HP419 leaking battery?

L

Leif Holmgren

Guest
Hello Group!

I have a HP419 null voltmeter that I use occasionally. Now it's however
been stowed away for a few years due to a major rebuild of my workshop.
I recently found that it was leaking something. Taking it apart I found
something that looks like a small battery that was leaking some fluid.

Measuring the voltage over it shows nothing, but you can read on one end
a + sign, the figure 3 and "U.S.PAT 2712565 AT.AL." The thing is 25mm
diameter and about 18mm long. And no, it's not one of the NiCd cells.

Is this some kind of reference cell? Is it a "standard" 3V cell, and can
it then be replaced with just about any Lithium or Silver-oxide cell?

/Leif
 
Robert Baer wrote:
Interesting, buut it reads "U.S.PAT 2712565 ET.AL." _not_ AT.AL.
Click to expand...
Sure, but late at night the keys on my keyboard start moving around ;-)
I actually looked up the patent number and found some very nice scanned
pages from 1953. It was too much to read but I guess it was all about
the mechanics and very little about the chemistry, even if I saw the
words mercury and potassium on my quick look.

/Leif
 
Gary Schafer wrote:
If you would be interested in getting rid of the 419 I am looking for
one.
Click to expand...
Sorry Gary, not for sale, not even to a ham like myself.

73
SM4RPQ/Leif
 
It is probably a mercury battery. No longer available.

If you would be interested in getting rid of the 419 I am looking for
one.

73
Gary K4FMX

On Sat, 27 Nov 2004 11:11:44 +0100, Leif Holmgren <nobody@nowhere.com>
wrote:

Hello Group!

I have a HP419 null voltmeter that I use occasionally. Now it's however
been stowed away for a few years due to a major rebuild of my workshop.
I recently found that it was leaking something. Taking it apart I found
something that looks like a small battery that was leaking some fluid.

Measuring the voltage over it shows nothing, but you can read on one end
a + sign, the figure 3 and "U.S.PAT 2712565 AT.AL." The thing is 25mm
diameter and about 18mm long. And no, it's not one of the NiCd cells.

Is this some kind of reference cell? Is it a "standard" 3V cell, and can
it then be replaced with just about any Lithium or Silver-oxide cell?

/Leif
Click to expand...
 
On Sat, 27 Nov 2004 11:11:44 +0100 Leif Holmgren <nobody@nowhere.com>
wrote:

I have a HP419 null voltmeter that I use occasionally. Now it's however
been stowed away for a few years due to a major rebuild of my workshop.
I recently found that it was leaking something. Taking it apart I found
something that looks like a small battery that was leaking some fluid.

Measuring the voltage over it shows nothing, but you can read on one end
a + sign, the figure 3 and "U.S.PAT 2712565 AT.AL." The thing is 25mm
diameter and about 18mm long. And no, it's not one of the NiCd cells.
Click to expand...
It's a Mercury battery that is used when you use the Null function to
buck the input voltage to give you an infinite input impedance. That
mercury cell is NLA, so you need to replace it with something else. I
removed mine, and its holder, and replaced it with a plastic Radio
Shack single AA holder. I put a rechargable alkaline battery in mine,
because the don't self discharge, but you could use anything that puts
out 1.2-1.5V. Note that it's probably important to use a plastic
holder to maintain the necessary isolation from the chassis.

I just attached my holder with a good quality double stick foam tape.

I don't believe I had to change any resistor values to make this work.

The 419 is an interesting little meter.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
Jim Adney wrote:
On Sat, 27 Nov 2004 11:11:44 +0100 Leif Holmgren <nobody@nowhere.com
wrote:

I have a HP419 null voltmeter that I use occasionally. Now it's however
been stowed away for a few years due to a major rebuild of my workshop.
I recently found that it was leaking something. Taking it apart I found
something that looks like a small battery that was leaking some fluid.

Measuring the voltage over it shows nothing, but you can read on one end
a + sign, the figure 3 and "U.S.PAT 2712565 AT.AL." The thing is 25mm
diameter and about 18mm long. And no, it's not one of the NiCd cells.

It's a Mercury battery that is used when you use the Null function to
buck the input voltage to give you an infinite input impedance. That
mercury cell is NLA, so you need to replace it with something else. I
removed mine, and its holder, and replaced it with a plastic Radio
Shack single AA holder. I put a rechargable alkaline battery in mine,
because the don't self discharge, but you could use anything that puts
out 1.2-1.5V. Note that it's probably important to use a plastic
holder to maintain the necessary isolation from the chassis.

