[volt-nuts] Traveling Standards

[Charles P. Steinmetz]

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.)

You also might include a charger, so that the recipient can top off 
the battery before measurement (if necessary) and before return 
shipment.  All measurements would be made with the charger disconnected.
Best regards,

Charles