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Problem with digital vernier

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Dusty25/07/2011 14:43:15
498 forum posts
9 photos
I have discovered that my digital vernier(Mitutoyo) reads .002 oversize at .500
i.e. .502. Using a set of slip gauges I have checked and all other tests at various dimensions have been bang on. Has anyone else had this problem or can explain why? Needless to say it is not the .500 slip that is wrong, I have made stacks of slips to .500 and I still get the same result .502. I await an answer with bated breath.
blowlamp25/07/2011 14:50:55
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1885 forum posts
111 photos
Does it zero OK and what does it read at say an inch or two of slips?
 
 
Martin.
John McNamara25/07/2011 15:35:09
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1377 forum posts
133 photos
Hi All

Just a quick note re button cells for digital calipers
There is more than one type for the same physical size of a commonly used cell.

Over the weekend I happened upon a new still in the box, but dead caliper for small change at a market. I noticed it had an LR44 battery installed while the packaging said it required an SR44

LR44 (Alkaline) has a capacity of 150ma

SR44 (Silver oxide) has a capacity 30% higher 200ma (The second site below says 175ma) and a voltage of 1.55v
As this was the recommended cell I purchased one and the caliper now works.

There is also a third type PR44 (Zinc air) with a much higher capacity 600ma that appears to be attractive but note the voltage 1.35v – 1.4 v

There are also differences in the discharge characteristics of the cells see the following wiki.
http://en.wikipedia.org/wiki/LR44_battery
 
Or here (commercial site) I have no connection with it.
http://www.sr44.com/

So it pays to read the manual and get the recommended type.

Cheers

John

Edited By John McNamara on 25/07/2011 15:47:41

Dusty25/07/2011 17:18:35
498 forum posts
9 photos
Martin
Yes zero's O.K. and reads bang on at .400 .600 and 1 inch 2 inches 3 inches, just not at .500. It is not about feel either, I have been using mike's and vernier's etc for over 50yrs, if that were the case I would be getting varied readings, which I am not. It is a complete mystery.
Steambuff25/07/2011 17:29:15
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544 forum posts
8 photos

Have you got a spec of dust etc on the 'rails' or a small scratch around the 0.5 point ?
 
I assume another vernier reads the 0.5 slip OK
 
Dave
 
Martin W25/07/2011 19:15:57
940 forum posts
30 photos
John
 
You list a variety of batteries with differing chemistries used to generate power. If my memory serves me correctly the one you really don't want to use for long term power is the zinc air chemistry. They normally come with a peel off tab to stop the air getting in and when required this is peeled off to allow the chemical reaction to start. Their advantage is they have, when compared to alkaline or silver technologies, a much higher power density but their Achilles heel is that they will continue to react with air even when no power being used. This limits their life drastically and as such they are used primarily in applications where a very small power source is required to operate continuously i.e. hearing aids or one shot applications as they have a good shelf life before the seal is removed.
 
For uses where the battery is only used intermittently over an extended period I would use silver oxide or lithium units depending on the application.
 
Cheers
 
Martin
Steve Garnett25/07/2011 19:24:43
837 forum posts
27 photos
Posted by Dusty on 25/07/2011 17:18:35:

Yes zero's O.K. and reads bang on at .400 .600 and 1 inch 2 inches 3 inches, just not at .500.

I just checked the spec of a typical Mitutoyo digital vernier, and they state that the accuracy (not resolution) when new is +/- 0.001". So with the best will in the world, assuming that it's not brand new any more, it's not that far out of spec. But since this is a capacitative pickup device (I think, just better built than the cheap ones), it really doesn't need anything much on the measurement surface to make a difference to the readings in any given place. My suggestion initially would be to clean the upper surface carefully and evenly, and see if that makes any significant difference