I just attached my holder with a good quality double stick foam tape.

I don't believe I had to change any resistor values to make this work.

The 419 is an interesting little meter.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
Click to expand...
Interesting, buut it reads "U.S.PAT 2712565 ET.AL." _not_ AT.AL.
 
Jim Adney wrote:
....
Shack single AA holder. I put a rechargable alkaline battery in mine,
because the don't self discharge, but you could use anything that puts
out 1.2-1.5V.
Click to expand...
Rechargeable alkaline??? Never heard about these before. Will start
looking around.

Are you sure you used one? There is a "big" figure 3 on it, but I guess
that could be some size identification since the voltage for a single
mercury cell should be 1.35V which makes 3 volts impossible.

How about the voltage drop over time? Have been looking around for data
on batteries and they all specify a rather large drop over time compared
to what could be expected from a mercury cell. Is it connected so that
the exact voltage is irrelevant as long as it does not drop too much
when reading the nulling voltage instead of 0.

Regarding isolation, is that a requirement due to the rather high
impedances involved, or is one or both of the poles separated from the
ground of the rest of the meter?

The 419 is an interesting little meter.
Yes, and apart from an AVO-6(?) with a stuck needle the only analog
Click to expand...
instrument around. Sometimes analog is really superior the digital stuff.

/Leif (SM4RPQ)
 
On Tue, 30 Nov 2004 22:31:43 +0100 Leif Holmgren <nobody@nowhere.com>
wrote:

Jim Adney wrote:

Shack single AA holder. I put a rechargable alkaline battery in mine,
because the don't self discharge, but you could use anything that puts
out 1.2-1.5V.

Rechargeable alkaline??? Never heard about these before. Will start
looking around.
Click to expand...
Ray-O-Vac sold them for a few years, but they are now discontinued.
You just need something that has a good shelf life, so you're not
opening the meter to replace it all the time. It's not recharged by
the meter like the other cells, either, so you have to take it out and
replace/recharge it.

Are you sure you used one? There is a "big" figure 3 on it, but I guess
that could be some size identification since the voltage for a single
mercury cell should be 1.35V which makes 3 volts impossible.
Click to expand...
Okay, I checked my manual, and the original cell is shown as 1.35V.

How about the voltage drop over time? Have been looking around for data
on batteries and they all specify a rather large drop over time compared
to what could be expected from a mercury cell. Is it connected so that
the exact voltage is irrelevant as long as it does not drop too much
when reading the nulling voltage instead of 0.
Click to expand...
Droop with time is irrelevant, because you trim the bucking voltage to
get just enough to buck the input signal to zero. Then you measure
your adjusted "bucking voltage." The only droop that might matter
would be that amount that occurs in the 10 seconds between bucking and
measuring. That should be insignificant.

Regarding isolation, is that a requirement due to the rather high
impedances involved, or is one or both of the poles separated from the
ground of the rest of the meter?
Click to expand...
All of the above, I believe. The bucking cell floats in the middle of
the circuit, and the meter is so sensitive that you will find that
tiny amounts of leakage will show up in the oddest places. I had to
carefully take apart the input binding posts and clean off the plastic
pieces to get the leakage down. I cleaned the range switch wafers with
alcohol and still couldn't get it to calibrate in the lower ranges.

I actually gave up on this meter, thinking that it had some fatal flaw
that I hadn't found. When I went back to it a year later, it was FINE!
I suspect that I had just touched something and left a fingerprint
that finally evaporated enough that it was no longer important.

Sometimes analog is really superior the digital stuff.
Click to expand...
I couldn't agree more. ;-)

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
Leif Holmgren wrote:
Jim Adney wrote:
...

Shack single AA holder. I put a rechargable alkaline battery in mine,
because the don't self discharge, but you could use anything that puts
out 1.2-1.5V.

Rechargeable alkaline??? Never heard about these before. Will start
looking around.

Are you sure you used one? There is a "big" figure 3 on it, but I guess
that could be some size identification since the voltage for a single
mercury cell should be 1.35V which makes 3 volts impossible.

How about the voltage drop over time? Have been looking around for data
on batteries and they all specify a rather large drop over time compared
to what could be expected from a mercury cell. Is it connected so that
the exact voltage is irrelevant as long as it does not drop too much
when reading the nulling voltage instead of 0.

