I have always wanted to check the calibration of the oil temperature and cylinder head temperature (CHT) sensors, as there are a number of ways that the indications could be in error. I could have a bad sensor, or a wiring problem, or I could have put the wrong values in EIS-4000 (each type of sensor has a different characteristic of voltage vs sensed value, so the EIS-4000 has to be programmed to match the type of sensor used).

Earlier today someone asked a question on the Yahoo Lycoming List about using boiling water to calibrate sensors. They were worried about the fact that the boiling point changes with altitude. That prompted me to find a table of water's saturation pressure vs temperature in an old copy of Steam Tables that I had when I was studying engineering. Now that I had collected the knowledge I needed, I decided to get the task over with.

I removed the oil temperature and CHT sensors, and cobbled together a way to secure a propane torch below a metal coffee can. I put some distilled water in the can, inserted the sensors, and let the readings stabilize. Then I fired up the torch and heated the water. The oil temperature stabilized at 100°C, and CHT 1 and 3 both went to 98°C. I moved the water and torch over to the other side of the engine, and repeated with CHT 2 and 4 - they read 98°C as well.

The elevation of our house is about 375 ft. The altimeter setting reported by the closest airport was 29.81 inches of HG. The pressure altitude varies approximately 1000 ft per inch change of altimeter setting. Standard pressure is 29.92, giving a difference of 0.11 inch, roughly equivalent to 110 ft. Thus the pressure altitude was about 485 ft (the exact correction for altimeter setting gives 478 ft, so the rule of thumb isn't too far off). An on-line standard altitude calculator shows that the pressure would be 995.6 mb for a pressure altitude of 485 ft (that calculator agrees exactly with my calculations, so it can be trusted). A set of on-line steam tables shows that a pressure of 0.9956 bars (i.e. 995.6 mb) gives a saturation temperature of 99.5°C - i.e. the boiling point of water would be 99.5°C. So the oil temperature is reading bang on, and the CHT are more than close enough.