Dusty25/07/2011 19:52:38
498 forum posts
9 photos
Steve
15yrs old at least, I have cleaned it and in view of John McNamara's post I have had a look at the battery. Yes, you've guessed it an LR44 instead of the SR44, by chance I had a spare LR44 which I inserted and lo and behold my .500 slip read .5005. End of story, oh no, I then repeated my earlier tests and found that the readings I got earlier were no longer. Well some were but others were out by .0015. I will try a new SR44 battery in the morning and see if that improves things, but it looks as if my vernier is suffering from old age and needs replacing. Many thanks Guys I just needed a push in the right direction, these electronic gizzmo's are a mystery to me.
Harold Hall 125/07/2011 20:43:26
418 forum posts
4 photos
As you have slip gauges I would try building up the value say 0.490, 0.491, 0.492, ,etc. and up and past 0.500 to see just where the error starts and stops. Also, does it build up from 0 error to 0.001 and then 0.002 and then back down again. 0.002 may not even be the maximum error,

If clean, it would appear to me that the track ( or whatever it is called) could be locally faulty.
 
Even if past gaurantee why not have a word with Mitutoyo. Having carried out the above tests you will have more info to approach them with.
 
Harold
Steve Garnett26/07/2011 01:14:08
837 forum posts
27 photos
Well, we just did some local tests. What we have discovered rather bears out the cell information above. Digital calipers/vernier gauges hardly draw any current, so from this point of view, the capacities of the cells aren't really an issue - but if you drop the supply voltage, then things rapidly go astray. When we dropped it down to about 1.3 volts using a lab power supply, the device we were testing this on simply wouldn't function any more - and the accuracy decreased noticeably.
 
Without checking this very carefully I can't be absolutely sure, but I suspect that when the battery volts fall, the internal oscillator which excites the plates becomes less stable - or its output falls off a bit. Either of these could easily affect the ultimate accuracy of the sensor system, but since the plates in it (both for the stator and the slider) are etched, it's unlikely that inherently they will alter. The whole device is still subject to the vagaries of temperature though, and it's well known that anything that affects the capacitance measurement locally (like liquids) will upset the readings.
 
But if it's not physically damaged, then I don't see why the errors would increase with age. It's a non-contact system so there's nothing to wear out as far as the measuring part of it's concerned. But, if you do as Harold suggests, just bear in mind that the accuracy spec I mentioned earlier is only +/- a thou, and that's all you could hold them to!
Bogstandard26/07/2011 04:44:04
263 forum posts
You will find these very good for your silver oxide batteries
 
 
Also, I don't know if you realise it, digiverns are not recognised as a precision instrument. There are just to many variables in their use, and should only be used for rough checking of sizes, especially not of critical finished sizes, that is the job of a micrometer..
 
Bogs
mick26/07/2011 08:25:48
421 forum posts
49 photos
Just a thought, when you have the jaws together can you see any light between them, ie. do the jaws appear to be touching at the top and not at the bottom. If so there should be a tiny adjusting screw on the top face of the body, this can be used to bring the jaws back together.
Dusty26/07/2011 08:42:57
498 forum posts
9 photos
Hi Bogs
You are of course right about the inherent acuracy of these, and no I don't use them for finishing components to size. On this occasion I became aware of the error, and once you start looking you want to find an answer to the problem. It may be that we all suffer a similair problem but are not aware of it during normal usage of the tool. Had I been bothered to get out my hole gauge I might never have found it, but as the hole did not need to be that accurate, I cut corners!
Mick
There is not a problem with the jaws closing tight. The problem as outlined above is the way the thing reads at the moment.
John McNamara26/07/2011 09:20:34
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1377 forum posts
133 photos
Hi All
Abbe's Law defines the problem; a Caliper measures at an offset from the actual point of measurement.
 
A general Google search on "abbe's laws measurement error"
 
From the search above: This document describes it rather well and other measurement errors as well.
 
Also this one from Mitutoyo (Wait a few seconds for it to load)
 
Cheers
John
 
 

Edited By John McNamara on 26/07/2011 09:25:10

Steve Garnett26/07/2011 11:09:15
837 forum posts
27 photos
Posted by Bogstandard on 26/07/2011 04:44:04:

Also, I don't know if you realise it, digiverns are not recognised as a precision instrument. There are just to many variables in their use, and should only be used for rough checking of sizes, especially not of critical finished sizes, that is the job of a micrometer..
 

I'm sure that's why Mitutoyo state the accuracy as only being +/- 0.001", which is rather less than for a lot of their other measuring devices...

