Poor Man gets his candies (6X weston cells)

[MisterDiodes]

Yep.  Compared to a Zener Vref, these just aren't very good.  They look cool sitting on a museum shelf though.

The other fun thing these do is if you ever draw over around 100uA (the threshold varies, but usually around that) for any length of time (Maybe you accidentally brushed the test leads together and realize the safety resistor was not in the circuit) you might as well throw out the cell after that.  Even if the cell voltage looks like it almost recovers after that high current event, the discharge rate is now so unstable it's not really useful any more.  Which you might find out about a few months later when you're monitoring your discharge trend lines.

A real Barrel o' Laughs when you realize that happened to you.

You'll see this warning on the various old standard cell manuals - and they aren't kidding.

[Conrad Hoffman]

FWIW, I had a pretty good collection of Eppley un-saturated cells. They just don't last forever and eventually I sent all of them to hazardous waste. I also had 3 good saturated cells in a JRL oven. It served me well for several years, but the temp control was primitive and I didn't like having that much mercury and cadmimum around, so it met the same fate. Nothing wrong with it, except it was a standard cell. Solid state is the only reasonable way to go. My Analogic (Data Precision) 8200 voltage source uses the LM399 and is as good as just about anybody needs. Naturally you can do better as some of the projects here show, but in any case the standard cells have had their day in the sun and are best retired.

[HighVoltage]

Another reason why you definitely don't want to put them in parallel is their different output over temperature.
The temperature dependency of the WestonCells is really significant and not always the same.

Here are two of my Weston Cells measured over night.
The first 6 hours during day/evening with activity in the room
The second 6 hours during the night

Although the output voltage is following the temperature profile, the amplitudes of change between the two cells is very different.

[HighVoltage]

Yes, #1 is special! This one always had this value, since I had it.

I only had those two cells at hand last night, may be I will repeat the test with my other weston cells.

[HighVoltage]

Hello HighVoltage,

 i can't understand what are the two spikes?

regards.

The spikes have nothing to do with the cell.! Probably not a perfect wire connection or a DECT phone call that came in. This was not really a good scientific setup and I need to repeat it.

May be I will hook up 3 of my good cells tonight to 3 different 34461A volt meters. (I have never done that before.) Usually I just take a quick measurement once a month and enter them in to the spread sheet.

[HighVoltage]

If I want to have absolute values correct in comparison, I usually use one meter only.

But in this case I just wanted to see the different change rates over time and temperature and then 3 meters should be ok. The great thing about 3 x 34461A meters: I have 3 graphs captured from the screens without any PC or software involved.

[fcb]

how about locating the weston cell in a gallium triple point cell (primary reference for 29.7646°C)
and then turning this assembly into a voltage reference standard - something like a composite
temperature / voltage standard. note that the temperature 29.7646°C is within the working range
of a weston cell.

lets call this standard a DPS - a dual primary standard for temperature AND voltage. and let us locate
this cell into a measurement equipment like hp 3458a (next generation)

RoHS??

[fcb]

RoHS??

let us forget RoHS for a moment.

regards.

Actually, for all the hassle it caused at the time, I do believe it makes sense. The less lead/cadmium/mercury/etc.. in the environment the better - how can anyone argue against that?

Weston cells contain cadmium AND mercury.

[HighVoltage]

Ok, I did another test over night and here are the pictures of the measurements

3 x Keysight 34461A
Temperature drift is around 1 degree C during this time
All hooked up with the same type of twisted pair low EMF cables

Weston-Cell_1_10h
SPAN: 52uV

Weston-Cell_2_10h
SPAN: 49uV

Weston-Cell_3_10h
SPAN: 223uV

Based on this test, that I had never done before like this, I think the Weston-Cell_3_10h with the larger pk/pk must have a problem.
But this problem has not shown up in my monthly recordings.

In general, these are three good cells and still show a little different behavior and therefore should never be hooked up in parallel.
 
I will keep this test running for another 24 h

[Alex Nikitin]

Weston-Cell_1_10h
SPAN: 52mV

Weston-Cell_2_10h
SPAN: 49mV

Weston-Cell_3_10h
SPAN: 223mV

Microvolts, not mV!

Cheers

Alex

[HighVoltage]

Ooops, what a typo, corrected above.
Thanks, Alex.

