Poly carbonate digital calipers

[Tozguy]

Take two identical dial calipers that are mechanically capable of an accuracy of +/- .001'' and resolution of .001'' with graduations of .001'' on the dial, if one reads 1.236'' and the other reads 1.235'' on the same piece what do you make of it?

 

[woodchucker]

Depending on the actual size, using a certified caliper, if the size is one of those readings, then the accuracy for both is in spec. If the actual size is 1.234, then only one is in spec.

 

[Tozguy]

You are correct. It is misleading advertising. Nothing new about that, whatever you are looking for. The only way to make it through the mine field is to understand well enough to know how to see through the spin...

And this is what I see through the spin:

The number of decimal places in a readout is meaningless if the instrument itself cannot in fact resolve measurements to that number of decimals or to that fineness of graduation of the readout.
Just like how a speedometer that reads to 250 mph does not garantee that the car can go that fast.

The design of a sliding caliper does not lend itself to making measurements more precise and accurate than +/- .001'' (.002'' total). Plastic or composite calipers probably have special applications (i.e. measuring magnets or delicate materials) but might not be as durable, as accurate nor as precise as stainless ones.

We should be aware how easy it is to be lulled into relying on a digital calliper, that reads to four or five decimals, for accurate and precise measurements. In fact the actual size of the object being measured might be quite different than what the readout is showing.

For work to the last .001'' or less, a micrometer should be used.

Some good reading here: http://www.hobby-machinist.com/threads/metrology-101.22521/

 

[woodchucker]

And this is what I see through the spin:

The number of decimal places in a readout is meaningless if the instrument itself cannot in fact resolve measurements to that number of decimals or to that fineness of graduation of the readout.
Just like how a speedometer that reads to 250 mph does not garantee that the car can go that fast.

The design of a sliding caliper does not lend itself to making measurements more precise and accurate than +/- .001'' (.002'' total). Plastic or composite calipers probably have special applications (i.e. measuring magnets or delicate materials) but might not be as durable, as accurate nor as precise as stainless ones.

We should be aware how easy it is to be lulled into relying on a digital calliper, that reads to four or five decimals, for accurate and precise measurements. In fact the actual size of the object being measured might be quite different than what the readout is showing.

For work to the last .001'' or less, a micrometer should be used.

Some good reading here: http://www.hobby-machinist.com/threads/metrology-101.22521/

I think there is a mistake in the link. The 1. should be +/- .5 I think, the decimal is not holding any precision other than units, since there is nothing after the decimal.

 

[Ebel440]

I see no reason to buy them really. You can get metal ones for 10$. With the plastic you would have more flexing and faster wear. Unless you need them for a specific use where plastic may be of some benefit

 

[graham-xrf]

Precision? accuracy? And how to tell if there are games going on in the software!
It gets harder unless we doggedly insist on being hard-heads about it. I guess the temptation among those sales oriented folk is overwhelming, and gaming the software to give you a nice warm "Gosh - that must be accurate" feeling is only one small step away from going the whole 9 yards like Volkswagen did with the software "recognizing" that an emissions test was under way.

For me, it is often about angular resolution using encoder kit in satellite tracking Earth station antennas
Stuff like this --> http://www.renishaw.com/en/optical-encoders--6433
One of the products offers 26 bits, and for an extra £15, one can have 32 bits resolution, of a full circle.
No surprise then that a computer sampling at 40 times/sec (in between lots of other tasks) sees all the digits to the right of the true accuracy dancing about randomly.
Worse, the numbers are being crunched by a hard-working DSP (Digital Signal Processor), hard-programmed hardware to do trigonometric math on these useless extra numbers.
1/(2^32) = 1/4294967296 = 2.32830643654e-10 . Try 0.3 milli-arc-seconds! Is that about the height of a pencil in NY, if one is in London? I don't know. It is beyond the ability of the calculator trig function.
OK - so it strains credulity. Maybe there is some kind of very special application that requires it.. (might be?)
So I try the 26-bit product. Keep in mind these things are very expensive!

1/(2^26) = 1/67108864. Even then, the display will not allow those random digits to stand still.
We have to discard the last bit because of the +/-1 bit uncertainty about the state of the least significant bit.

Eventually, finding that the communication to encoder protocol to the servos can only do 22 bits.. Hmm the digits still dance about sampling noise!
1/(2^22) = 1/ 4194304 (of a circle), or 85.83 micro-degrees, or 0.309 arc-seconds. Better than the theodolite!

OK - we can have the system working fine, but that does not stop the sales guys bragging that it has (looking at the encoder) 32 bits accuracy.
In fact, the seismic vibrations of me stomping about on the concrete floor were making the last two digits jump!

Getting the terms used correctly may be near impossible amid the sales culture (yeah - mostly telling sort of lies!)

 

[bfd]

don't measure anything hot with them they melt bill