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Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Unless you have a God given calibration current source I don't see that Ohm's law helps a lot. Anyway I plan to make use of that overlay but I need my house to be warm and stable enough for the Weston cells to give a sensible voltage first. (or I might have to ovenise them; I don't think a single 2N3055 will cut it somehow). Then I need to address their output resistance which is about 1K.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Do you really need to adddress the high outupt impedance of Weston cells? I thought they were always measured at zero output current, into a balanced bridge for instance?
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Good point. It specifies <150pA so that shouldn't be a big problem. The Weston cells are not in the first flush of youth but that shouldn't matter much; they are designed to be stable (perhaps not as stable as I hope). One in a set dated as being made in 1970 gave 1.018886V last time it was tested in 1990. This ref gives the life as 60 years if they are the right sort and well treated. I can also cheat. I have an electronic "pretend weston cell" with a low source impedance and reasonable stabillity which I might be able to check at work and use as a transfer standard.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Glad the overlay has at least one user
You get saturated and unsaturated Weston cells. Saturated cells are more accurate and last longer but are more temperature sensitive so need a constant temperature bath. Unsaturated cells are less temperature sensitive but drift more per year and have a shorter life. I've heard that when they get too old the out put becomes 'unstable' but I can't quantify that. Mine was manufactured in the early 60's and seems stable to a certain number of decimal places, but varies far too much for a 6.5 digit meter in ambient conditions. I haven't tried real temperature control yet. The advantage of your large selection of cells would be if you built a temperature controlled oven for them you could monitor them all over an extended period and weed out the bad (drifting) ones. Thanks for that Weston Cell link. I like this too
Ohms law. To find I,V or R you need to know the other two. I have a calibrated voltage reference but can't trust my resistors or current measurements so I'm stuck. I can determine resistive ratio's very accurately however so that's all I'm working on (precision wise) for now.
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Precision Voltage Reference
