Reply to: RE>low gauges
This is about a week old, but I think my Internet access is back....
******************************************
Scott Paisley complains that his TR6 gauge is reading low....
Scott, as it happens, I have spent the last few days working on this very
problem. I have a TR4 electrical gauge that I would like to use on my '2,
simply because it looks original. Perhaps you, and the rest of the net, would
be interested in what I've found so far.....
The "Jaeger" electrical temperature gauge is a very simple instrument. It
has a bimetallic strip with a piece of resistance wire wrapped around it. The
wire is heated by current passing through the gauge on its way to the heat
sender. Now, the resistance of the heat sender is inversely proportional to
the water temperature. At 212 degrees F, the resistance is approximately 34
ohms. At room temperature it is 300-something ( my notes are at home ). So
the higher the temperature goes, the lower the resistance of the sender, the
higher the current in the circuit, the hotter the bimetallic strip in the meter
gets, and the further the meter deflects.
One disadvantage of a system like this, is that it is highly supply-voltage
sensitive: the supply voltage has just as much control over the current
through the meter, as the resistance of the sender! And the battery voltage in
a running automobile can vary from about 12 to 15 volts. So the makers don't
run such gauges right off the battery line. Instead they run it off some lower
voltage, which is regulated by a little electro-mechanical gizmo called a
"voltage stabilizer". This produces pulses such that the average voltage is
constant. If the battery voltage rises, the gizmo makes shorter pulses.
All the above is fairly elementary, but here's where it gets interesting:
these meters are ADJUSTABLE. There are a pair of screwdriver slots visible
from holes in the back. You adjust them by pushing them back & forth. One
slot adjusts the position of the meter in relation to the bimetallic strip: I
would call this the "zero adjust". The other one adjusts the spring tension
that works against needle movement. This one, I would call "sensitivity
adjust", although these two adjustments are no doubt interactive.
This weekend I built an electronic gizmo that produces a steady, regulated
voltage for my gauge. The only trouble is, I still don't know what voltage to
adjust it for ( Pat, where are you? ).
To aid in calibrating my gauge, I am first going to find out the resistance
of my sender at all temperatures of interest. Then I will build a test jig
with resistors and a selector switch, producing the appropriate temperatures
with a turn of a knob. Then, hopefully, I can dork around with the gauge until
it reads right :-).
One last thing: I was quite impressed by the SW gauge that I took out of
the car. It responds fast, smoothly, and gives repeatable readings. If only
it didn't *look* so wrong, I would definitely leave it in. I believe it to be
a superior system to the bimetallic electrical gauges....
- Jerry
|