[volt-nuts] Traveling Standards
John Devereux
john at devereux.me.uk
Fri Aug 26 18:27:20 UTC 2011
"Charles P. Steinmetz" <charles_steinmetz at lavabit.com> writes:
> Bob wrote:
>
>> I can see the advantage of [shipping the reference under power], but
>> as an example the lm199ah draws about 15 mA at 25C. If exposed to
>> low temperatures during shipment it will draw more. Perhaps this
>> could be reduced if the part is put in a thermally insulated box. A
>> one week shipment (out and back), at 15 mA, would require a 2600
>> mA-hr battery. Certain not impossible, but somewhat heavy.
>>
>> With the goal of 10ppm overall uncertainty, will the thermal
>> hysteresis be a limiting factor if the unit is shipped un-powered?
>> How long should it be powered at the remote lab before a measurement
>> is made?
>
> When one is trying to achieve repeatability in the single-digit ppm
> range, I would not leave anything to chance -- and retrace
> (hysteresis) is always a matter of chance, particularly if the device
> has been powered off for a long time (several days) and may have been
> exposed to significant temperature shifts during that time. In my
> view, the best approach in this instance is to use something like a
> 5Ah gel cell (to ensure relatively little voltage droop during the
> transit time), with at least one stage of preregulation. The total
> package should be under 2 kg and should easily fit into a USPS
> medium-size flat-rate box that ships for $11 (this assumes that you
> and the recipient are both in the US). UPS should be similar. (If
> $22 for a round trip is a deal-breaker, we might conclude that one
> wasn't very serious about the project.)
[...]
Hi Charles,
I think you will end up on Fox news and the Reference will end up
"defused" with a controlled explosion :)
I am still thinking LTZ1000 is the way to go on balance. It has all the
complications Andreas pointed out. But I think:
- stable without keeping it always powered up
- stable with RH
- stable with ambient temperature (compared to others)
- stable with respect to orientation, less likely to be affected by
handling(?)
Does anyone know if the ebay / Chinese LTZ1000 are the real thing?
E.g. 250823975393.
Regards
John
--
John Devereux
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