[Shop-talk] Micrometers

[David Scheidt]

On Oct 29, 2009, at 20:04, Doug Braun <doug at dougbraun.com> wrote:

> The $19.95 HF 6" digital caliper has the interesting property
> that if you turn it off, the display blanks, but it otherwise keeps  
> working.  for example, if you turn it off, move it, and turn it on  
> again, it correctly indicated the new position.  In fact, I think  
> the battery will last just as long if you leave it on all the time.
>
They all do that. It's a requirement of the multiple pole printed  
capacitor. If the circuit doesn't watch all the time it can't tell if  
it's moved. The power use when the display is off and it's not moving  
is really low. The display is low power but higher than the resting  
circuit.

While the digital calipers are pretty well to the sort of junk that  
dmages the gears of a dial, they're vulnerable to liquid that changes  
the capacitance of the linear encoder. Some are better sealed than  
others.


> I have one, and it's incredibly handy.  Everybody ought to own one.
>
> Doug
>
> --- On Thu, 10/29/09, David Scheidt <dmscheidt at gmail.com> wrote:
>
>> From: David Scheidt <dmscheidt at gmail.com>
>> Subject: Re: [Shop-talk] Micrometers
>> To: "J. F. Juhas" <james.f.juhas at snet.net>
>> Cc: shop-talk at autox.team.net
>> Date: Thursday, October 29, 2009, 12:06 PM
>> On Thu, Oct 29, 2009 at 10:45 AM, J.
>> F. Juhas <james.f.juhas at snet.net>wrote:
>>
>>> I have a book on precision measuring instruments that
>> describes how digital
>>> calipers work, and although I don't recall it clearly,
>> I believe it
>>> suggested this technology is simple and inexpensive
>> and so most bargain
>>> brands can be as good as the name brands.  I'll
>> see if I can locate the
>>> portion of the book and send it to you tonight.
>>>
>>
>> Yes.  No moving parts.  There's a printed pattern
>> on two printed circuit
>> boards.  A sensor detects the change of capacitance as
>> the two parts move
>> relative to each other.  With standard PCB fabrication
>> methods, you can get
>> something like a tenth (one 10,000th of an inch) accuracy
>> in 6" board.  Some
>> apparently use induction, but the idea is the same.
>> The circuit involved is
>> about a second semester project these days.  Only
>> precision machining the
>> caliper needs is to make sure it runs square.  A cheap
>> digital caliper
>> (assuming it's not bent or something) is more accurate than
>> an expensive
>> non-digital one.  Progress is cool, sometimes.
>>
>> -- 
>> David Scheidt
>> dmscheidt at gmail.com
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