[VintageNut]

hello friends

according to the certificates which came with these cells, the cells were manufactured
in february 2004 and when measured at 20 degC were 1.01862 volts.

according to the table provided for temperature correction with these cells, the measured
value (hp 3468a - calibration unkown) was 1.01828 at 26 deg, corrected for 20 deg = 1.01855 volts.
(i need to get a calibrated meter)

this translates into roughly -5 ppm / year drift average over 13 years (this includes a *VERY* rough ride by these cells).

do these numbers make any sense?

comments required from people experienced with weston cells.

best regards.

The cells should not degrade in voltage if the cells were treated properly.

The difference between the original measured voltage and the later measured voltage is more likely adifference between the temperature of the initial measurement and the temperature of the later measurement.

How was the later measurement corrected for temperature? Do you know the temperature coefficient(s)? The temperature dependency may not be linear.

20 degrees Celsius sounds kind of cold for a calibrated measurement.

[HighVoltage]

I only got my first own Weston Cell in 2016, so I have only data for about 1 year.
And during this time I have not seen a drop in voltage.
The changes I have measured are only due to temperature variations.

Out of the Conrad Hoffman document from above, I copied this paragraph:

The NBS recorded data on approximately 600 un-saturated cells. About 5% showed an increase of e.m.f. with time, the average change being 28 µV/year. The remaining 95% decrease in e.m.f., at an average of 85 µV/year. Of that group, nearly half changed by more than 50 µV/year, and one fourth by more than 100 µV/year. That data suggests that certification at yearly intervals is required to insure .01% accuracy. The expected life of un-saturated cells is 7-14 years.

My oldest cell is from 1971 (46 years old) and still performing well with a voltage above 1.018V

 

 

[tggzzz]

My oldest cell is from 1971 (46 years old) and still performing well with a voltage above 1.018V

My two oldest date from 1949 (68 years old), and are currently (ho ho) 1.018711V and 1.017747V - and 40uV/degreeC

[HighVoltage]

Ok, here is the same test setup again after 48 h

Weston-Cell_1_48h
SPAN: 117uV

Weston-Cell_2_10h
SPAN: 113uV

Weston-Cell_3_10h
SPAN: 444uV

It really seems my Cell 3 has a problem.

[tggzzz]

My oldest cell is from 1971 (46 years old) and still performing well with a voltage above 1.018V

My two oldest date from 1949 (68 years old), and are currently (ho ho) 1.018711V and 1.017747V - and 40uV/degreeC

could you let me know the temperature when you made the measurement on the 68 year old cell,
and what was the cell marked / certificate voltage / temperature of measurement on the cells when it was marked 68 yrs ago?

regards.

Room temp was around 16C, and it was measured with an uncalibrated Solartron 7081, best guess reading ~20-30ppm high.
https://entertaininghacks.wordpress.com/2016/12/28/a-weston-standard-cell-an-introduction-to-voltnuttery/#more-852

[HighVoltage]

... and now i understand the importance of 1.018v output on reference standards.

regards.

Yes, that was a long transition from Weston Cells to DC standards.
This is also one of the reason why Null-Meters have been so important for so many years.

Even my old Fluke 731B has a 1.018 and a 1.019 output plus a delta knob to bring it
to zero with a Weston Cell on a Null meter.

I think the next generation Fluke 732C will be the first one without the 1.018V output.

[tggzzz]

i wonder why these wonderful beasts were retired? the drift rates are better than 732a/b !
and predictable (most of the time; that is, -ve drift).

The problem is saturated Weston cells are far more fragile (physically and electrically) and unpredictable than modern equivalents. My other two are in spec, but nowhere near as good.

[VintageNut]

This week I am taking some measurements on a Fluke 731B while the temperature in my house will vary from 65 degrees F to 80 degrees F. It is fall and there will be some cold mornings and warm afternoons. It is a good time to find out the temperature sensitivity of the 731B.

From the cold side of the week so far, the number is about 0.74 ppm / degree C.

If your Weston cell varies 40uV / deg C, then it is 50X more temperature sensitive than a Fluke 731B.

In addition, your 1.018xxx cell requires a KVD to use for just about any voltage calibration purpose.

And, you cannot draw current from the Weston cell. The 731B can be used with calibrated resistors to create a well known current for calibrating a picoammeter or an electrometer.

