Digital Calipers – Can you rank these from the measurements?

[SillyOldDufferModerator @sillyoldduffer]

By fair means and foul I now own four different Digital Calipers. They cost me £4.99, £9.99, £30 and £50. The £50 caliper was significantly reduced from the usual price as a special offer.

Only the two expensive calipers came with a calibration certificate. They both claim ±0.02mm.

I measured a precision parallel five times in each dimension. I took care to measure the dimensions at the same point on the parallel each time. The precision parallel came with a certificate claiming 100 ±0.01mm, 6 ±0.02mm and 24 ±0.005mm

The calipers and parallel were cleaned before the test. The order of testing was shuffled by rolling a dice. The temperature was 19℃.

I also recorded the zero reading before and after each set of five readings. I did not reset zero during each round of 15 measurements.

Results

Average of Five Readings (mm)

'100mm' '6mm' '24mm' Zero at end

Caliper A 99.916 6.012 24.042 -0.01
Caliper B 99.922 6.038 24.028 0.02
Caliper C 99.952 6.028 24.070 0.00
Caliper D 99.880 5.976 23.968 -0.01

Standard Deviation (mm)

'100mm' '6mm' '24mm'

Caliper A 0.086 0.008 0.051
Caliper B 0.018 0.013 0.030
Caliper C 0.019 0.013 0.031
Caliper D 0.014 0.011 0.013

I find it difficult to work out from the readings which Caliper is which. The strong implication is that, for home workshop use, expensive Digital Calipers are not good value for money.

Are you able to identify the expensive Calipers and can you rank the calipers according to which was the best deal?

Of course it's possible that I've only proved that good calipers are wasted in my clumsy paws. Even if that's true, I still conclude that my money could have been spent better. I'd be grateful though for any comments that might improve my technique.

Cheers,

Dave

Edited By SillyOldDuffer on 26/09/2016 13:36:34

[SillyOldDufferModerator @sillyoldduffer]

[ASFParticipant @asf]

I only use a caliper for a quick measurement. If I need a good fit the micrometer comes out.

As far as what is a best buy, its what the pocket can afford.

[Tony Pratt 1Participant @tonypratt1]

Digital calipers from my experience are not a 'precision' measuring tool, although my Mitutoyo is I would say within .01/.02 mm, but I am surprised how far off the dimensions your selection is.

Have you checked your parallels for size?

Tony

[SillyOldDufferModerator @sillyoldduffer]

Posted by Tony Pratt 1 on 26/09/2016 13:42:48:

…

Have you checked your parallels for size?

Tony

Well in so far as I can with limited equipment!

With a cheapo 0-25mm micrometer I consistently get 6.00mm and 24.015, which might be telling me something about the micrometer's accuracy close to maximum.

The micrometer's ratchet and big flat anvils help with consistency. In comparison Calipers are much more vulnerable to tilt and pressure errors.

Dave

[richardandtracyParticipant @richardandtracy]

I tend to agree with you, there is no noticeable difference between them.

With callipers, you are lucky to hold them straight enough to be seriously concerned about the accuracy of any digital unit, it being rather better than the holding accuracy.

The biggest thing I can see to choose between any current calliper is

1. Material (plastic frame can distort if pushed hard or wear, stainless steel doesn't so much) and
2. Battery life. Purely dumb mechanical ones will work as well in 10 years as they do today. A battery powered one cannot usually be left for years with one battery.
3. Solar cells are coming in to help battery life.

Buy direct from China on E-Bay, this is the cheapest way for any of them, and most digital callipers use one of 3 Chinese made sensor units.

Regards,

Richard.

[steamdaveParticipant @steamdave]

When I use my digital calipers, I try to hold the jaws closed on the object being measured. The thinking being that this will prevent the jaws being tilted and thus giving a false(r) reading.

