How to Select an Automotive Gauge Sensing Point
The gauge sensing point can have a big impact on the accuracy of an automotive gauge.
Gauge Sensing Point Purpose
The purpose of the gauge sensing point is to provide a window into the process stream for measurement of the desired parameter. For example, for engine oil pressure, the process stream is the engine oil, and the desired parameter to measure is the pressure.
Selecting a Gauge Sensing Point
The sensing point can impact gauge accuracy. As a simple example, let's again consider engine oil pressure. Suppose we have a normal passenger car engine with a wet sump. We place the engine oil pressure sensor on the bottom of the oil pan. Sure, we are measuring engine oil pressure -- but it is likely not the engine oil pressure we want to measure. We would more or less just end up measuring the engine oil static liquid head in the oil pan using the bottom of the oil pan as a sensing point. So we must be specific in our definition of the parameter we want to measure.
Be Specific in the Definition of the Parameter Being Measured
So let's be more specific. Suppose we say we want to measure the engine oil pressure supplying the engine oil galleys. Now we are better equipped to select an appropriate sensing point.
Looking the engine over, we discover the original equipment oil pressure switch on the engine block. Using this sensing point seems logical -- it is easy to access, we don't have to drill and tap to use the sensing point, and we can use a tee to keep both the original oil pressure switch and our new oil pressure sender for our new oil pressure gauge.
On a typical engine, the original equipment pressure sender or switch is going to be located in the process stream after the engine oil filter, somewhere around where the main oil galleys go into the engine block.
So now that we've taken a look at engine oil pressure as an example for selecting a sensing point, let's take a look at a different example involving engine coolant.
Engine Coolant Sensing Location
Now as an example let's say we would like to measure engine coolant temperature. But let's be more specific -- let's define the parameter we would like to measure as "engine coolant temperature at the water pump intake".
Measuring at the water pump intake will tell us the temperature of the engine coolant entering the engine. Now to measure at this point may be tricky; most engines are not going to have a convenient threaded port in this location.
So we take a look at the factory engine coolant temperature sensor location. We find this sensor is on the block near where the coolant discharge to the radiator is. This location would tell us the temperature of the engine coolant exiting the engine. (Typically the engine coolant discharge temperature is the selected parameter to measure rather than the engine coolant intake temperature).
Now we know that modern electronic fuel injection controlled engines typically utilize the value of the engine coolant temperature sensor in the air/fuel calculations, so we decide we don't want to mess with the factory sensor.
Instead, we decide to weld on a bung at the nipple for the engine coolant discharge to radiator hose.
Conclusion
So we've seen that in selecting a sensing point for a gauge for a car, truck, or other piece of equipment, we need to be specific in the parameter we want to measure. But at the same time we must be realistic in what sensing points are practicable to access or install.
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