Digital Calipers - Can you rank these from the measurements?

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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.

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.

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

Wait until you see my findings

In respect of battery eating, some years ago I came across a site where a contributor having suffered from this problem did some on/off tests and discovered that only one of the electronic calipers actually switched off the device. I think this was the Mitutoyo. All the others did was to switch off the display.

Also, the point about batteries is worth shouting loud and clear. There are two technologies of small button cells, alkaline (eg LR44) and silver oxide (eg SR44), of which the silver oxide is a slightly higher voltage than the alkaline and which also lasts longer. The problem I have found is that although they are nominally the same, shopkeepers don't know the difference, don't understand the difference, and take umbrage if you try to correct them. So now if I'm offered alkaline, I say "No thanks", and walk out. My advice is to stay well clear of the alkaline cells for these single cell devices.

About the testing I did and temperature. When I did my tests, I left all devices overnight in the same warm room in order to have them all stabilized at the same temperature, not necessarily 25 Deg C, but whatever the heating was set at (probably 20 - 22 deg C). The three standards actually have a plastic part by which they are to be handled, whilst the instruments were mounted in the plastic jaws of a small table vice which itself had been left overnight. I thus attempted to reduce any temperature effects to a minimum.

Peter G. Shaw

ps. Anyone a pair of LR44 cells dating from February 2010? No? Thought not!

Edited to add post script

Edited By Peter G. Shaw on 27/09/2016 11:31:27

Something I don't go into much in my article but I should have made more of - for a digital caliper to work at its best you need to adjust the gib screws.

Neil

Shouldn't need saying Neil.

Peter

I have one of those too. It's kept upstairs for rough work when I'm playing electronics.

The main trouble with this one is reading the vernier. The picture demonstrates parallax error very well - the camera must have been off centre. Actually, from directly overhead the reading is closest to 24.05, but it's hard to tell the difference between this and 24.1 or even 24.15. A magnifying glass helps.

Much better than a 6" rule though.

Dave

I didn't even know they have gib screws that can be adjusted!

Ignorance is bliss.

Dave

It's a fair cop Michael. The sample size is far too small to be more than 'interesting'.

When posting results I try to be open and honest about the data and how it was collected. It's amazing how easy it is to unconsciously mislead oneself, which is why I'm a great believer in peer review even if it's, ahem, robust.

Cheers,

Dave

Got to take the base material & form into account too.

I regularly turn plastics for pen making. The ratchet click load on my mic is way, way too compressive for a thin wall plastic tube (frequently <1mm) as found on some designs of barrels & caps and it distorts the tube like anything. It often ends up with me getting a better measurement with a calliper. I try to go to a precision of around 0.02mm as that is getting to the limits of machinability on many acrylic alloys. To get closer than that reliably you have to do lathe sanding.

Regards,

Richard

Over the last few years I’ve noticed lots of debate on cheap (ebay - £8.99) vs expensive Mitutoyo calipers, and to be honest I think you are all neglecting to mention the biggest and most important difference. Cheap calipers are impossible to drop no matter how hard you try. Whereas you only have to look at expensive calipers and they will slide off the bench and crash onto the concrete floor.

Tell me I’m wrong ….

Edited By David Cambridge on 27/09/2016 12:59:46

Edited By David Cambridge on 27/09/2016 12:59:57

Edited By David Cambridge on 27/09/2016 13:02:52

Good point Richard.

I've just finished collecting data after working with each callper to learn how hard I need to press the thumb-wheel to get consistent results.

Three of the calipers work best with firm pressure, certainly enough to deform a plastic tube. The fourth (the £9.99 model) works best with a much lighter touch and would be much more useful for your needs than the others.

Neil's comment about adjusting gibs could be relevant to the amount of thumb-wheel pressure needed.  More testing to do!

Dave

Edited By SillyOldDuffer on 27/09/2016 14:01:20

I spent time learning just how much thumb-wheel pressure to apply to each Digital Caliper. A bit of practice and careful positioning of the jaws makes quite a difference. Three Calipers work best with the same firm pressure, the fourth (£10 model) works best with a gentle touch.

Unlike the first round of measurements, the Calipers all spent the night in the same room to make sure they were soaked to the same temperature, 18℃.

I took extra care to clean the Calipers generally and also by sliding clean paper between the jaws. Even though the Calipers were cleaned yesterday I was able to remove more dirt. The precision parallel and worktop were cleaned and clean paper was laid on top. I wore new latex gloves and a clean apron.

I took care not to touch the parallel or Calipers more than necessary. As it took 20 seconds to enter the result of each measurement the Calipers had some time to cool off after handling. This is based on Neil's hint that the electronics are temperature sensitive.

