How To Calibrate Your Tiger Instruments
By Cullen Bennett
B9472658
September 17, 1999
Most all of the instrument gages are built the same way internally. The only difference is what has been stenciled onto the faceplate. Most highly damped gages (those that have slow movement of the needle as opposed to an instantaneous needle movement) use the same type bimetallic strip inside them. They also have a small "heater" wire wrapped around the strip to cause it to bend when heated and thereby move the needle.
The more it is heated, the more the strip will bend. In the case of the Fuel gage, there is a "sender" mounted inside the gas tank. This sender is nothing more than a wire wound resistor that has a shorting slider attached to the float mechanism. When the fuel us high, the shorting slider makes a ground connection close to the top of the resistor thereby producing only a small resistance to be in series with the Fuel gage. This small resistance allows a relatively higher current to flow in the gage which causes the "heater" wire inside the meter to heat up and force the bimetalic strip to bend which moves the needle to read toward the "full" side of the meter.
The whole process works the same way (but in reverse) for an empty tank where a relatively higher resistance (in the sender) causes less current to flow and produces less heating in the "heater" wire inside the gage.
The sender in the Temperature gage however uses a different technique to produce a resistance that changes. In this case the desired effect is to produce a resistance that changes as a function of engine temperature. The internals of this sender contains a semiconductor material that changes resistance with temperature. The absolute value of the resistance and the amount of change are very exact and repeatable from one unit to another.
This material is called a Thermistor. It can be made by have either an increasing or decreasing resistance value as a function of temperature. In the case of the sender in our cars, it is said to have a negative temperature coefficient. This means as the material is heated, the bulk resistance decreases. In the automotive application a lower sender resistance causes more current to flow in the circuit and the internal heater inside the meter gets hotter causing the needle to read higher. In both cases you can see that the amount of accuracy is dependent on just about everything.
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