Exhaust gas temperature measurement.            Home            18-04-2009

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Introduction.

Measuring the temperature of hot gases is not that easy especially when the temperature and speed of these gases are continuously changing.

Due to the properties of heat transfer at and in the thermocouple the measured temperature at changing condition will always have some delay to the actual temperature.

In general the smaller the thermocouple the better it will follow the actual temperature but at the expense of mechanical stability. In many stationary industrial application thermocouples with a diameter of 0,5, 1 mm are used with success. Looking to exhaust gas thermometers one sees in general  a diameter of 3 mm.

The consequence of this 3 mm diameter is a relatively large measuring delay

e.g. in air with 2m/s the T0,9 is for 0,5 mm 8 sec.; 1 mm 17 sec and for 3 mm 64 sec. for the used type of thermocouples.

The consequence is that the indicated temperature of these very precise thermocouples is far from precise during temperature change.

At the moment I do not have delay values for diesel engines at working conditions.

From the literature safe temperatures are between 250-680 Celsius, 720 Celsius is upper limit. 

Engine can be switch off at temperatures below ??? Celsius.

 

Components.

Thermocouple:

Display:

Greisinger GTH 2448/1 about 56 euro

Adapter plate with Thermolok seal glands:

Metal gasket:

Land rover LRJ100000; experimental, should be better then no gasket; time will show.

Indicative measurements.

The 3 mm and a 1 mm thermocouple are place at the same time instantly in a hot air stream in a tube.