Mercury Battery [233]

Advert

New Replies

Report Bug

Help/FAQs

Mercury Battery [233]

This topic has 9 replies, 3 voices, and was last updated 7 September 2012 at 21:55 by Michael Gilligan.

Viewing 10 posts - 1 through 10 (of 10 total)

Author

Posts
7 September 2012 at 16:55 #22206
Michael Gilligan
Participant
@michaelgilligan61133
Looking for a Substitute …
Advert

7 September 2012 at 16:55 #97911
Michael Gilligan
Participant
@michaelgilligan61133
I purchased a scondhand TESA GT-21 linear measurement probe [see Album for technical details] together with an Analogue display unit [for which I can find no details].

The "meter" was designed to take a 4V Mercury Battery

… type Mallory TR233R or Eveready E233

… Note: strictly this should be 4.2V

These Batteries are [for good Health & Safety reasons] no longer available, so I am looking for a substitute. I wrote to TESA UK, but have received no reply.

The dimensions of the 233 battery are a little larger than a cluster of three AAA cells, so it should be simple enough to make a physical replacement.

I have tried three Alkaline cells, but these produce 4.5V and, although everything works, the front-panel control will not Zero the meter. … There is also a "Cal" pot on the side, but I haven't touched that yet.

I suspect that the Voltage is critical; and this would be why they used a Mercury Battery in the first place.

So … Can anyone suggest a small Voltage Regulator that would produce a stable 4.2V from three Alkaline cells?

Thanks

MichaelG.

P.S. If anyone has the original instruction leaflet, I would be grateful for sight of it.
7 September 2012 at 19:35 #97917
Peter G. Shaw
Participant
@peterg-shaw75338
Depending on the current drawn, probably the easiest may be a 4.3V zener with a suitable series resistor but this would fail as soon as the cells cumulative voltage dropped too far as they discharge. There is also the permanent current drain which would ensure a quicker discharge of the cells.

Another idea would be to use one or two Schottky diodes in series (or one or two germanium diodes if they can be found) to drop the voltage by a small amount. This would mean that as the cells discharge, the output voltage would always be that bit less than the cumulative cell terminal voltage.

To be honest, it's so close that I would suspect either a circuit malfunction or a recalibration is required. If you want to attempt a recalibration, make the position of the adjustment pot so that it can be returned back to that same position if it doesn't work.

One of the problems with devising a voltage regulator is the low voltage available, ie 4.5v max. Now if you were to completely change to power pack to say a 9V PP3 type battery, then a voltage regulator becomes much easier to design and can be set to whatever output voltage you desire. But this depends on the space available.

Regards

Peter G. Shaw
7 September 2012 at 20:01 #97923
Michael Gilligan
Participant
@michaelgilligan61133
Thanks for your thoughts, Peter

I was heading towards "start with a higher input voltage, and regulate that" … but was hoping that there might be a suitable LDO (Low Drop Out) regulator available.

I didn't actually mention, but the "meter" is, I believe, an AC bridge circuit.

MichaelG.

Useful Discharge Characteristics [taken from the Wikipedia article]

Mercury batteries using mercury(II) oxide cathode have a very flat discharge curve, holding constant 1.35 V (open circuit) voltage until about the last 5% of their lifetime, when their voltage drops rapidly. The voltage remains within 1% for several years at light load, and over a wide temperature range, making mercury batteries useful as a reference voltage in electronic instruments and in photographiclight meters.

Edited By Michael Gilligan on 07/09/2012 20:07:09
7 September 2012 at 20:08 #97926
Billy Mills
Participant
@billymills
Michael you have a PM .

Billy.
7 September 2012 at 20:20 #97929
Michael Gilligan
Participant
@michaelgilligan61133
Billy,

I just read your Messages … thank you.

I will check later tonight, to see if it matters whether the probe is bottomed-out.

MichaelG.
7 September 2012 at 20:42 #97930
Michael Gilligan
Participant
@michaelgilligan61133
Here is some concise information about the GT 21 probe, at Long Island Indicator.

… Certainly looks worth persevering with.

MichaelG.
7 September 2012 at 21:25 #97933
Michael Gilligan
Participant
@michaelgilligan61133
For anyone else with Mercury Battery difficulties:

This paper is worth reading.

MichaelG.
7 September 2012 at 21:32 #97934
Billy Mills
Participant
@billymills
Yup, it's a linear variable inductor sensor, cut down version of a linear variable differential transformer. It has a uniformly wound coil tapped in the middle. A core -about half the lenth of the coil- slides inside the coil. When the core is in the middle of the coil both halves have the same inductance so when you form the bridge with two equal resistor arms the bridge balences and there is no output from the tap and the middle of the two resistors. When the core moves it increases the inductance in one half coil and decreases the other so there is an output from the bridge, the magnitude and sign give the displacement of the core. It is a half bridge transducer because the two resistor arms are in the box- not in the probe.The bridge is driven with typically 1-10V at 5-50KHz.

The arrangement is very simple and very widely used in go/no go rigs. The sensitivity can be very high because you can use a lot of amplification and phase sensitive detection, the limits are thermal, seismic noise, air movement and other exotics. Spliting a micron is simple. The three cell supply will probably work fine, the sensor does not make precision displacement measurements, it is a comparitor, plus or minus from the centre position so you would never want to null the endstop position. LVI's and LVDT's are very easy to make, even home made ones work very well and can be made with good linearity by fiddling the pitch of the windings.

Billy.

Edited By Billy Mills on 07/09/2012 21:34:55
7 September 2012 at 21:55 #97936
Michael Gilligan
Participant
@michaelgilligan61133
Billy,

A very helpul summary, thank you.

I begins to sound like the Battery Voltage is probably not so critical, after all.

I was wary because of the known calibration problems with camera light-meters; but in a bridge it shouldn't really matter [should it?].

You are, of course, right about not wanting to null the end-stop position. The only trouble is; it's even less "zero-able" when the probe is displaced. … I think it's time to try adjusting the CAL pot [on the reasonable assumption that someone has probably been there before me].

Thanks again Billy, and Peter

MichaelG.
Author

Posts