How To Calibrate Your Tiger Instruments
By Cullen Bennett
September 17, 1999
Two of the instruments on the dash of your Tiger were considered critical enough to have a voltage stabilizer (so it is called) supplying power to them. They are the Fuel gage and the Temperature "indicator". I use the term "indicator" very deliberately in this case. The original voltage stabilizer for these instruments was developed by Lucas Electric, and for that period in time was probably the best that they could produce for a production environment.
Theory of Operation:
The internals of this stabilizer consists of a bimetallic strip (a strip of metal that has been laminated together that has a different type of metal on each side of it.) The intent is to produce a strip of metal that bends all by itself when heat is applied to it. This is due to the difference in thermal expansion for the two metals. A strip of this metal is inside the Lucas stabilizer with a small "heater" wire wrapped around it and a set of electrical contacts on one end.
In theory, the heater wire is connected in series with the bimetallic strip. The contacts are normally closed which allows for dc current to pass through the arrangement and "heat" up the small wire. This, in turn causes the bimetallic strip to bend and open the switch contacts and stops the current flow through the heater wire and allows the strip to start to cool. When the bimetallic strip cools off sufficiently the contacts close again and start the whole process over again.
By carefully adjusting the spacing on the electrical contacts an on/off ratio of the voltage applied to the heater wire can be set to produce an "average" output voltage that is less than the battery voltage applied to the input of this device. In the case of the instrument stabilizer, this "average" voltage is somewhere around 10 Volts. The thought being that if the battery voltage goes higher while charging the small heater wire, it will heat up quicker and open the contacts longer. This would cause the voltage to the output to be less and conversely, the lower the battery voltage. The less the heating and the output voltage is applied, the longer it would take to make the "average" stay about the same. In theory, if everything were working exactly perfectly the "average" voltage would always stay the same. But that never exactly happens due to many reasons.
The major source of error is the bimetallic strip itself. As the ambient temperature under the dash of your Tiger warms up or cools down, it will affect how well the stabilizer operates. The problem with the gages is that if the voltage supplying power to them fluctuates, the needle readings themselves will change with no other changes in the system. To compound the problem, each gage would like to have a different voltage applied to it for the reading to be accurate. The linearity or "tracking" from the bottom of the reading to full scale would require that the "sender" and the gage were exactly matched in their characteristics. This case very seldom occurs.
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