Dave
The Emerald Isle

[John RuddParticipant @johnrudd16576]

Posted by steamdave on 26/09/2016 14:33:17:

When I use my digital calipers, I try to hold the jaws closed on the object being measured. The thinking being that this will prevent the jaws being tilted and thus giving a false(r) reading.

Dave
The Emerald Isle

Ditto……

I always use the same set of calipers on a project, this avoids any mismeasurement error….

For greater accuracy, out come the micrometers but generally work to within a thou or two….

[Peter G. ShawParticipant @peterg-shaw75338]

Somewhere on this site I have published some results of the various calipers I own, but can't now find it. I used Mitutoyo test gauges for 25mm, 50mm, and 75mm.

One electronic caliper reads consistently slightly low. The other was all over the place. Both have this silly idea of being plus or minus 1 on the least significant digit which in my opinion rather makes a nonsense of the readings.

A dial caliper also read consistently low whilst a 0.05 vernier caliper appeared dead accurate.

The biggest problem with the electronics is that they eat batteries. As a result, for best resolution I now use a 150mm Starrett dial caliper reading to 0.01mm, a resolution also of 0.01mm and with a good accuracy (I can't remember the figures), something equivalent to the micrometers I have. I have kept the vernier – it's useful for measurements which don't need that high a resolution. The low reading dial caliper was given away and I have bought a 100mm 0.02mm resolution dial caliper for use around the lathe. Being shorter it's easier to get in to the work.

I don't use the electronic calipers unless absolutely necessary.

I too hold the jaws on the work until I have locked them. For much the same reasons as Dave. Also as John says, using the same device every time would appear to give the best results although maybe not accurate in terms of absolute accuracy. But is absolute accuracy necessary? I suggest that relative accuracy, for home use, is satisfactory.

Peter G. Shaw

Edited for corrections.

Edited By Peter G. Shaw on 26/09/2016 15:30:24

[Brian HParticipant @brianh50089]

My old boss (Quality Manager) used to say that digital calipers were digital 6 inch rules and I think that's about right.

If you need to measure something accurately within the limitations of the amatures workshop then a decent mike, well maintained and properly calibrated, is the way to go.

[VicParticipant @vic]

I don't like the lag you get with cheap calipers but I don't like leaving the Mitutoyo's in the shed!

[SillyOldDufferModerator @sillyoldduffer]

Posted by Peter G. Shaw on 26/09/2016 15:29:35:

…

The biggest problem with the electronics is that they eat batteries.

…

Peter G. Shaw

Edited for corrections.

Edited By Peter G. Shaw on 26/09/2016 15:30:24

I didn't mention it before but one of the four eats batteries. It's the £30 model.

Otherwise I've had good luck with digital and batteries. The 3 scales on my mill are working fine after about two years and my two cheapest digital calipers are still good after a year. The newest caliper is fine after 3 months – perhaps too early to leap to a conclusion – but the £30 model had a new battery at the same time.

In another experiment I'd found the quality of LR44, SR44 and CR2032 cells to be very variable and had put 'greedy caliper' syndrome down to people being unlucky installing low capacity cheap batteries, old stock, or fakes.

But this Digital Caliper got a new cell from the same pack as the £50 model and must have bit the dust since I last used it a few weeks ago. Therefore 'Greedy caliper' syndrome must be a real thing – shame I don't know of any way of finding out if there's going to be a battery problem other than buying one and trying it.

Dave

[JAParticipant @ja]

My only experience with a cheap digital calliper was that it wore badly. After a few years of use there was a lot of rock between the two legs and it could not hold a reading. I then went out a bought an expensive Swiss digital calliper which has behaved very well for over ten years. It supplements my micrometers of which the 0-1" was bought when I was an apprentice over 50 years ago and has never given any problems.

JA

[AjohnwParticipant @ajohnw51620]

Looks like you have a pressure problem to me. What tends to happen is that more jaw is in contact with the part being measured which needs more pressure. Conversely if less area less pressure is needed and in that case with quality calipers that situation is more likely to give a more precise reading.

