Insane Hydraulics
Bold and audacious blog about fluid power
This is a very respectable set of two digital pressure gauges (SCJN-600-01 and SCJN-100-01) manufactured by Parker and sold under the brand Minipress:
We used to stock these kits back in the day, and I reckon some ten years ago, one of these bad boys ended up in the instrumentation section of our tiny warehouse. These kits are pretty nice, but a quality digital pressure gauge is a relatively expensive piece of measuring equipment, and folks who appreciate and/or have use for such instruments don't buy kits - they order individual gauges because it makes more sense. Fittings that are included in most of the kits are never enough and never what you need, and while the carrying cases look good, especially on pictures in brochures, they are often of lower quality - so paying an extra couple hundred bucks for a Chinese carrying case, half a dozen of fittings and a test hose is not worth it. I don't want to bash the creators of such kits - they do this because there's, obviously, a market for them, and it's perfectly fine, but I must say that while we sold a couple of dozens of individual Service Junior gauges, this kit never left the shelf.
I opened the kit and used the gauges a couple of times "on special occasions" (for example - when I needed "unspoiled" gauges to run some tests), but other than that - the case lay still for all these years - until last week - when I took out the 600-bar gauge, turned it on and saw that it began to randomly turn the backlight on an off by itslef and the buttons became irresponsive.
As soon as I saw this I knew what was going on because I saw this happen to clients' digital pressure gauges before, and I always thought that something like that would never happen to me or "on my watch", only... it totally did! I am sure that if you've dealt with these gauges for a while - you already know that I am talking about damage caused by the leaking AA batteries, which are very treacherously placed directly on the PCB of the gauge, without any physical separation whatsoever, so when the batteries leak, the "juice" pretty much shorts everything it can get to, especially if the gauge was stored face down.
That's right - both of the gauges became unusable because of the leaked batteries. Shame! While I, quite obviously, wouldn't have the audacity to sell them to anyone, I would be damned if I didn't try to revive them - because every hydraulic tech knows that there isn't such thing as "too many pressure gauges"!
Here are the tools and supplies that I gathered for this job:
The white vinegar that is sold around here as a cleaning fluid is basically a 5%-ish solution of acetic acid, and everything that I read online suggested that it was safe for applying directly on PCBs and surface-mount components, as long as it was neutralized and washed off afterward, but I decided that for starters I would use the vinegar on the battery holder and the zinc body, and hit the PCB just with the isopropyl alcohol and then see if the gauge would work or not.
I started with the 600-bar gauge because it looked a bit better than its 100-bar sibling. I de-soldered the battery holder and the wires of the pressure transducer, removed the PCB, and began the cleaning process:
The body and the battery holder were very easy to clean - the vinegar did a great job on them, but pretty soon I discovered that isopropyl alcohol alone was not enough for the board - I cleaned and dried it three times, and the gauge would still not work - it behaved as if the power button was being pressed, with the gauge turning off or turning the back-light on randomly by itself. It was then that I decided to give the vinegar a try.
I must say that applying the acid over the PCB made a lot of difference visually. Some of the battery juice leaked through the hole of the battery holder on the PCB surface (right next to the threaded insert), this is how this area looked when I cleaned it just with the alcohol, and this is how it looked as soon as I applied the vinegar:
You can see that the acid reacted with something - and even a bit of the green soldering mask got instantly removed in a small area. Then I cleaned the PCB with distilled water, then dried it with hot air, and then cleaned it with isopropyl alcohol "for good measure", and finally - the gauge worked, and the buttons became responsive! So, I quickly tested the gauge with PressureMaker II using just the built-in 1000-bar analog pressure gauge for my reference, and at first glance everything worked fine.
Then I grabbed my daily driver (which is another SCJN-600-01) and decided to see how the newly born gauge would fare against it - last time I tested these two gauges against each other the discrepancy of the readings was always within a single bar - and it was then when I saw that the repaired gauge was actually not good at all because it was booting up with an offset of negative 5 bar - something that it definitely didn't have before!
Apparently, when the reading offset happens to be below zero - the gauge just shows zero instead of a negative value. The zeroing function, however, still worked, and zeroing the gauge would bring its readings back to normal - but such a big offset worried me. Where could it come from?
I disassembled the gauge and once again inspected the analog section. One thing occurred to me - I never cleaned the flux residue.
The CHIPQUIK SMD291 that I usually use is supposed to be "no-clean," but some articles suggested that flux residue could indeed affect sensitive systems, so I carefully swabbed the flux leftovers off with isopropyl alcohol:
And, what do you know! The offset was gone and the gauge returned to its normal 0.0 - 0.1 bar boot-up condition. Sweet! Running it against my daily driver confirmed that the 1-bar max discrepancy I saw before was once again there (by the way, my daily driver, after some 6 years of use, boots up with an offset of +2.5 bar):
Now, it was time to do the same to the 100-bar gauge, which looked way worse. Check out the vinegar sizzling over the alkaline residues on the end cover:
I applied pretty much the same cleaning procedures to this gauge, and, once again - when I reassembled and hooked it up to my test rig - I saw a negative offset of almost 13 bar! This time zeroing function would not even work because the offset was above 5% of full scale, so it would just display the zero off-limit error message:
"It must be the flux residue once again!" thought I, but no matter how hard I cleaned the PCP with alcohol - the offset remained. Both the analog area of the PCB and the wires looked pristine, but the damned thing remained offset, and, to make things worse, the readings were pretty "jumpy" as well! I, however, soon found what was affecting the readings - somehow small portion of battery juice got onto the PCB of the traducer cell, and I could actually measure the parasitic resistance between the body of the cell and the wires:
I didn't risk swabbing the residues off with alcohol or vinegar because I was afraid of damaging the tiny wires connecting the pressure cell to the PCB, so I gently brushed the contamination off and then scraped around the board with a tiny flat screwdriver to remove potential conductive paths - and the parasitic resistance was gone:
Finally, after I assembled the gauge - it worked perfectly fine:
So, if you ever see yourself in need of repairing a SensorJunior gauge damaged by leaked batteries - this post, hppefully, can steer you in the right direction.