Re: 48-50 Bimetal Senders Retrofit Project

Posted by DavidPackard On 2024/5/12 0:18:24

I did a second round of K-S instrument testing on my ’48.

The purpose of this round of testing was to investigate whether the instrument panel gauges are essentially the same electrically. Beginning with the coolant temperature gauge a potentiometer was used in lieu of the sending unit. This potentiometer was adjusted to deflect the needle to mid-span. This test was repeated several times resulting in 30.8 ohm, 0.142 amp, and 36.1 ohm, 0.126 amp data sets. The second potentiometer setting was then applied to the oil pressure gauge which resulted in a needle deflection to approximately 45% of full scale (just under 40 psi). The resistance was then reduced until the current was 0.146 amp, and the needle indicated mid-scale (40 psi exactly). Results of the first test of the coolant temperature gauge indicated a heightened degree of non-linearity (needle position versus current) at mid-span. Seems that some type of test protocol will need to be created to explore whether hysteresis is part of resistance/needle position repeatability.

For both gauges the potentiometer was adjusted to produce equal voltage drop between the panel instrument and the simulated sending unit. The coolant temperature gauge appeared to have 14 ohms resistance, while the oil pressure gauge resistance appeared to be 18 ohms. In both cases that would be the resistance of the bi-metal heating coil. No other locations of voltage drop were either identified or investigated.

Back to the possibility of gauge hysteresis or sticking; during this and the first test the engine was not running, nor was the car in motion. Back in the days of aircraft equipped with ‘round gauges’ the instrument panels were frequently equipped with small out of balance electric motors intended to slightly vibrate the panel. Part of my training was to tap a gauge face twice before a reading was logged . . . not so much for digital readouts, but surely for round gauges. The out of balance motor was intended to forgo the need to tap individual gauge faces of an aircraft instrument panel.

My current conclusion is that the K-S panel gauges are equivalent when evaluated in needle deflection measured from ‘cold state’ versus electrical current (perhaps watts).

If anyone has a Patent Number for a 48-50 Packard K-S gauge, I’ll search for the original application for same. The technical description section of those applications normally contain insight on the uniqueness of the design features.

dp