There is always some oil about and that seems to be the culprit. The same effect can be noticed on micrometers when measuring bar as against the thickness across flat surfaces. The latter needs more pressure which is why ratchets are useless.

When accurate measurements are needed with either it's best to clean the anvils. One way of doing that is to grip some photo copier etc paper firmly and pull it out. Maybe more than once. Or if nice clean overalls use those but paper is better. Using a micrometer setting gauge is a pain. All needs to be dead clean and more pressure is still needed than when measuring the diameter of a bar.

The only accurate measurements I generally ever make with my calipers is to check reamers and taps etc for size. They can tell me for instance that reamers have a tendency to be over size these days. It's not much but it's there. Contact area in this case is very small which helps. I generally use a mic when it matters and find those are far more easily used when parts are on a machine.

I do think it was worth forking out for a set of Mitutoyo 200mm calipers because if I need the size I know that they are likely to be accurate but am aware that they can't always be used to measure as accurately as a mic can.

John

–

[mechman48Participant @mechman48]

Don't rule out body heat transference to affect last digit repeatability; i.e. how often have you slipped the digi calliper into your coverall / warehouse coat, top pocket for a few minutes while you carry out the next machining process, in the meantime your body heat has minutely transferred to & expanded the device enough to alter the last digit. I have done it many times but as I use mine predominantly as a comparator I expect some differential to occur, I mainly use a 3" DRO digi calliper in general, I also have a 4", when I needed more accuracy on final cuts out comes the Mitutoyo digi 0 -25 mm micrometer, as a double check I also use Starret / M & W analogue mechanical mic's for verification so I'm pretty well covered.

For any large measurements I also have a Mitutoyo 200mm digi vernier, in which the batteries last quite a long time, & it is always kept in its case in the lockup chest, I changed the battery beginning of the year & it's still going strong although I do switch it off after use ( of course, who doesn't &nbsp. my mill dro's are the same, change the batteries once per year. I only ever use SR44 & CR2032 batteries, I went down the cheap route with LR44's etc & have found they don't last anywhere near as long as SR44's.

JA … my micrometre's of which the 0-1" was bought when I was an apprentice over 50 years ago and has never given any problems…

I have one of similar age, though mine has multi scale reading on it … Metric & Imp. & although this has been relegated to the lock up as well, every few months I take it out open & close it, also pull a sheet of paper through the anvils, it still reads '0' when the anvils are closed.

George.

[Neil WyattModerator @neilwyatt]

Interesting.

The next ME contains the results of my own tests on digital calipers, using a 25mm test bar.

I found somewhat different results, the cheaper calipers were less accurate but this may have been down to wear.

I also tested a few other things as well, and to me the results were rather interesting.

Neil

[SillyOldDufferModerator @sillyoldduffer]

I look forward to getting the next ME so I can see what Neil's been up to. The more data the better.

By the way, the cheapest of my sample is also the oldest and it has been well used. Not only that but I dropped it on a concrete floor and then stood on it, cracking the display. It still works as well as the others.

Tomorrow I'm going to pick up on Ajohnw's comment 'Looks like you have a pressure problem to me.' It may be that the biggest cause of error is me.

I'm going to see if I can teach myself to repeatedly get 24.00 readings from the 24mm dimension by experimenting with how hard I push the thumb-wheel.

I suppose it won't be a surprise if I find that careful practice with a tool is necessary to get the best results out of it!

Cheers,

Dave

[HughEParticipant @hughe]

Has anybody compared a digital caliper with venier of similar quality?

As mention previously for accurancy l use a mic everytime.

My Lidle caliper is great for quick and rough measurements 0.1 mm batteries last for ever, I only use quality batteries.

