[volt-nuts] Traveling Standards

[Andreas Jahn]

Hello Bob,

Im experimenting for some years with different references.
For a traveling reference with 10ppm I would use a heated reference like a 
LM399.
You will find it in most of the serious 6,5 Digit Multimeters like HP34401A 
or Keithley 2000.

I use them battery powered (NIMH-Cells) with a voltage stabilizer.
(the heater voltage has some influence on the output voltage).

Time stability can be determined by building several of them pre-ageing for 
some 1000 hours
and then sorting out the extremes.
Of my 2 LM399s  one is running pretty close to 2 LTZ1000A references
the other (a desoldered part from a old measurement card) is drifting around 
by +/-25uV.

For unheated references like a MAX6250 you will have to determine the 
temperature
very exactly. I use NTCs mounted directly at the ground pin of the Reference 
to get
nearly the temperature of the reference with a resolution of 0.1 K.
When selecting some 3rd order compensated LT1027CCN8-5 references
you will be able to find some with a low TC near room temperature.

But all plastic references suffer from 2 things:

first: mechanical stress will shift output voltage.
So I solder only one pin directly to the pcb the others with thin wires
So for a traveling reference you will run into trouble.

2nd: when having calibrated the temperature out humidity will play a role.
On my LT1027 the humidity change is around 0.4ppm for each per cent of 
relative humidity.
so for a change of 30% you will not get your 10ppm level.
The problem is that the humidity shift has a long time constant. Which is 
around 4 or 7 days
with my 2 LT1027 references. So you will have to record the history.

With best regards

Andreas


----- Original Message ----- 
From: "Bob Smither" <smither at c-c-i.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Thursday, August 25, 2011 7:14 PM
Subject: [volt-nuts] Traveling Standards


> Fellow voltage aficionados ,
>
> If this has already been discussed - my apologies.  I could not find
> this topic in the past several months of list archives.
>
> Like many of us, I have an ever growing collection of voltmeters and
> related instruments.  Like the man with two clocks who is never sure of
> the time, I now have enough volt meters to have doubts about all of them.
>
> So - I was wondering if those on this list with really good voltage
> measurement capabilities would be willing to help those of us without.
>
> What I have in mind is creating a small voltage reference circuit based
> on one of the many available IC references that is stable enough in both
> time and temperature to use as a transfer standard.  I am thinking that
> the actual voltage is not important, just the stability.  Using the ICs
> that I am familiar with the actual reference voltage would be around 5,
> 7, or 10 volts.  Said reference would be mailed to a willing list member
> who would record his reading of the reference and mail it back.
>
> I don't have such a reference yet, but have breadboarded a couple and
> the idea seems like it might work.
>
> My first objective would be to obtain a reference that I could use to
> get all my meters to agree.  Since the best meters I have are an HP3455A
> (best accuracy about 20 - 40 ppm, and only for 24 hours) and an ancient
> (but very usable) Fluke 883AB (best accuracy is 100ppm), I would be very
> happy to have a reference that I could trust to 10ppm.
>
> So - two questions for the list:
>
> 1.  Does this sound feasible?  Am I overlooking anything that would keep
> me from being able to transfer a 10ppm known reference?
>
> 2.  Any list members that would be willing to help with this?  I
> envision mailing a small package with the reference in it along with an
> enclosed, postage paid, return box.  I am asking for a member that would
> take the reference, apply power, let it warm up, record the room
> temperature and the reference voltage to within 10ppm, and return it to
> me.  If you prefer to respond off-list - smither at c-c-i.com.
>
> BTW - I live in Friendswood, Texas (near Houston).  Any fellow nutters
> close enough that I could hand deliver the reference?
>
> Best regards,
>
> Bob Smither
>
>


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