Insane Hydraulics
Bold and audacious blog about fluid power
Take a look at this setup and tell me what you see:
Two identical digital pressure gauges (Parker SCJN-600-01) connected to the same spot and, naturally, reading the same pressure... What's so special about it? Well... nothing, so far, but let me throw you a curve ball now - these are the same gauges a couple of moments later - notice anything strange?
That's right - same static pressure but... different readings? And no, the pressure gauges are not broken! If you have no idea why this could be happening, you should definitely keep on reading.
This, my fellow tradesmen, is the Parker's very own firmware doing its thing, and by "its thing" I mean the mysterious filtering (a.k.a. damping) function it applies to the readings. Why do I say mysterious? Because none of the manuals I could find have a clear explanation on how exactly the gauge handles the said filtering, be it Parker, Stauff, Minipress, etc.., and if you think that it is simply calculating the average of a given number of pressure scans in real time - no - there's more to it than plain averaging, and if you don't know its quirks - it can bite you in the rear big time!
So, what does the official user manual say about the filtering option? Not much at all. The only thing you get is "you can set 7 different filter levels" and a picture of a screen with an explanation of how to set it, and that's it. In fact, you get not seven but eight "filter levels" - because the filtering coefficient can be set from zero to seven, with zero, supposedly, corresponding to "filter off," which is also a filtering level if you ask me, but I guess we can let this one slide. Anyhow, let us figure out how it works, shall we?
First - let us check the firmware versions (just for the record), and then set the filtering to "0" on one gauge and to "7" on the other:
The red gauge is "younger", and it carries the firmware n3.6, while the older Minipress has the n3.1. I tested both of them and didn't see any difference in the behavior of the filtering function, but the red gauge does come with an extra unit - mH2O (meters of water), and from what I found online, there are firmware versions that also have inches of water. I must say that the representation of the names of some pressure units on a segmented LCD feels like a puzzle to me. Can you tell what pressure units these gauges are displaying?:
The left one is the mH2O, and the right one is kg/cm². I confess it took me a while to figure it out, too.
But let us go back to the filtering/damping matter now. In an instrument, the objective of damping is to reduce the oscillation of an unstable reading, so that you can better see its average over a period that makes "human sense " - usually a couple of seconds. So, what do you think these two gauges would read (one with the filtering set to "0", and the other one to "7") if I connected them to a point that would go from, say, 2 to 20 bar in one tenth of a second?
You probably think that one of the gauges would instantly jump to 20 bar, and the other one would "lag behind", gradually reaching the 20 bar readout some moments later, and then when I would lover the pressure to 2 bar - the fast gauge would drop to 2 bar, and the slow one would "eventually get there" - but... that's not how it works for these digital gauges. In this scenario, you would see no apparent difference between the two gauges! Both of them would instantly jump to 20 bar, and then both of them would drop to 2 bar.
Here's an important thing to understand about the filtering function of the Service Junior pressure gauges - when a discrepancy between the displayed value and a currently scanned value becomes higher than a certain threshold - the gauge "discards" current filtering average and jumps to the scanned spike, where it "re-assumes" the filtering, and the "jump" is instant even with the highest damping setting.
In my experiments, I determined that the threshold is about 10 bar. But when the change in pressure is less than 10 bar, the damping works as you would expect. But boy, is the setting "7" slow! Super slow! I set the pressure to 9.7 bar and then turned the pump off. The reading on the red gauge went to zero immediately, but the filtered gauge got to zero... in 30 seconds!!!
If you use these gauges professionally (and I am sure you do because these Service Juniors are a hydraulic equivalent of the Fluke 87V), but somehow never use this fiction, I suggest you copy this test on your bench and "play" with the filtering setting to see how it behaves for yourself. In my opinion, filter settings above 3 are useless, and in fact, I have the filtering turned off on all of my gauges.
Now it is easy to see how a high filtering setting on this pressure gauge can mislead you when you need to monitor the dynamics of a pressure reading in a less-than-ten-bar range. For example - when you measure pressure in a line that has a bleed-off orifice, and you expect the pressure to go down relatively fast, and then see that is creeps down slowly - you would suspect a clogged bleed-off orifice, or if you are checking a low-pressure accumulator, and see the pressure going down slowly to zero, without the expected drop-off at the accumulator's pre-charge setting, you could mistakenly believe the accumulator is not charged.
So - don't forget to check the filter setting on your Parker digital pressure gauge, especially if you share it with others.
On the other hand, the fact that these gauges have the filtering function available with such an incredibly slow setting makes them great practical joke devices. That, and the "especially advanced pressure units," so the next time you want to prank a colleague of yours, borrow his ServiceJunior digital pressure gauge, set the filter to 7 and the units to meters of H2O, and then observe him scratch his head the next time he tries to use it!