Barometric pressure influence fluke 732B accuracy

[Theboel]

Hi,

I read at least two report about barometric pressure influence Fluke 732B accuracy,
Ok I can understand if temperature can changed the accuracy and or stability of Fluke 732B but how barometric pressure can made difference ? 

Any body can explain in plain English why ?

[dr.diesel]

One reason could be due to the reference being a sealed can, with no method to equalize with the outside.  Any pressure variance would every so slightly distort/stress the substrate inside.

[Theboel]

Hi dr.diesel,

got Your point, Thank You.

a rough calculation if someone move 732B from 1m above sea level to 1000m its caused almost 4uV difference
another Google search give approximately 1.7 hPa in daily barometric pressure
http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/diurnal.html
so naturally in daily pressure caused less than 2nV variation for 732B output.

if You dont calibrated Your 732B in Boulder Colorado (1655m) and move it to Jakarta (8m) there no need to worry about pressure difference 

[chuckb]

That's good information. Attached are two plots of the pressure change in my lab over 5 day spans. On November 18th you can clearly see the high pressure front moving through.

[Andreas]

Thanks,

but the 1,7 hPa difference is only for the daily earth rotation without the much larger part of the weather.

I have recorded 41 mmHg (= 55 hPa) span during the last years in my lab
(measured every morning so the extremes might be even larger).
With 20nV/hPa I would get 1.1uV change.

with best regards

Andreas

[Theboel]

Hi Andreas.

Now its something to consider if the reference "drift away"
1,1uV.

But where the "drift" come from are just from reference or also come from other component ?
Are this drift has long term effect ?
How to maintain pressure with mmhg accuracy ?

[Kleinstein]

Maintaining pressure with high accuracy is not not easy. Even a closed container could show variations from temperature or long time drift.

But one could measure the pressure and do a numerical correction if needed. The effect is not that large. In larger labs there usually is a air pressure reading anyway - just in case, to check for correlations.

[Theboel]

Maintaining pressure with high accuracy is not not easy. Even a closed container could show variations from temperature or long time drift.

But one could measure the pressure and do a numerical correction if needed. The effect is not that large. In larger labs there usually is a air pressure reading anyway - just in case, to check for correlations.

I think You made a good point, its time to log my reference include temperature and pressure, I hope plesa and TiN can include this parameter to their log project. 

[Kleinstein]

The logging project of TiN and  Plesa already includes a BME280 as the preferred  environmental sensor. So they include temperature, humidity and air pressure.

The change is really not that much: it more like extreme changes (e.g. storm to good wether) that could give you a 1 µV (0.1 ppm) change one might be just able to notice if one has a good stable reference that does not have the same influence. At that level there are not so many ref chips to choose from (old Motorola SZA2.., LTFLU, LTZ1000 and LTZ1000A and maybe old ZD based). So the main unknown is how the LTZ1000 is performing.

The thing is, that to do the correction one would need to know how much effect is there - such instruments usually don't have the number on it - though it might be a good idea for the high end ones. The only point where this might be important if you send a high end instrument for calibration to an very different altitude - so calibration in Bolder Colorado might not be the best option for every one at see level. Still this would be only 170 hPa or maybe 0.4 ppm.

[zlymex]

From what was said in the first paragraph, those old 732B employing SZA263 is much better than LTFLU-1, since there is no change for other critical components such as those hermetic PWW. Because LT makes the similar components, I guess that LTZ1000 will suffer the same as LTFLU-1 for pressure influence.