Help reading an hour meter (on some medical equiptment) - Page 1
Forum section: electronics / beginners

author: ResoluteElectro
author_rank: Newbie
timestamp: November 12, 2017, 06:44:21 am
content: I'd be extremely grateful if someone could help me read the hour meter in the attached photo. It's on a lamp on an endoscope light source. I haven't seen a meter like this before, and I can't see any obvious mark of the time used on the scale.Any help very much appreciated :-)

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 07:54:30 am
content: http://curtisinstruments.com/Uploads/DataSheets/Coulometers.pdfIt may not be their brand but its certainly the same Mercury electrolytic Coloumbmeter technology. If you cant see a dark dot in the Mercury column in the scale region, the maximum scale of this circuit (1500H) has been significantly exceeded and the indicator has been driven off scale and destroyed.N.B. A Mercury electrolytic Coloumbmeter can be reset by driving a current backwards through it, total charge determined by how far you need to move the indicator back. Don't exceed its maximum permitted current. If it is indicating >50%, it saves time to reverse it so you only have to drive it to the nearest scale end. Monitor it carefully as driving the indicator too far will kill it. Its not fast - several days to a week is typical for resetting from mid scale to the nearest end, but if the alternative is special-ordering and waiting for a replacement, setting up a PSU and a series resistor and a time switch for the final cutoff and leaving it running at the back of the bench may be viable.

author: ResoluteElectro
author_rank: Newbie
timestamp: November 12, 2017, 08:00:00 am
content: Brilliant, thanks Ian, just what I needed.

author: Gyro
author_rank: Super Contributor
timestamp: November 12, 2017, 10:03:54 am
content: Alternatively, you could try just removing it and refitting the oposite was round. Sometimes they are socketed, but it could well be soldered.

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 10:50:36 am
content: I'm not seeing the separator anywhere in the Mercury column. Once its jammed against the end electrode, the electrode starts eroding which is likely to FUBAR the seal. Attempting to reverse it is likely to be unsuccessful as there is almost invariably some distortion of the capillary next to the electrode, the electrolyte composition will have changed due to th erosion of the electrode and the very slight solubility of Iron in Mercury is likely to result in particles forming at the Mercury surface jamming the other end of the separator.

author: Gyro
author_rank: Super Contributor
timestamp: November 12, 2017, 12:46:23 pm
content: That's quite possibly so (depending on whether it's very recently hit the 'stop'). My thinking was that as it's probably toast anyway, there's nothing to loose. Leaving it at the current polarity may cause leakage. The other alternative, of course, is to remove it completely.

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 01:08:08 pm
content: You are right there's nothing to loose for the indicator, however I wouldn't do it in-situ as I don't want to have to follow up after about 50H of operation to be certain its not going to leak electrolyte or Mercury. I'd try to figure out the max permissible current, pull it and put it on a suitable supply with a series resistor and check and see if you get lucky and it moves. You'd want to get it somewhere near the scale end mark to start with. If it doesn't reset, its FUBARed and needs to be removed as the seal may be compromised.

author: SeanB
author_rank: Super Contributor
timestamp: November 12, 2017, 06:19:00 pm
content: Current for those is really tiny, around 10uA max IIRC, and they really do not like anything over 100uA of current through them. Matsushita and Sony love them as small, reliable and not noticed hour meters, with them also being easy to scale with the one high value resistor, and being able to run off almost any DC supply rail with ease.Remove the board with it, and just leave the plug disconnected, and it will be fine. Probably soldered in, and if the unit is still supported fully by the manufacturer it will also be a service part as well.

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 06:37:52 pm
content: Its scaled for 1500H total duration, but is probably specced at 1000H duration. If its got a simple series resistor from a voltage source, you can divide that resistor by 15 to get it to run at a 100H duration rate, from the same voltage. That would be about as high as its safe to push the current. If its salvageable, as it cant be off scale by much over 5%, if that, you should be able to drive it back to the scale end in a single working day - check it hourly with a magnifying glass.

author: helius
author_rank: Super Contributor
timestamp: November 12, 2017, 06:43:24 pm
content: See this video on a Soviet mercury coulometer:https://en.wikipedia.org/wiki/Mercury_coulometer