Insane Hydraulics

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Testing Another Cheap Digital Pressure Gauge Kit

Today I am testing another "economical" digital pressure gauge kit, which looks quite promising at first glance:

The gauges (0...100 bar, 0...250 bar, and 0...400 bar) bear the brand name "Ritherm", and even though there's no model reference on any of them, they look a lot like the "3330 intelligent digital pressure gauge" listed on the manufacturer's web site. The gauges come in a plastic box, and along with the gauges, there are six M16x2 test fittings (1/4NPT, 1/8NPT, 1/4BSPP, 1/8BSPP, 7/16UNF, and 9/16UNF) and two (not three, for some reason) DN2 630 bar capillary test hoses with a 1/4BSPP gauge adapter on one end and the standard M16x2 female on the other. The Rehau test hoses seem to be of a very decent quality, by the way. There's also an instructions manual and a "product certificate":

The gauges, apparently, are a step up from the very simple Ritherm I tested last time - they can display pressure in more units (bar, psi, MPa, and kg/cm²), they seem to have an analog bar graph, which is very welcome, the screen has a backlight, and there's even a snubber orifice:

The manual states the accuracy of "1% (specific reference to Product Label)". I suppose they wanted to say 1% full scale. However, there are no labels with accuracy figures on any of the gauges.

Another interesting thing: these gauges have an adjustable filtering constant, which is (arguably) a useful feature. By the way - older versions of the ubiquitous ServiceJuniour gauges didn't have it, but newer ones do - so your Parker gauge may actually have this feature, in case you didn't know. For example, my everyday driver gauge has the firmware version "3.6" and the filtering constant can be changed from 0 to 7. It's on 1 all the time, and I don't think I ever needed to change this setting, but it's still important to know that it is there, because if somebody enters into the menu and accidentally sets it to, say, 7, you may start wondering why the read-outs "feel different"...

As usual, I ran the pressure tests using my Parker 100 bar and 600 bar SCJNs as reference gauges:

Before I present my "discoveries", let's get the accuracy out of the way first. Here are the pressure logs:

...below 100 bar:

SCJN-100-01 Ritherm 100 bar Ritherm 250 bar Ritherm 400 bar
1.0 0.8 0.9 0
2.0 1.9 1.9 0
3.0 2.8 2.9 2
4.0 3.8 3.9 3
5.0 4.9 4.9 4
6.0 5.8 5.9 5
7.0 6.8 6.8 6
8.0 7.9 7.9 7
9.0 8.9 8.8 8
10.0 9.9 9.8 9
20.0 20.0 19.7 19
30.0 30.0 29.7 29
40.0 40.0 39.7 39
50.0 50.2 49.7 49
60.0 60.3 59.9 59
70.0 70.3 70.0 69
80.0 80.5 80.1 79
90.0 90.6 90.2 89
100.0 100.8 100.6 100

...above 100 bar:

SCJN-600-01 Ritherm 250 bar Ritherm 400 bar
125 124.5 124
150 149.5 149
175 174.5 174
200 199.6 199
225 224.8 224
250 249.8 249
275 274.9 274
300 --- 299
325 --- 324
350 --- 349
375 --- 374
400 --- 399
425 --- 425

As you can see, the advertised 1% accuracy is absolutely there. So, just like with the other cheap gauge, you can't say that they don't work, however (there's always a "however") the "promising features", unfortunately, turned out to be duds.

First of all - the counts. The 100-bar gauge has a resolution of 0.1 bar, which gives it 1000 counts and is OK for a cheap pressure gauge. The 250-bar gauge also has a 0.1 bar resolution, which actually gives it 2500 counts, and, once again, this is OK (respectable even). I have no problems with these two, but the 400-bar gauge has a 1-bar resolution, which is... sad. Only 400 counts! It could be better. And it also can't read pressures below 3 bar. In this day and age, even the cheapest digital pressure gauge should do better than that!

But let's forget about the resolution and look at the bar graph now. I could totally forgive the low count figure if the analog bar graph worked - but... it does not! This is a big deal, and, actually, an issue with many cheap instruments that advertise the presence of an analog bar graph indicator - the bar graph simply mirrors the reading of the main display at the same refresh rate (about 3 Hz in this case - more about the refresh rate below). This is rubbish, and I don't know why they even bothered with it. I would rather have them ditch the bar graph altogether and use larger digits. Gentlemen at Ritherm, listen up: an analog bar graph is only useful when it has a high refresh rate (at least 20Hz). The one that mirrors the main reading is like a security officer with a replica BB gun in the holster. It looks good but it's utterly useless!

A few words about the filtering constant (which can be set from 1 to 10).

Here's a quote from the manual:

"...Press On/off key + back-light key at the same time, it will display 'r-03', indicating that the current filter constant is 03. The number can be set by pressing key button, and the range can be modified from 01 to 10. After setting up, press the backlight key to exit. Note: The smaller the filter constant, the faster the digital change, the slower and more stable the digital change..."

Well... It kind of works, but you will never use it, because, by the looks of it, it uses the same 3Hz reading rate (and now I strongly suspect that the gauge is actually taking about three readings per second from the pressure cell - but I'll be able to tell for sure when I peek inside one next week). At such a low scanning rate the "filtering" makes readings very sluggish. If you set the constant to 10 - it takes something like 10 seconds for reading to "get there"! Proper filtering, in my opinion, is only possible in a high scan rate system (100 Hz scan rate or higher), and, once again purely in my opinion, it should affect the reading refresh rate - and, by the way, this is exactly how I implemented it in my pressure measuring App, and it works very very good. For the record - even the ServiceJunior's firmware does not do it right. For these gauges, the setting of 5 seems to work OK-ish.

There's also a bug in the firmware which sometimes changes the units when you change the filtering constant.

And, as far as I can tell, there's another bug that affects the refresh rate of the reading about every 10 read-outs or so. You can see it when the pressure is constantly changing in one direction. For example - if it's increasing, you can see the number on the screen increase at a certain rate, but at certain periods there's a "hick-up" and the reading freezes for an extra second or so.

The backlight looks good in the dark, but as is the case with all battery-powered instruments with screens, it is timed, which makes it virtually useless, because it lights up the screen for about 5 seconds (about 20 seconds for my Parker gauge). This is OK for a wristwatch but is not OK for a measuring instrument. In my opinion, the "eternal back-light" should be an option in the menu. In this gauge's defense - I am yet to find a digital gauge that does the backlight right.

So... What would these gauges be good for then?

I would reserve this kit for static pressures only - things like charging accumulators, for example, where one can benefit from the higher resolution and better precision of a digital instrument. I like the plastic carrying case, the fittings, and the test hoses, but I would gladly trade all this stuff for a fast scan rate and a functioning bar graph.

I can not recommend this kit as a professional hydraulic diagnostics tool. The key word here is "professional". It can be used for diagnostics, of course, and it definitely beats no gauge at all, but it is not a professional-grade instrument. Shame. It looked so promising in the beginning...