My Eppley cell was manufactured May of 1975.

There is a sticker that states that the cell was 1.01918 on May of 1985.

It has degraded about 1.2 uV/ month since 1985.

So, if you want to do sub-ppm work, you have to calibrate the cell often.

It is fun to play with this Weston cell, but not useful for calibration work.

[Conrad Hoffman]

When I wrote my page on the things, I tried to be very specific as to what applied to saturated vs. un-saturated cells. I seemed to have a lot more info on the un-saturated type. The reason is that properly constructed saturated cells don't seem to have any inherent degradation that limits their life. It's the users that damage them. I'm pretty sure they can go more than 100 years if the materials were pure, the oven temperature is low and people stay the heck away from the terminals.

Un-saturated cells, OTOH, have a finite life. Certainly all Eppley un-saturated cells are useless by now. I've no idea who might have produced them more recently, but odds are that all unsaturated cells should be retired. They may well produce almost 1.018 volts, but the value won't be stable over time, even if the temperature is. That's why a minimum voltage is required for certification. IMO, some of the cases were really pretty, especially the Weston models, and can make a nice case for a solid state reference. Just install a DC jack in the rear and run it off a wall wart. The cases are thermally lagged with aluminum and even copper on the older ones. You could ovenize it if you wanted.

(BTW, my page was decent when I wrote it, but the discussion here and pages people have done since, really add to the total information.)

[VintageNut]

This week I am taking some measurements on a Fluke 731B while the temperature in my house will vary from 65 degrees F to 80 degrees F. It is fall and there will be some cold mornings and warm afternoons. It is a good time to find out the temperature sensitivity of the 731B.

From the cold side of the week so far, the number is about 0.74 ppm / degree C.

If your Weston cell varies 40uV / deg C, then it is 50X more temperature sensitive than a Fluke 731B.

In addition, your 1.018xxx cell requires a KVD to use for just about any voltage calibration purpose.

And, you cannot draw current from the Weston cell. The 731B can be used with calibrated resistors to create a well known current for calibrating a picoammeter or an electrometer.

My Eppley cell was manufactured May of 1975.

There is a sticker that states that the cell was 1.01918 on May of 1985.

It has degraded about 1.2 uV/ month since 1985.

So, if you want to do sub-ppm work, you have to calibrate the cell often.

It is fun to play with this Weston cell, but not useful for calibration work.

i agree with most of what you said, but consider these:-

1. long term *stability* (not tempco) of around -1 ppm/year, stabilities in 100's of ppb/year (!) are reported.
(Instrumentation Reference Book by Walt Boyes)

2. low noise to the extent that they *can* be calibrated down to 40 ppb (!) against a JJA.
(edwin g. pettis, misterdiodes)

3. very well defined tempco curve.
refer to an equation which came with the cells in one of my earlier posts.

4. yes the tempco is *horrible*, -40uv / degC near 20 degC (but reaches zero at around 3.1 degC)
so a cooled weston cell at around 3.1 degC will have zero tempco and could make a *very* decent standard.

5. un-saturated variants (your Eppley cell) have a *lot* lower tempco, but they have a *horrible* drift of around -85uV / year.

6. last but not the least. look at the *simplicity*

other than above, i agree with most of what you say, it is *definitely* fun to look at a wonderful piece of work
from our past and wonder what have *we* been doing. if we throw the technology available today at this cell
(or some variant) i am almost sure that the next generation of measurement technology can benefit.

regards.

It may be picky semantics, but your *horrible drift * is just predictable degradation in my opinion. Being able to measure this degradation gives me confidence in my 731B and my KVD and my ability to measure small ppm drift over time.

My Eppley cell is no where near 85uV/yr degradation. If the cell is useless below 1.018 V, then my cell may still be useful 30 years from now. It was manufactured 42 years ago. Not bad!

My point is that very old solid state standards are more useful than a Weston cell and less costly. What you will spend on a temperature regulated oven is less than the cost of an ancient but more useful zener reference.

