precision pressure gauges. Unfortunately, I didn't have either... So, as always, I had to improvise, and work with what I had at hand - and you'll see how in a minute...
The pressure gauges that I used for these tests were: two Parker SensoControl ServiceJunior SCJN-600-01 (one new and one with months of use), one very neat looking Parker pressure test kit, composed of an SCJN-600-01-MP and an SCJN-100-01-MP pressure gauges (several years old but hardly ever used), and last but not least - a 6 bar Bourdon tube type pressure gauge (brand new, but with a couple of years of sitting on a shelf).
The basic idea for the test was:
a) to use the analogue pressure gauge as my low range reference and see how the digital pressure gauges behave in the range between 0 and 6 bar
b) then, since I have a 100 bar pressure gauge with the declared precision of 0,5 bar (0,5% FS), which is, in theory, 6 times better than that of the 600 bar gauge, - to use it as the pressure reference to check the other three 600 bar gauges in the pressure range between 6 and 100 bar
c) and then, in the end, see how the three 600 bar gauges compare to one another in the range of 100-400 bar. This part, obviously, can't give me the information about the gauges absolute precision, since I don't have a reliable pressure reference in that range (well, I did, actually - but this will be revealed in my next post), but I would still like to know how their readings would differ, at least to get an idea about their calibration repeatability.
If you ask - why 400 bar and not 600? - well, the answer is - it's the limit of my pressure inducing "apparatus", which I built from scrap, and which is based on a 350 bar hand pump and parts from a very old pipe bending machine that I salvaged from scrap about 10 years ago, and which were lying quietly in the corner waiting for their hour to shine...
Behold, my friends! I present to you - the Stormbreaker! Oh, wait, not that one, that's the other one... Yes, the Pressure Maker, in all its glory! Since it's a 350 bar pump I, naturally, took it to 400 bar. But when I took it over 400 - I felt that something in this ingenious contraption was about to bend if I kept pushing it, so I decided to keep all the pressure tests within the respectable (especially by scrap metal standards) 400 bar limit.
I bet you've never laid your eyes on something like that before... I can tell you one thing - it does its job. It may be ugly and makeshift - but it allows you to build pressure and keep it stable for as long as you need it.
Now for the test results. Starting with the most interesting 0 to 6 bar range. None of the pressure gauges were able to read very low pressures, so the analogue pressure had its needle at the 0.2 bar mark when the 100 bar digital pressure gauge was the first to "wake up" and show something, but above 1 bar its readings were spot on! However, I was really surprised to discover that two of the three 600 bar pressure gauges showed a 0.1 bar discrepancy from the readings of the analogue gauge, and even the pressure gauge Nº1, that differed the most, did it very consistently, and if you look at the graph - you will see that the reading slope is the same, the line is simply offset by 0.2 bar.
Very, very respectable, Mr. Parker!
|Analogue Gauge 6 bar||SCJN-600-01 Nº1||SCJN-600-01 Nº2||SCJN-600-01-MP||SCJN-100-01-MP||Diff A-B||Diff A-C||Diff A-D||Diff A-E|
Let us move on to the higher ranges:
|SCJN-600-01 Nº1||SCJN-600-01 Nº2||SCJN-600-01-MP||SCJN-100-01-MP||Diff A:B||Diff A:C||Diff: B:C||Diff D:A||Diff D:B||Diff D:C|
As you can see, now the discrepancy is slightly bigger. At the end of the 100 bar scale, our 600 bar gauges deviated 0.6 - 1 bar from the reading of our 100 bar reference gauge. Still not bad at all! I am OK to measure 100 bar with 1 bar error - perfectly usable.
You can see that the slope of the 600 gauge lines is slightly divergent from the 100 bar gauge line, which is why I am choosing the 600 gauge with the least divergence for my future high-pressure references (at least for now).
Even so - the results show that these digital gauges have incredible repeatability. You can see that they were definitely calibrated according to the same lookup tables, since their readings are so close, especially for the gauges Nº1 and Nº2!
Now to the 100-400 bar range:
In the 100 - 400 range - you can see that the results between the first and the second gauges never differ more than 0.1-0.4 bar, which is excellent! The slopes are also equal for all intents and purposes. It seems to me that these pressure gauges are worth the money you pay for them!
So, to conclude, I take the following from these tests:
1) Are these digital gauges any good? They most certainly are!
2) Can you reliably measure very low pressures (below 1 bar) with a 600 bar range 0.5% FS precision digital pressure gauge? - No, you can't (I bet you already knew that). You definitely need a good analogue or digital gauge with a much lower range.
3) Can you reliably measure low pressures (1 - 10 bar) with a 600 bar range 0.5% FS precision digital pressure gauge? Above 1 bar - yes you can (I bet you didn't know that) but with a caveat - your usable resolution will be about 0,2 bar at best, and you will have to test your particular pressure gauge against a known analogue low range gauge before you can do that - the fact that these digital pressure gauges have such decent repeatability allows you to do that even if the "line" is slightly off-set. If you find out that your pressure gauge is, say, consistently 0.2 bar above the actual reading - grab a permanent marker and write this down on the back of it, and do mental calculations next time you use it to take low range pressure readings. I would still recommend getting eventually a lower range gauge for your toolbox.
4) Can you reliably measure pressures above 10 bar with a 600 bar range 0.5% FS precision digital gauge? You absolutely can. For most real-life scenarios these gauges are accurate enough.
All the three gauges that I tested showed absolutely repeatable results with precision much higher than the declared 0,5 percent. Do bear in mind that pressure measuring cells have wear and "drift" over time, so I would advise checking your digital pressure gauge every other year or so if you really want to keep track of its low range precision.
5) What about pressures above 100 bar? - Don't worry about calibration or anything else - even if your pressure gauge has drifted a couple of bar - it's still accurate enough to adjust a 300 bar relief valve setting.
Which pressure gauge should I buy then? - you may ask. Well - that's a difficult question to answer. Your budget will dictate, naturally, but if you can only buy one and intend to work on closed loops - get the 600 bar model and don't look back! You can always get cheap analogue gauges for case pressure measurements anyway.
If you are positive that your troubleshooting or adjustment target will never require such a high range, get a 400 bar model - you'll get a better resolution.
If you can get two gauges - a 600 bar + 100 bar, or the 400 + 100 bar combo would be a sound choice.
That is - if you want to go with "official" pressure gauges. Because in my next post I will show you how all of these pressure gauges stack up against various industrial pressure sensors coupled to a smart wireless transmitter.
You also can also download the Open Office spreadsheets with the data:
All of the gauges that I used in this test were either brand new or with only months of use. But I will repeat the test with an old ServiceJunior when I get my hands on one (my last "old" gauge died on me at the beginning of this year...). I still want to know if "older" and "abused" gauges lose some of their "low range capacity..."