Re: Current measuring resistor calibration

Bill Sloman wrote:
On Jun 26, 12:26 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 25 Jun 2009 18:28:00 -0700 (PDT),Bill Sloman

<snip>

And nobody would accept his current cals as traceable.
If he has his own Thompson Lampard capacitor, they wouldn't need to -
it's a primary standard in its own right.
If you build it exactly right, dimensions and all. Nobody who buys
precision electronics would trust or buy from an amateur who makes his
own standards.
If they did, he wouldn't be an amateur, which maes this a less-than-
useful claim.
They would think he's crazy, and rightly so.
Only if they failed to understand why national standards laboratories
exist in the first place. For a self-proclaimed supporter of the free
market, you seem to be strangely insistent on the sort of rigid
central supply of calibration services that characterised the state-
controlled economies of Eastern Europe until recently.
I am not bound by your absurd definitions of what I am.

There are lots of cal labs around. But I wouldn't use one that can't
trace their measurement chain to a national standard. What's the point
of having a free market in the definition of a volt?

It costs money to have your secondary standard calibrated against a
primary standard. Make the service a monopoly, and the sole supplier
can charge what they like.

Your skill set seems to be centered on reading papers and fantasizing
about building stuff.
I've actually built stuff from time to time, and when I do it, it
works. Not everybody has the skill of disciplined fantasy.
You do the OP a disservice when you make wild, impractical suggestions like this.
Read what I actually suggested that he should do - posted about an
hour after your post, but long before I saw your post - where I
advvise him against building a Thompson-Lampard calculable cross-
capacitor.
He should buy a calibrated digital multimeter, which has lots of other uses.
Read what he has posted - he has bought several, and is uhappy that
they don't stay calibrated.
I think he has other problems. And nothing but a primary standard
stays calibrated forever.

He hasn't told us why he hasn't sent them back for recalibration.
A couple of good lab-type shunts might be handy too. They can be had on ebay and sent to a local
cal lab for certification.
I suggested that he buy a top of the line four terminal reference
resistor from Vishay. When I last looked they were at at least an
order of magnitude cheaper than another calibrated digital multimeter.
But then he needs a good voltmeter too. Why not just buy a dmm that
does it all?

As I've mentioned, he does seem to have already gone down that route.
Telling him to do it again isn't useful advice.

Get real.
As in encouraging the OP to rush out an buy yet another calibrated DVM
when he isn't happy with ones he's got?
As I said, I think he has other problems.

And that's you excuse for telling him to do what he has done before,
which didn't seem to work?

Pointing him at precision "coaxial" AC bridges built around ratio
transformers may not seem realistic to you, but it does at least offer
him another way of tackling his problem.

If he's desperate enough to contemplate building a Thompson-Lampard
calculable cross-capacitor, he's presumably desperate enough to
contemplate the rather less demanding task of getting hold of or
building the necessary ratio transformers

I have several of them, 6-digit ratio transformer boxes, flea market
gadgets, apparently linear to a few PPM as far as I can tell.

They should be good to one part in ten million, if properly built.

But not very useful for resistance calibration and totally useless for DC
current.

You use them to set up an AC-excited resistance bridge (a Blumlein
bridge) which allows you to compare a wide range of resistors with a
single secondary standard resistor with fairly spectacular precision -
national standards laboratories have been doing this for many years,
and publish at length on the subject.

I think he needs to straighten out the gadget he's trying to build and
calibrate.

Too true. Since you posted this message, you've got into the
discussion between Petkovic, John Fields and MooseFET, which does make
it clear that Petkovic doesn't seem to understand the idea of using a
bridge to compare two resistors, which happens to be the gadget he
should be trying to build and calibrate.

---------------------------------------------------------------------------
Bill,
I will build a Blumlein bridge to find out exact value of my resistors and it solves problem of calibrating one range but as it is evident in my original post, I also need that when I change current measuring resistors, I read the same value in different current ranges.
As I've mentioned I've used 0.1% resistors but this problem persists, so in addition of knowing exact value of my resistors I have to find a way
to eliminate this problem.
---------------------------------------------------------------------------
.

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