Hello
Put my CHT leads into boiling water at sea level and got a reading of 200 deg F (93 C) and 22 deg F (-6 C) with them in ice. Is this within spec and what tolerance is acceptable?
MGL under plug sensors that I set up as type "K". All 4 cylinder readings are within 1 or 2 degrees of each other. reading on Xtreme and Challenger Lite are the same.
CHT Accuracy
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Re: CHT Accuracy
Thermocouples are relative temperature measurement devices so you have to be careful what you are looking at.
The thermocouple creates a very small DC voltage that depends on the difference between the hot side of the cable (that contains the thermocouple) and the cold side of the cable - the point it screws into the RDAC.
The RDAC has a sensor inside the box very close to the connectors that measures the temperature at that point and adds this to the value from the thermocouple to get an absolute temperature.
Other systems may elect to add a fixed value - 20 degrees C or 0 degrees C are common here - this effectively is the case for simple analog instruments as they cannot do the compensation. Some newer digital instruments also assume a fixed temperature to add.
The internal sensor is affected by the self heating of the RDAC itself so we tend to calibrate this sensor after giving the RDAC 15 minutes to warm up. The temperature differential between the inside and outside of the RDAC tends to be the same over a large temperature range as it consumes constant power.
The compensation goes out the window if you extend the thermocouple cable with any other cable (except if you use K-type cable). You have now formed new thermocouples which will add/subtract additional voltages and the RDAC will not be able to measure the temperature at your junction.
If extending the thermocouple with K-type wire - the professional approach would use special plugs made from the same materials. However we find that using simple automotive crimp connectors work well as the new thermocouples formed are "self canceling" as the junctions tend to be at the same temperature and polarity is opposing.
The thermocouple creates a very small DC voltage that depends on the difference between the hot side of the cable (that contains the thermocouple) and the cold side of the cable - the point it screws into the RDAC.
The RDAC has a sensor inside the box very close to the connectors that measures the temperature at that point and adds this to the value from the thermocouple to get an absolute temperature.
Other systems may elect to add a fixed value - 20 degrees C or 0 degrees C are common here - this effectively is the case for simple analog instruments as they cannot do the compensation. Some newer digital instruments also assume a fixed temperature to add.
The internal sensor is affected by the self heating of the RDAC itself so we tend to calibrate this sensor after giving the RDAC 15 minutes to warm up. The temperature differential between the inside and outside of the RDAC tends to be the same over a large temperature range as it consumes constant power.
The compensation goes out the window if you extend the thermocouple cable with any other cable (except if you use K-type cable). You have now formed new thermocouples which will add/subtract additional voltages and the RDAC will not be able to measure the temperature at your junction.
If extending the thermocouple with K-type wire - the professional approach would use special plugs made from the same materials. However we find that using simple automotive crimp connectors work well as the new thermocouples formed are "self canceling" as the junctions tend to be at the same temperature and polarity is opposing.
Re: CHT Accuracy
Rainier,
Thank you very much for the detailed reply, forgot to add that leads were not joined.
Thank you very much for the detailed reply, forgot to add that leads were not joined.
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stephen.hayne
- Posts: 1
- Joined: Sun Jun 23, 2024 6:53 pm
Re: CHT Accuracy
Revisiting this issue - I have a 9-cylinder Russian Radial engine (M14P) with type-k thermocouples that I tested to 202F with boiling water (close enough for me as I live at 5000' elevation). My problem is that I think the Blaze TC-6 is reading LOW CHTs, e.g., 125C when the cylinder is really 190C.
The front cockpit has the TC-6 and the rear has the old stock single CHT gauge (which reads 190C)...
Any calibration I can do, or suggestions?
Stephen
The front cockpit has the TC-6 and the rear has the old stock single CHT gauge (which reads 190C)...
Any calibration I can do, or suggestions?
Stephen
Re: CHT Accuracy
That sounds very much like you are using K-Type probes but have selected J-Type probes.