9X WESTON CELLS - Advice required - Page 1
Forum section: electronics / general

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 06, 2017, 08:52:33 am
content: Hello All,I have a chance to acquire 9X Weston Cells (Saturated, NEW), should i acquire them?regards.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 06, 2017, 10:05:13 am
content: I have 4 good Weston Cells by now and find them very fascinating.But I also bought a few bad once, so watch out for that.If you have a chance to measure them before you buy them, take a good DMM, set it to High-Z input (> 10 GOHM input impedance) and measure the idle voltage. If they are above 1.018 Volts you are good.But what do you mean they are "New" ?

author: tggzzz
author_rank: Super Contributor
timestamp: July 06, 2017, 11:22:10 am
content: Some designs are relatively fragile, and like to be kept upright. Be aware they contain "dangerous" chemicals such as cadmium and mercury.Whether they are useful to you is up to you to decide.If you have a high resolution stable voltmeter, you could use them as 40uV/degreeC thermometersBeware of two problems: becoming a volt-nut, and that if you have more than one clock you are never sure of the time.

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2017, 11:30:19 am
content: Plenty of clocks to average though. 9 cells would provide lots of opportunities for back to back null comparisons. The whole lot would need to be closely thermally coupled in a thermally stable container. Quite a lot of work there!Maintaining all the comparison records could be a full time job in itself (I'm sure NPL and NIST had someone whose job that was). A Volt-nut's dream!

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 06, 2017, 01:58:29 pm
content: Hello and thanks all.should I take your comments as a yes?regards.

author: 0.01C
author_rank: Regular Contributor
timestamp: July 06, 2017, 02:14:04 pm
content: I bought one from ebay ,but I dare not use it

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2017, 02:21:24 pm
content: That's a really wierd arrangement, they're all wired in series!Normlly you would bring each cell out on its own pair of terminals so that they can be compared (back to back) with a high impedance null detector.Hard to say whether they are saturated or unsaturated cells. They look as if they have been shaken though - are those blobs of Mercury in the middle of some of the tubes? They should all be constrained down at the bottom of each cell as part of the construction.

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2017, 02:26:48 pm
content: Quote from: zhtoor on July 06, 2017, 01:58:29 pmI think it was more of a 'Be aware of their care and feeding needs and potential pitfalls'.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 06, 2017, 03:26:16 pm
content: If they're saturated, they have a high temperature coefficient and need an oven. Without that, they're not very useful. Those are the ones that really need to be kept upright. If not abused, they last near to forever.If they're unsaturated, the tempco is lower, but their life is limited. Most unsaturated cells that have ever been produced are now bad. I'd never buy an unsaturated cell.I got rid of all my standard cells some years ago, not wanting that much contaminated mercury in the house.

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 06, 2017, 05:53:28 pm
content: Quote from: Conrad Hoffman on July 06, 2017, 03:26:16 pmthanks.the cells are SATURATED and reportedly never used in original packing / metal jars designed to bemaintained in an oil bath, each with a temperature measurement thermometer port.i thought that doing back to back nulling and measuring relative drifts could enable one to maintain a pretty decent (old)primary standard.regards.

author: manganin
author_rank: Regular Contributor
timestamp: July 07, 2017, 02:10:42 pm
content: Quote from: Gyro on July 06, 2017, 02:21:24 pmThat is to produce stable voltages higher than a single cell. Each cell measured against a reference cell and the total voltage is simply the sum of the individual cell voltages.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 07, 2017, 02:50:06 pm
content: Yes, compare back to back at a known temperature and you can maintain a pretty good voltage standard. Just be careful when transferring and scaling the voltage that you don't draw any current.