Electronics > Metrology

Weston Standard Cell

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jesuscf:
A few days back I found a couple of Weston Standard Cells at work.  The cells were made by The Eppley Laboratory.  Both are "CAT. NO. 100". One of them was obviously damaged.  The other one works: about 1.01871V!  (The nominal voltage when new was 1.01930V) I contacted the The Eppley Laboratory inquiring about the approximate age of the cell.  Tom Kirk from The Eppley Laboratory thinks the cell is from the early 80s.  He also provided me with some fantastic documentation about the Standard Cells The Eppley Laboratory used to manufacture.  With his permission I am attaching it to this topic.  Unfortunately I can not upload the whole pdf because is larger than 1MB, so I split it into two smaller parts.  Note: in the attached picture the input resistance of both HP34401A multimeters is set to ">10 GOHM".

edpalmer42:
More than you ever wanted to know about Standard Cells:

http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS84.pdf

All the records from Eppley were donated to The Vintage Radio and Communications Museum of Connecticut.  If you want to find out the history of your cell, they can send you a copy of the original factory test results.

http://www.vrcmct.org/special-displays.html

The sample that they're showing on that page happens to be one of mine!  :)

zlymex:
I have two Western cells and I love the noise free and the stability(if properly maintained). I know someone in the metrology field that prefers cells over solid states. However, the very large tempco may problematic.

btw, set 34401A to 1V range will get better results.

manganin:

--- Quote from: zlymex on March 08, 2016, 02:25:31 am ---I have two Western cells and I love the noise free and the stability(if properly maintained). I know someone in the metrology field that prefers cells over solid states. However, the very large tempco may problematic.

--- End quote ---

There are still a few labs that use Weston cells as their primary reference. And others that have kept the old set as a backup even though not regularly maintained.

From the hobbyist point of view the problem is that buying a good cell is almost impossible nowadays. Unless you get a powered cell enclosure directly from a closed cal lab and hand carry it to your home lab.

Buying from Ebay or from equipment dealer is not an option. High temperature will ruin the cell, low temperature can ruin it, any temperature change affects the behaviour. Turning the cell upside down even for a few seconds can ruin it. Some cells fully recover, some don't.

In one year time scale the drift rate of some good cells can be assumed constant, but in the long run the change is not linear. And monthly comparison is needed to make sure that nothing is going wrong. Sometimes there is a reason for the instability, but often it just happens.

Interesting to play with, but I think we should be very happy that the LTZ1000 exists.

zlymex:

--- Quote from: manganin on March 09, 2016, 06:18:59 pm ---
--- Quote from: zlymex on March 08, 2016, 02:25:31 am ---I have two Western cells and I love the noise free and the stability(if properly maintained). I know someone in the metrology field that prefers cells over solid states. However, the very large tempco may problematic.

--- End quote ---

There are still a few labs that use Weston cells as their primary reference. And others that have kept the old set as a backup even though not regularly maintained.

From the hobbyist point of view the problem is that buying a good cell is almost impossible nowadays. Unless you get a powered cell enclosure directly from a closed cal lab and hand carry it to your home lab.

Buying from Ebay or from equipment dealer is not an option. High temperature will ruin the cell, low temperature can ruin it, any temperature change affects the behaviour. Turning the cell upside down even for a few seconds can ruin it. Some cells fully recover, some don't.

In one year time scale the drift rate of some good cells can be assumed constant, but in the long run the change is not linear. And monthly comparison is needed to make sure that nothing is going wrong. Sometimes there is a reason for the instability, but often it just happens.

Interesting to play with, but I think we should be very happy that the LTZ1000 exists.

--- End quote ---
Got it. I seldom test my western cells but I now have an alternative way to get a low noised reference that exceeds all commercial solid states.

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