Electronics > Metrology

From the bin!-- weston cell +meas. data & lastly Muirhead Cat. data sheet

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My cursory reading about weston cells is that you do not want to source any current from the cell. Even a very high impedance DMM will draw some current.

The cell is supposed to be measured deferentially. The way I measure my Eppley cell is with a 10V Fluke 731B, a Kelvin Varley divider and an ordinary DMM as the null meter. The DMM is a 6 1/2 digit KE2000 on the 100.0000 mV range. It can resolve 0.1 uV.

My Eppley cell is from 1975. The most recent voltage measurement is 1.018731 in June of this year. A faded sticker on the cell states that the cell was 1.01918 in 1985. It drops 1 to 2 uV per month. I have had this cell for 1 year.

With a high impedance meter like the K2000 it does not make a big difference whether you do a normal measurement or do it differentially. The input current is something like less than +-50 pA plus an parallel resistance of more than 10 Gohm (often way more), which would be less than 100 pA at 1V. So one can not even be sure about the sign of the current at less than 1-2 V.  Currents in the pA range should not be a real problem - when the standard cells where up to date, the voltmeter currents where more in the µA to nA range. If you are that paranoid about currents, keep the cell really dry ( < 40% RH) to reduce surface leakage at the cell itself.

It would be a good Idea to check the input current before, as leakage can be higher if contaminated.


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