Regarding isolation, is that a requirement due to the rather high
impedances involved, or is one or both of the poles separated from the
ground of the rest of the meter?

The 419 is an interesting little meter.
Yes, and apart from an AVO-6(?) with a stuck needle the only analog
instrument around. Sometimes analog is really superior the digital stuff.

/Leif (SM4RPQ)
Click to expand...
I would suggest using a silver cell; the voltage is higher and they
hold up quite well.
One of the good things about a mercury cell, was that its terminal
voltage was purported to be a fixed, known value to 4 digits (if
unloaded from day one), and that it could be used as a transfer standard
for 6 digits (if i remember correctly).
Now the environmentalist guys have subverted the constitution and
outlawes mercury cell standards (partial dictatorship).
Maybe one of the lithium-based cells would be useful; someone
suggested to me the LiI cell.
Does anyone have any real-world data to support (ore negate) that
possibility?
 
Jim Adney wrote:
On Tue, 30 Nov 2004 22:31:43 +0100 Leif Holmgren <nobody@nowhere.com
wrote:

Jim Adney wrote:

Shack single AA holder. I put a rechargable alkaline battery in mine,
because the don't self discharge, but you could use anything that puts
out 1.2-1.5V.

Rechargeable alkaline??? Never heard about these before. Will start
looking around.

Ray-O-Vac sold them for a few years, but they are now discontinued.
You just need something that has a good shelf life, so you're not
opening the meter to replace it all the time. It's not recharged by
the meter like the other cells, either, so you have to take it out and
replace/recharge it.

Are you sure you used one? There is a "big" figure 3 on it, but I guess
that could be some size identification since the voltage for a single
mercury cell should be 1.35V which makes 3 volts impossible.

Okay, I checked my manual, and the original cell is shown as 1.35V.

How about the voltage drop over time? Have been looking around for data
on batteries and they all specify a rather large drop over time compared
to what could be expected from a mercury cell. Is it connected so that
the exact voltage is irrelevant as long as it does not drop too much
when reading the nulling voltage instead of 0.

Droop with time is irrelevant, because you trim the bucking voltage to
get just enough to buck the input signal to zero. Then you measure
your adjusted "bucking voltage." The only droop that might matter
would be that amount that occurs in the 10 seconds between bucking and
measuring. That should be insignificant.

Regarding isolation, is that a requirement due to the rather high
impedances involved, or is one or both of the poles separated from the
ground of the rest of the meter?

All of the above, I believe. The bucking cell floats in the middle of
the circuit, and the meter is so sensitive that you will find that
tiny amounts of leakage will show up in the oddest places. I had to
carefully take apart the input binding posts and clean off the plastic
pieces to get the leakage down. I cleaned the range switch wafers with
alcohol and still couldn't get it to calibrate in the lower ranges.

I actually gave up on this meter, thinking that it had some fatal flaw
that I hadn't found. When I went back to it a year later, it was FINE!
I suspect that I had just touched something and left a fingerprint
that finally evaporated enough that it was no longer important.

Sometimes analog is really superior the digital stuff.

I couldn't agree more. ;-)

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
Click to expand...
Concerning cleaning with alcohol, that is fine, BUT.
One has to dry out the surfaces; if phenolic then some of the bulk as
well.
To be safe, a whole day for drying seems to suffice.
However, be advised that if the cleaning was not good enough (still
leakage - but lower than before), then do it again.

If you dunk parts for cleaning, the container itself must be clean -
use those "throw-away" single serving cups (or similar) fresh out of the
sales container; rinse once with a small amount of alcohol, then fill
for dunking.
Let part soak for a few hours, dump the alcohol and replace for a
10-15 minute soak, and then dump the alcohol and replace for a short
(self) stirred rinse.
Shake off the alcohol, do *NOT* attempt to dry with any cloth of any
sort (feelthy!); air dry (may warm up to help - but *not* hair drier).

If you cannot dunk, then use a cleaned pump spray bottle filled with
alcohol for dousing the part(s); douse & partly dry, repeat 3-4 times
then final dry.
If the parts are really doity, use a new brush (paint brush or
toothbrush as appropiate) for scrubbing nasty areas with alcohol before
the above cleaning regimen.
Keep the brushes clean the same way...
 
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