differential pressure controller 30 to open the supply of control air from line 32 into line 34 and to the control portion 36 of valve 38 purge gas supplied over line 42 thus passes through the valve 38 to line 40 and into the flare gas line as shown however the point of entry of the purge gas is preferably down line from the position of the sensors so as not to affect the measurement of temperature of the gas flowing through the flare line fig2 is a second view of the apparatus 10 of fig1 taken across the plane 2 2 of fig1 it shows the flare gas line 12 with arrows 44 indicating the flow of the flare gas the flange cover 18 supporting the two gas cells 20 and 22 are clearly shown it will be clear that fig1 is a view taken across the plane 1 1 of fig2 referring now to fig3 there is shown a second embodiment indicated generally by the numeral 50 fig3 shows an apparatus similar to that of fig1 namely the flare gas line 12 the side pipe 14 flange 16 and flange cover plate 18 here again there are two temperature sensors one is a thermocouple 58 inserted into a thermowell 52 of thin metal so as to respond rapidly to the temperature of the gas flowing past the thermowell 52 along the inside of the flare gas line 12 a second identical thermocouple 56 inside of an identical thermowell 52 is provided however the second thermowell is completely covered with thermal insulation 54 so as to delay heat transfer from the gas to the thermowell metal 52 and.