Bogstandard26/07/2011 11:25:07
263 forum posts
I wasn't trying to stir up an argument over the inaccuracy issue, but there are a lot of people out there that swear by their digiverns to give a true reading, when in fact they are far from it at times.
 
Of course to most people within a couple of thou can be quite acceptable.
 
Having worked in a metrology lab for a while, we used to have digiverns in for calibration, which of course, by the way they work can't really be done. Basically, when a new set came in, we ground off the internal measuring fingers on the top as they are as useful as teats on a bull, being unable to give any sort of accurate reading, and we stuck a label on the unit to say it was not to be used for final measurements on a finished article.
With the more expensive ones, you could sometimes get them to read a little more accurately by adjusting the pressure on the phos bronze contact spring, but on cheap varieties, because the slots that it runs in was so badly 'hacked' out, there was no chance.
 
 
John
Dusty26/07/2011 14:20:24
498 forum posts
9 photos
I have now installed a new battery of the correct type and given the calliper a good clean with a few drops of meths. Lo and behold it now reads to within .0005 across the whole range.
RICHARD GREEN 226/07/2011 21:05:37
329 forum posts
193 photos

Are we saying that the vernier calipers that use the vernier scale with lines is going to be more accurate ( with a magnifying glass ) for internal and external measurements than a dial type or a digital type ?
Dial types can jump a tooth on the rack sometimes due to swarf or dirt getting inside,
I've used all types for 35 years, but always use the 'line' type for critical measurements.
 
Richard.
Ian P26/07/2011 21:09:22
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2747 forum posts
123 photos
Posted by Bogstandard on 26/07/2011 11:25:07:
I wasn't trying to stir up an argument over the inaccuracy issue, but there are a lot of people out there that swear by their digiverns to give a true reading, when in fact they are far from it at times.
 
Of course to most people within a couple of thou can be quite acceptable.
 
Having worked in a metrology lab for a while, we used to have digiverns in for calibration, which of course, by the way they work can't really be done. Basically, when a new set came in, we ground off the internal measuring fingers on the top as they are as useful as teats on a bull, being unable to give any sort of accurate reading, and we stuck a label on the unit to say it was not to be used for final measurements on a finished article.
With the more expensive ones, you could sometimes get them to read a little more accurately by adjusting the pressure on the phos bronze contact spring, but on cheap varieties, because the slots that it runs in was so badly 'hacked' out, there was no chance.
 
 
John
I had to read this twice before it sank in.
 
You seem to be saying that you damaged someone elses property just so they could not use part of the instrument as it was designed to be used?
 
If you received an instrument for calibration that could not be calibrated surely the correct thing to do was just return it. If you put a sticker on anyway stating that it was not to be used to measure a finished article what's the point of calibrating anyway?
 
Sometime ago I was involved in calibrating exhaust gas analysers so that they complied with UK MOT regulations. The analysers themselves have to be calibrated with gases of known (certified) composition and part of the engineers kit was a digital multimeter that also had to be 'calibrated'. These were sent to a NAMAS company who checked and certified the multimeters. The multimeters were not expensive items and had NO adjustable or calibrate-able settings so all they could do was to certify that they read values in accordance with the manufacturers specification. The stupidest thing was that invariably the stickers were applied to cover the battery compartment thus preventing batteries being replaced without it having to be (expensively) recalibrated.
 
As most, probably all, digital callipers cannot be calibrated there seem little point in anyoned sending them for calibration only to get them back, less use than they were before.
Bogstandard26/07/2011 21:31:36
263 forum posts
You are quite right Ian, ALL personal measuring equipment that came into the factory HAD to be sent for calibration in the metrology department, so people tended to leave them at home and use the section supplied ones instead. God help anyone who was caught using an uncalibrated tool.
 
What is the point of attempting to get standardisation into a working environment if you have 'pirates' hanging about that cannot be controlled.?
 
All my own precision tooling stayed at home.
 
Someone also mentioned dial verniers.
 
If looked after well and kept clean and very lightly lubricated , they can be very accurate indeed to within their displayed tolerances. I swear by my 8" imperial and 200mm metric ones, but only after checking their calibration before use (very easily done). But one thing I don't do is use the internal measuring nibs at all.
 
John
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