In the end, enjoy what you like.

cheers

[Edwin G. Pettis]

While the NIST will calibrate a Weston Cell to .04 PPM uncertainty, the cost is prohibitively expensive and the restrictions are just as bad, you can't ship a cell in for this calibration, it must be hand carried, if you want to know all of the requirements just check with the NIST.  Don't forget this cell is full of chemicals considered hazardous so the shipping restrictions are going to be very expensive.  Maintaining one of these cells to anywhere near that .04 PPM uncertainty is going to be expensive and as Conrad mentioned there are a lot of difficulties in doing that.  It wasn't directly mentioned anywhere that I could see on the NIST website but I rather doubt that .04 PPM uncertainty lasts for very long.  You can probably e-mail the NIST and ask them about the requirements and restrictions.

Compared to today's solid-state Vref standards, the cells are very obsolete and fragile with many more restrictions for use.  As I mentioned before, probably the only advantage those old cells had over today's Vref is a fairly low noise level but that is hardly a reason to use them now.  Batteries can supply a lower noise level if it is really needed and they can be measured to a very high accuracy for short term use.

Overall, a LM399 or LTZ Vref will be cheaper in the long run and much easier to maintain, the Weston cell is an interesting piece of technology but it has seen its day, you might be able to maintain your set of cells as a secondary standard but forget using them as a primary, they just don't make the grade now.

By the way, you can't make any definitive statements about measurements made with uncalibrated equipment of unknown accuracy and drift, those are nothing more than approximate at best.

[MisterDiodes]

2. low noise to the extent that they *can* be calibrated down to 40 ppb (!) against a JJA.
(edwin g. pettis, misterdiodes)

4. yes the tempco is *horrible*, -40uv / degC near 20 degC (but reaches zero at around 3.1 degC)
so a cooled weston cell at around 3.1 degC will have zero tempco and could make a *very* decent standard.

5. un-saturated variants (your Eppley cell) have a *lot* lower tempco, but they have a *horrible* drift of around -85uV / year.

6. last but not the least. look at the *simplicity*

Yes it is fun to learn from the older technology, but the real take away you should be focusing on:  Realize how much more practical, useful and safe a Zener Diode Vref is in comparison.  There is absolutely no reason to go to the hassle and risk of a wet cell - especially when you have LM399's / LTZ's available...  In that sense we do not throw out old technology.  You use the knowledge of old technology like this: You be VERY thankful every day you don't have to go back to the "Bad Old Days of Wet Cells" and that's why you treasure a known, stabilized Zener Vref that is so much better than any wet cell.  Especially when you're the one paying the shipping and cal lab invoices.

"Simplicity"?  The cell is simple in construction (depending on where it can be assembled legally), but far from it for actually using it, let alone transport legally - at least in more developed countries with strict hazmat rules.  Really you'll want a KVD and Null Meter to go with those cells - if you want to maximize lifetime and lowest current drawn at each measure.

Don't take that .04ppm possible measure uncertainty (at NIST) of a standard cell too far out of context - that comes at an -extremely- high cost and hazmat risk, at least in developed countries.     Even if you can find a lab willing to take one in for measure these days.  What is possible is not the same thing as what is practical.

If you're not calibrating them often (at high expense) then they aren't going to be much use as being a traceable absolute value in your lab - just another unknown voltage source with a (hopefully) predictable discharge rate.

Looking at it a profitable business expense / risk:  There is really not much use for any wet standard cell these days...More for entertainment value if anything.  Of course it's fun to see something decades old still function at some level - but a Zener Diode is easier, safer, more forgiving and much more profitable to own from a business standpoint.

Have fun!

[Conrad Hoffman]

My understanding of the bad old days was that you hand carried your saturated standard cells to NBS. My Julie oven had fastenings and a shoulder strap- remember, you can't invert the cells, so you can't ship them at all. The recovery time from inversion is near to forever, and it can permanently damage some types of cells. the Julie oven had a strict warning on top of the box against inversion. You'd get a bus ticket for you and a seat for the cell oven. You'd also have to carry two small lead-acid cells to run the 24 volt oven. You'd make the trip, taking several days if necessary, then drop off the cells and let NBS do their thing. When done, you'd repeat the procedure to get them home. Some weeks would have to be allowed before trusting the cells after any travel, nor would NBS do anything until they settled down. I think people must have had too much time on their hands back then. 

[HighVoltage]

I still had the instruments hooked up to one of my Weston Cells.
Here is a good graph to show the huge dependence between temperature and output voltage.

- Delta Temp:      5.7 °C
- Delta Voltage:  246 uV
- Which comes down to about 43 uV/°C