Hugh

[Enough!Participant @enough]

Posted by mechman48 on 26/09/2016 19:54:49:

Don't rule out body heat transference to affect last digit repeatability; i.e. how often have you slipped the digi calliper into your coverall / warehouse coat, top pocket for a few minutes while you carry out the next machining process, in the meantime your body heat has minutely transferred to & expanded the device enough to alter the last digit.

There it is again. Given that it takes over a 100 deg F temperature rise to expand an inch of steel by a thou I have serious doubts.

Sorry about the units

[HopperParticipant @hopper]

In industry only ever sawcalipers used for "rough" measurements. Mikes were always used for the precision stuff, eg any diameter turned on the lathe that needs to be within a thou or so, shaft fits etc

Regardless of the accuracy of the calipers (and ours were Mitutoyo verniers so probably pretty good) it is regarded that they just don't have as good a "feel" as a mike when taking a careful measurement. Also, using the ratchet on a mike was frowned upon as calibration of the ratchet is never reliable. A good tradesman was expected to have his own inbuilt calibrated feel when using the mike, developed from practice.

Edited By Hopper on 27/09/2016 02:43:49

[MuzzerParticipant @muzzer]

Posted by SillyOldDuffer on 26/09/2016 13:31:49:

Are you able to identify the expensive Calipers and can you rank the calipers according to which was the best deal?

Are you going to tell us then?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 26/09/2016 13:31:49:

…

Average of Five Readings (mm)

…

Standard Deviation (mm)

'

I'd be grateful though for any comments that might improve my technique.

Cheers,

Dave

.

Dave ,

Several good points have been made already, but I think your statistical techniqe might also need improving.

… I'm very rusty in these matters, but, I think you need a much bigger sample size.

'Standard Deviation' presumes a Gaussian Distribution, and I don't think you can reasonably demonstrate that with a set of five readings. If memory serves, the first check would be that the Mean, Median, and Mode are 'identical'.

Bear in mind that for a high confidence level, your 'tram-lines' will be at Mean +/- 3 Standard Deviations.

MichaelG.

[john carruthersParticipant @johncarruthers46255]

I sent off for the free Mitutoyo reference tables and basic guide on metrology; interesting and quite a lot to it. Not just a quick clamp with a caliper, (although that is usally good enough in my shed).
Scrupulous cleanliness, gloves for temperature control.. etc .. etc.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Muzzer on 27/09/2016 08:04:10:

Posted by SillyOldDuffer on 26/09/2016 13:31:49:

Are you able to identify the expensive Calipers and can you rank the calipers according to which was the best deal?

Are you going to tell us then?

Yup, I've added the prices.

Average of Five Readings (mm)

'100mm' '6mm' '24mm' Zero at end

Caliper A 99.916 6.012 24.042 -0.01 £9.99
Caliper B 99.922 6.038 24.028 0.02 £4.99
Caliper C 99.952 6.028 24.070 0.00 £50.00
Caliper D 99.880 5.976 23.968 -0.01 £30.00

There's a give away in the numbers about which might be the expensive one. Being an 'absolute' type, it held zero correctly, while the others wandered. That's a useful feature.

As to which was the best deal, on those numbers it's the cheapest.  This is assuming my data is accurate and I'm not just measuring poor technique or dirt etc.

Dave

 

Edited By SillyOldDuffer on 27/09/2016 11:25:00

[Neil WyattModerator @neilwyatt]

Posted by Bandersnatch on 26/09/2016 21:59:39:

Posted by mechman48 on 26/09/2016 19:54:49:

Don't rule out body heat transference to affect last digit repeatability; i.e. how often have you slipped the digi calliper into your coverall / warehouse coat, top pocket for a few minutes while you carry out the next machining process, in the meantime your body heat has minutely transferred to & expanded the device enough to alter the last digit.

There it is again. Given that it takes over a 100 deg F temperature rise to expand an inch of steel by a thou I have serious doubts.

Sorry about the units

Wait until you see my findings

[Author]

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