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How The Gauges Work


Captain Obvious

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Many moons ago I promised to describe how the gauges work...

So in order to understand how the gauge system works, I think it would be a little easier to first understand how they DON'T work. I think everyone already has a good handle on this simple system, but it's an important place to start.

Here's a pic of a simple (uncompensated) gauge design. Wrap a heating coil around a bimetallic strip and mechanically attach that strip to the linkage of a gauge needle. Flow current through that heating coil and to a sender unit who's resistance reflects the level of whatever you're trying to measure (temp, oil, fuel, whatever).

As the resistance of the sender unit changes, the current through the heating coil changes and that changes how hot the bi-metallic strip gets.

The lower the sender resistance, the higher the current.
The higher the current, the hotter the bimetallic strip gets
The hotter the strip, the more the needle moves.

Here's a pic:
gaugetheory1.jpg

This simple uncompensated gauge system does "work", but it is subject to a few real-world outside influences that can affect the accuracy:

First, since the whole thing works on the temperature of the bimetallic strip, the gauge will read differently on a hot day than on a cold day.

And second, since the gauges are powered by the battery system of the car, changes in that system voltage will affect the gauge readings. The gauges would read differently sitting with the engine off than they would with the engine spinning at 3000 RPM when the alternator has kicked up the voltage a bunch. The system voltage can vary from about 12V to over 14V and the gauge readings would change as the voltage varied.

So the basic gauge system above sorta works, but these two real-wold effects are undesirable. If the above basic gauge system makes sense, I'll get into how they compensate for those two real-world effects.

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So the basic gauge system described above works, but there are inaccuracies caused by ambient temperature (cold day vs. hot day), and variations caused by system voltage (low alternator output voltage vs. higher alternator output voltage. In order to compensate for those real world conditions, the gauges are a little more complicated than the simple system described above.

There are two "stages" to the compensated gauge. One of those stages is he simple gauge we already talked about above. The other stage is the compensating or regulating stage. It's job is to compensate for changes in ambient temperature and varying system voltage.

The compensation stage consists of another heating coil wrapped around another bimetallic strip. This bimetallic strip forms an electrical switch such that when the strip heats up, it breaks contact. And when it cools down, it re-establishes contact. The trick is, that it makes and breaks contact to it's own power source and the power source for the simple gauge stage.

The result is that this second strip/heating coil combo will make and break the power source such that it will always achieve the same average temperature. Of course the temperature is rising and falling some, making and breaking connection, but the AVERAGE is always the same.

Colder day? Power to the compensation strip will have to be on longer to reach the temp that bends the strip to the point where it breaks the switch connection. Hot day? Just the opposite. Power will be on for a shorter time.

Low system voltage? Again, power will have to be connected longer in order to heat the strip to the desired temperature,. And conversely, if the voltage is higher, it'll take less time.

The result is that the compensation stage creates an ON/OFF/ON/OFF pulse train whose duty cycle and frequency will change depending on the ambient temperature and system voltage. This effectively creates a voltage source that will always supply a constant amount of POWER to the gauge system under all conditions.

Clear as mud? Connect that constant power source to the simple gauge from above, and you have this:
gaugetheory2.jpg

If you put your key in and turn it to "ON" you should see the gauges start to rise. Don't start the car. Just pick a needle and watch carefully. You'll see the needle start to rise, but probably before it reaches it's final position, it'll pause... Then start rising again. Then pause again. This will continue until it finally reaches it's final position. And even then, if you watch carefully, you'll see the needle actually wiggles a tiny bit.

This effect can most easily be seen with a needle that's moving well above minimum like a full tank of gas. That pausing and wiggling is the compensation stage opening and closing.

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Here's a pic of one of the gauges. You can see the two different heating coils wrapped around two different "U" shaped bimetallic strips. Interesting to note that internets research indicates that "U" shape is part of the compensation as well. The concept is that the unheated side will compensate some for changes in ambient temperature. "They say" it doesn't get rid of all of the temperature based effects, but it helps some. Between that "U" shape and the compensation stage, the gauges seem to be really stable.

So this is an example of the compensated gauge with the two stages:
gaugetheory3.jpg

And here's a closer-upper pic of the compensation stage showing it's parts:
gaugetheory4.jpg

The switch opens and closes to keep the temperature of the compensation strip at a constant temperature (ave). If you put a Voltmeter on the sender unit, you'll see that it isn't a steady voltage, but is instead a square wave.

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Pretty cool how they used to combine simple basic mechanical and physics concepts together to accomplish a goal.  Today it's all converting lines on paper to lines on silicon.

I came across this guys Youtube channel a while ago.  Seems to be a traveling electronics engineer with free time.  Here's one of his videos, he takes things apart and explains them.  I am not claiming to understand what he's saying, but things can be learned.

 

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