I took twelve measurements each of the 24mm dimension and graphed the variation from assumed truth, ie ±24.00mm

• The red line is the most expensive caliper. It is by far the most consistent but it reads 0.01mm high. Mean 24.011.
• The blue line is the cheapest caliper. It reads closest to 24mm but has a tendency to read 0.01mm low. Mean 23.997.
• The yellow line is the £30 caliper. It's not quite as consistent as the blue line and reads slightly lower too. Mean 23.987.
• The green line is the worst performer. It's the £10 DIY store caliper. Mean 24.015 but notice that wavy line which is not good.

The surprise is just how good the £4.99 caliper bought from Lidl is.

I actually took four sets of measurement for each caliper:

1. 24mm measured using half the jaw depth
2. 6mm measured using half the jaw depth
3. 24mm measured with the jaw tips
4. 6mm measured with the jaw tips

Measuring with jaw tips only looks to be less accurate and it also seems as if measurements taken at 6mm are more accurate than those taken 24mm. I haven't done the graphs yet but will post them later if they're interesting enough. It would be nice to see the expensive caliper measuring equally well across the range whilst the cheaper ones don't.

+1 for everyone using micrometers for accurate work!

Cheers,

Dave

Edited By SillyOldDuffer on 27/09/2016 16:19:53

Edited By SillyOldDuffer on 27/09/2016 16:21:05

I'm glad some one else knows that the ratchets are dubious. It's worth bearing in mind the area and pressure effects as well. Bar is generally easy. Flat surfaces not so easy and in practice will need more pressure.

Telescopic gauges are interesting. I found that to get a real size for the diameter of a bore they need to be pretty tight fit when they are swung through on the actual diameter. As the ends are domed they are more or less point contact devises.

I have an old set of Kanon verniers. They are very precise but have the same problems as the digital ones. Used correctly they will measure to 1 thou and a very rough guestimate of over or under can be made. They were supplied for toolmaking so would be to workshop standard.

John

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It helps to have realistic expectations when testing calipers:

www.mitutoyo.co.uk/hometabs/top-rated/500-196-30

Only the green one is outside the Mitutoyo +/- 0.02mm spec on that graph.

Neil

Interesting though this discussion is, there are aspects being missed, I think....

First there is no such thing as real accuracy. You can go on getting finer and finer measurements until you are counting atoms, and then it goes all quantum. When we get to .01mm measurements what we are really seeking is matching components, which is about fit and tolerance. Any of these devices will do this as long as we use the same device in the same way on each.

Second thought: I was always taught that the only purpose of the last digit is to interpret the previous one, so we can understand how it is rounded, and by how much. A digital calliper reading 1.00mm could be rounding up .99 which is probably good, but might be rounding up 0.901, since some electronics only measure the passing of a certain point. We instinctively round up on .5, but electronics need more instruction than that. Good kit will measure to two decimal places more than it will tell you, in order to compute the best answer it can - that might be what the more expensive tools are doing (but probably more scientific tools). When we use analogue equipment, our eyes are doing this for us whether on a micrometer or a 6" ruler.

Third thought: what we pay more for is probably quality control. Expensive kit should be more reliably accurate to the appropriate level: replicable and repeatable ( although still within a range of accuracies). They should also be more reliable mechanically. On the other hand, some of the pound shop devices will be accurate enough for NASA, but not many. When you pay more, you are probably paying for the ones they through away because they don't meet the standard.

Finally..is a 'however'..... it is common to see things that look identical at a wide range of prices. They may not be the same, having higher spec or tolerances that aren't obvious, but the nagging feeling is that they are the same. Try looking for Wigglers and you will see anything from under a fiver to over £40, in many cases using exactly the same photograph. How do we know that the £30 calliper isn't just a £5 calliper with good PR?

Edited By Matthew Reed on 27/09/2016 20:56:41

Edited By Matthew Reed on 27/09/2016 20:57:36

I wonder if a Mitutoyo in practice would comfortably be within spec across the range? It's possible that their ±0.02mm claim is conservative and actual performance is better.

I'm too mean to buy a Mitutoyo out of curiosity. Are there any Mitutoyo owners out there prepared to test their Caliper as I have and share the results?

Thus far I'm concluding that cheap digital calipers are better value for money than dear ones in a home workshop.

But I don't think this means that expensive calipers are a complete waste of money. My £50 example is smooth and comfortable to use. It feels good. I suspect that it, and other quality types, are likely to have longer working lives (David Cambridge apart), and be less likely to inconveniently conk out in a busy industrial setting.

Proving that expensive calipers last longer than cheap ones is too difficult for me.

Looking forward to seeing your ME article.

Dave

My article will include a Mitutoyo test certificate with internal and external results, but you will have to wait five weeks

N.

I agree. Seems to me there are two bad mistakes you can make when spending money:

• Wasting it by buying too cheap
• Wasting it by buying on reputation only or because it's "reassuringly expensive".

Wigglers are a cracking example of something I suspect to be overpriced. Unless I'm doing something wrong a dress-makers pin with a blob of Plasticine out-performs my shop-bought metal example.

How do we know that the £30 calliper isn't just a £5 calliper with good PR? We don't. Unless someone already knows, the only way to find out is to test a lot of them.

Dave