Strange digital caliper behavior

One of the digital calipers I have which has worked perfectly for about 3 or 4 years will not zero now.

At switch-on the display does show 0.00 but it the zero button is pressed without moving anything it sets the display to -0.02

Its totally consistent and always -0.02 Oddly when the zero button is held down it shows 0.00 but goes to -0.02 when released.

I have reasonable understanding of electronics and how these Chinese calipers work but I cannot work out what could cause this fault. I'm pretty sure its not mechanical as its the same when its locked solid.

Ian P

PS I've not identified the supplier but wondered if anyone else has seen the same problem

You might try taking the calipers apart and cleaning everything, but that's just clutching at straws. My cheap ones have lots of foibles, my 30 year old Mitutoyo's never give any trouble at all.

Ian,

This is not a definitive answer, but it may possibly point you in the right direction:

Have a look at this page: **LINK**

http://www.pcbheaven.com/exppages/Digital_Caliper_Protocol/

At 2 March 2012, 10:30:16 user Hans U. Meyer wrote:

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MichaelG

I think the site MG linked to is one where I learned some of the info about they work, I've just had quick look but not yet found anything relevant.

Dismantling and cleaning the calipers (which I have just done) has not changed them at all. I did see that that it has three solder-bridge jumper options (only No2 was linked) so now I'm curious and might experiment. (Maybe one of then disables the annoying auto-off feature)

Ian P

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Sorry for wasting your time, Ian

... I had assumed that Hans Meyer’s first two paragraphs might be relevant.

MichaelG.

This may seem a little obvious?? But just try leaving the caliper on a radiator or similar overnight. Go buy a new best quality battery and fit this after little dab of mild solvent on the terminals. - Remove the battery, after use on the chinese ones, if you have the patience?

I have six or seven cheapies that occasionally crash and you may be surprised how often they can revived by drying.

Dependent on your calipers, it may be that you may just need to reset the origin.. ..not all have this facility, but some have an "origin" button, and others have a small hole to access the reset switch with a paperclip or similar..

It's just a reset for the original "0" calibration..

Perhaps try zeroing the caliper with something between the jaws (a micrometer standard or a gauge block would be ideal) to help determine the source of the error. If the zero still jumps by .02 the problem is probably in the read head, if not it may be something on the scale. If the reading goes to the size of the object between the jaws -0.02 it could be switching between abs and inc modes.

Brian G

I guess you were wondering if it could be a rounding error?

Having written a program to read the protocol, it isn't as the resolution of the signal is several times that of the display.

Try cleaning and a fresh battery.

Neil

my genuine Mititoyo Abosolute now srarts at 16.8888, nothing I can do will change it - given up on it!

Worth a try, condensation causes electronics many problems, temporary and permanent. Cheap calipers are often associated with poor battery life, which is odd because measuring current they draw shows micro-amps. A decent battery should last a year or two.

One cause of poor life is the owner fitting the wrong battery, notably LR44 instead of SR44, or saving money with old-stock pound-shop specials, or fakes. However, I suspect the cells usually go flat because cheap calipers aren't sealed as well as their expensive brethren. Damp penetrates inside, and the resulting film of water holds dirt and tends to discharge the battery. Over time condensation will cause electrolytic corrosion, thoroughly upsetting the electronics as well.

I like Brian G's suggestion that the gauge is flipping mode, or that the read head is confused. Both could be due to damp induced corrosion / dirt. The problem may be too small to detect with the naked eye. Drying out may fix it, but removing corrosion may require a deep clean of the entire unit. Pay special attention to exposed areas like the head and to less obvious areas like the switch contacts and circuit traces. Due to the construction, it may not be possible to get inside without breaking something - these are not designed to be maintenance friendly.

Is it worth the bother? I suggest it's time to take advantage of the very best feature of cheap Chinese Tools, which is they are cheap. This one is well out of warranty. Replace it. If there's any suspicion a slightly damp workshop is the cause, keep the new caliper in a sealed box with a silicone absorber.

Dave

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I didn’t really think-it-through that far, Neil

It’s just that I remembered Meyer’s system was based on 0.2 inches scaling, and that he used BCD rather than straight Binary for his calculations. [whereas the Chinese scales natively use Binary]

Ian’s 0.02mm error suggested that there may be some related computational problem[s] in his caliper.

This exhausts my ability to contribute to the diagnosis.

MichaelG.

I had one of the cheap ones, Aldi/Lidl, do something similar, but with different numbers.

I sprayed some neat IPA inside it, and blew it out with a low pressure airline.
I then left it on a radiator to dry out, cleaned battery terminals with a fibreglass pencil and fitted a new battery.

It now seems to work OK, other than having a bodged battery door, as I lost it somewhere at a steam rally.

Bill

p.s. There is something a little odd about the way some of them work. Of the cheap ones, one make auto turns on when you move the slider, another doesn't, but if you move the slider, and then turn on, it shows the correct reading.
One of my Genuine Mitutoyo ones zero/resets via the on off switch, and that's the only facility to zero it.
The other one as well as having an on/off button has a separate Zero button as well as a separate Origin one, so two extra buttons over the first model.

Edited By peak4 on 21/12/2019 12:52:50

Mine had a fit of reading full scale at turn-on. A nuisance, until I reset the zero when at actual full extension. Behaved itself again, and no problem since. Fingers crossed it keeps on behaving.

Worth asking mitutoyo? (But probably not if it is a genuine mititoyo and not actually a mitutoyo).

India Pale Ale eh? I would never have thought of that one. Sounds good though.

Edited By Bandersnatch on 21/12/2019 16:36:14

It is with deep embarrassment that I have to admit I have wasted everybody's time on my caliper problem!

To cut a longer story short, it WAS the battery.

The calipers are ones that are somewhat better than the usual Aldi and Lidl offerings and came from a respected advertiser on this site. They use a 3 Volt Lithium cell which I think I have replaced once since I bought them. What fooled me was that the calipers with the usual 1.55 Volt LR44 type button cell, give warning of the battery state by the fact that the display becomes feint. The display and all operations on this caliper were completely normal until this weird 'not setting to zero' fault occurred, I did have a spare battery but never thought to try that first.

This pair of calipers are on my desk and have never been in the workshop so I knew it was not a damp problem. This evening I was going to dismantle them and check the board carefully under magnification but after taking out three of the four screws, paused and remembered the advice I was given here so tried the battery.

Surprisingly the 3Volt coin cell measured 1.64 Volts so I am amazed that it functioned as well as it did.

Thanks everyone.

Ian P