[TR] Tr3, conversion from capillary tube to electrically sensedwater temperature gauge

[Randall]

> I though the TR3 gauge was
> domed but the TR4 gauge is flat (but I may be mistaken and am glad to be
> corrected)

There was a change point fairly early in TR4 production, where the glass
went to flat and a shroud was added over the needle pivot.  Moss gives the
change point as CT11308.

And yes, the TR4 sender fits the TR3 housing, even though the threads and
sealing method are not the same.  I have not tried a TR3 bulb in a TR4
housing, but it looks like it should work.

The "capillary" tube is mostly filled with ether, a very volatile substance.
The tiniest crack will let it leak out, ruining the gauge.  I have also had
several movements fail from overheating.  One of the posters on another
forum has suggested that the movement comes apart inside when needle hits
the top of the scale, but I have not investigated that.

As a side note, I installed a cheap aftermarket gauge in the Ferguson TEA20
tractor I bought last summer.  While prepping it for a show, I noticed a
spot near the new temp gauge and tried to wipe it away.  Only to discover
that the ether was dripping out of the gauge!  The tube had apparently
cracked just moments before, probably from the tube being wiggled a little
during cleaning.

-- Randall