Suggested method of EGT input calibration / verification.
Quick hint: if EGT reading goes down (often rails to 0) when sensor temp goes up, swap the EGT sensor polarity.
Needed for calibration:
- 2 resistors ( with a ratio of 100-500, like 4k7 and 1Mohm ) or sensor in gas-flame
For offset-calibration, only an alligator clip is needed.
Round device typical EGT calibration value: 157 (0x9d).
If calibrating, start from this.
Calibration is for device input, NOT the sensor. No need to calibrate if using with a different sensor.
Recently (since 2008-03) EGT input is calibrated.
(the default value gives smaller reproducible error than the error related to sensor placement).
Calibration is useful verification any time one .
Before adjusting offset, make sure egt calibration has a reasonable (near 157 value, say +- 8). If it's way off, like 0 or 255, you will have to repeat offset step later, so better start with a reasonable offset.
- attach 0mV to input (alligator clip between white and green)
- start with offset =255. (EGT will read a bit high)
- decrease offset (offset -= 2 for -1C) to a value until it displays the cold junction temp, which is the end of green compensational cable (the green and white wires at the alligator clip) (eg. +27C)
- This step is easier if EGT is displayed on bottom, but can be watched in megatune.
offset: 0-255 (device dependent, no typical value)
Now follow to EGT calibration step:
EGT calibration with 15-45mV signal
A voltage divider (or hot EGT sensor) can be used to provide a small appr 30 mV signal. Maybe divide from a 4V signal (eg. temporarily steal from wbo2 pump- ),
- 1k between green wire and GND
- 100k between green wire and 4V
result: 4V * 1k / (100k + 1k) = 39.6mV
- white: 0mV
- green: +39.6mV measure this with DVM (digital voltmeter)
This 39.6mV is equivalent to 39.6 * 25C + 27C = 990C + 27C = 1017C
(where +27C is the cold junction temperature).
Adjust EGT calibration until reading is 1017C.
That's all. Attach the alligator-clip again (0mV) and verify that cold-junction temperature is still OK. (if offset was adjusted with a weird calibration value, it might be off now: than go back to offset adjustment).
Take note of the offset and calibration values for future reference (with permanent pen on the side or on a white tape on a cable).
Ice / Boiling water - because of the 100C (=4mV) not as precise as the 15-45mV signal above
Set the offset to 255 and the calibration to 157.
Adjust the calibration until you get a 100degree difference between the ice and boiling water - then use the offset to set the temperature so that it reads 0 in ice, and check the boiling water temperature to ensure that the temps are scaling properly.