Insane Hydraulics

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Testing the Cheapest Digital Pressure Gauge I Could Find

In this post, I am pressure-testing the cheapest pressure gauge I could find to see if it can be used and trusted at all.

This is the test setup and gear:

I will be using three time-proven Parker digital pressure gauges for my pressure references - The SCJN-100-01-MP (MP for Minipress) with a 0...100 bar range, and two CSJN-600-01 (0...600 bar range) gauges. The 100-bar model with its better resolution will be my reference for lower pressures, and the 600-bar gauge "wearing" the red boot will be my main reference for pressures up to 600 bar (simply because this is the gauge that I use every day). And of course, I will be using the Pressure-Maker II as a convenient "tabletop pressure source".

The main goal of this test is to see if the cheap Ritherm 3305 with a 1000 bar range can present readings comparable to the readings of the Parker digital pressure gauges, but this test will also inadvertently answer two additional questions:

- How good is the long-term stability of the simplest version of the Parker SCJN gauge range?

- Does the zero-offset that some of these digital pressure gauges seem to develop over time affect their accuracy?

The long-term stability question is interesting, because the Minipress branded gauges were manufactured in 2014, and my go-to gauge - in 2019. I've tested these gauges against each other before, and I did see that their readings are extremely close and their accuracy is much better than the advertised 0.5% full-scale - so I want to re-check the readings to see if anything has changed since the last test.

The zero-offset question has been nagging me for quite some time because my "everyday driver" gauge has developed a zero offset of 2.2 bar (meaning that it reads 2.2 bar when I turn it on and I have to manually zero it every time). But the Minipress gauge hasn't developed any offset at all, most likely because it is less used, and I have been wondering if the manual zeroing "does any harm" to the gauge's precision. I know that 2 bar is insignificant in probably 99% of cases but I still want to know the answer, and I especially need to know if I can trust the gauge when I use it to read pressures below 10 bar - a 2.2 bar error is nothing for a closed loop transmission, but it makes a hell of a difference for a shaft seal, for example.

So, first I checked how both of the 600 bar gauges and the Ritherm read the pressures under 10 bar in 1-bar increments, and pressures between 10 bar and 100 bar in 10-bar increments, using the 100 bar range gauge for my reference, and then I checked how the Minpress 600 bar gauge and the Ritherm read pressures from 100 to 600 bar in 50-bar increments, using Parker's 600 bar gauge for reference. Here are the results:

SCJN-100-01 SCJN-600-01 SCJN-600-01-MP Ritherm 3305
1.0 0.9 1.0 0
2.0 1.9 2.0 2
3.0 3.0 3.0 3
4.0 4.0 4.0 4
5.0 5.0 5.0 5
6.0 6.0 5.9 6
7.0 7.0 6.9 7
8.0 8.0 7.9 8
9.0 9.0 9.0 9
10.0 10.0 10.1 10
20.0 20.1 20.0 20
30.0 30.1 30.1 30
40.0 40.2 40.2 40
50.0 50.2 50.1 50
60.0 60.2 60.2 60
70.0 70.4 70.3 71
80.0 80.6 80.4 81
90.0 90.7 90.5 91
100.0 101.0 100.7 101
150.0 150.2 150
200.0 200.1 200
250.0 250.0 250
300.0 300.1 300
350.0 350.0 351
400.0 400.0 401
450.0 449.9 451
500.0 499.9 502
550.0 549.7 552
600.0 599.5 602

And here are some pictures of the tests:

All of the gauges were zeroed at the beginning of the test. I also checked Ritherm's zeroing function - and it works OK (although I would like some sort of visual confirmation that zeroing has happened - like a flashing reading or a short message on the screen). You can even zero it with positive pressure, and then it will display the negative sign next to the reading when the pressure falls below the new "floor":

My comments.

First and foremost - The Parker gauges, once again, proved themselves to be worth every penny. I call them "Fluke 87Vs" of pressure gauges. Their value is not in especially advanced features or extreme precision, but rather in a simple and functional combination of components (a Wika's TTF-1 thin-film pressure transducer and an 8-bit micro-controller ) that has been time-proven - which is the best (and only) kind of proof you need to determine if an instrument is good for professional use.

Secondly - my heart is relieved now because I know that the annoying zero offset of my gauge does nothing to its accuracy - all I need to do is not forget to zero it when I turn it on before hooking it up to a system.

But let us get back to the "star of the show" - the Ritherm 3305.

First of all - let us talk about its accuracy. It is very good, especially given the 1000-bar range. It is better than the announced 0.5% full scale, and in the 600 bar range, the readings mostly fell within 1 bar from Parker's readings, getting closer to a 2-bar discrepancy at 600 bar. If the slope continues - it may be about 3-4 bar off at the end of its range, which is totally acceptable.

The gauge also does not read pressure below 2 bar (which, once again, is perfectly OK for a cheapo gauge with a 1000 bar range).

The resolution is not great - there are no decimals, it is merely a 1000-count gauge, and given the fact the micro-controller of the gauge has a 16-bit ADC - it could be better. At least in theory - because there's no information about the origin of the pressure transducer in it.

But I can tell you one thing - this gauge is soooo sloooow! When I drive the pressure piston of the Pressure Maker II with the hand wheel (not the crank) - I can change the pressure pretty fast, and you can tell immediately that it takes the Ritherm several seconds to "pick up" the correct reading. This pressure gauge is absolutely not suitable for measuring dynamic pressures - which, unfortunately, makes it useless for diagnostics of hydraulic equipment.

Like I said before, this gauge is basically an analog gauge with a better resolution. Theoretically - one of the places one could use such a gauge would be in a press, a hand pump, or, maybe, in a piston-based strain measuring device. However, since I don't know the origin of the pressure transducer inside, I would not recommend using it in a lifting application (or at least mount a tiny snubber orifice in the traducer inlet, so that if the membrane bursts - at least the load does not fall fast).

So - if you need to measure a stable pressure with a 1 bar resolution, and you are 100% positive that even if the gauge's membrane ruptures there will be no harm done, you can use this gauge. If there's a dynamic component to the pressure you need to measure - forget about it! Get yourself a nice Parker gauge and be done with it.

Now - an add-on of sorts. Since the Pressure Maker II is capable of reaching 1000 bar, I decided to see if I could take the Ritherm for a full-scale spin, and I got to about that much..:

...when a thought crossed my mind: "...hmm, I kind of forgot that I used a short capillary test hose instead of connecting the gauge directly to the Pressure Maker, and I wonder if the hose can take it...", and then I looked at the test hose and thought - "Oh crap!" - when I saw the capillary hose "sweating" - i.e. covered with tiny and slowly growing oil patches:

Yep - it seems that I binned this one all right. I wonder if this is the high-pressure oil permeating the thermoplastic tube in multiple spots, without rupturing or opening a pinhole big enough for the pressure to drop, or is it the tiny amount of oil trapped between the inner tube and the outer skin that was now being "squished out" through cracks by the unusually "pumped up" inner tube - but in any case - I decided not to crank it up more.