[volt-nuts] Traveling Standards

[Andreas Jahn]

> I can see the advantage of this, 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.

Practically I can power a LM399 with 8 NiMH cells (9,6V 2000mAH)
for about 2 Days before I need to recharge.
I tried better thermal insulation but this has no significant influence.

> 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?

On my LM399 with a 6,5 Digit instrument (10uV resolution)
you will not see any thermal hysteresis.
(except when comparing 2 of them in a higher resolution range).

To the diagram: 
Y-axis is the output of my 5V  24 Bit ADC in mV. X-axis is time in minutes.
(integration time 1 minute so measurement noise is around 1uV at ADC-input)
The reference is attached to the ADC by a precision 2:1 divider (LTC1043)
Voltage before power off 3431.174 * 2 mV
1 minute power off
1 minute value out of diagram
about 15 minutes warm-up time
about 1.5 uV * 2 = 3uV Hysteresis.
2nd power off for one minute off gives additional shift of 0.5 * 2 = 1uV.

So I would recommend a power-up time of at least 1 hour or so.
The instrument which is used to measure the value will need this too.

With best regards

Andreas





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