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3.2.1. T.G.S. 711 response to hydrogen, carbon monoxide and methane The conclusions to be drawn concerning the variation in

response of the T.G.S. 711 sensor to hydrogen, carbon monoxide and methane with heater voltage, illustrated in graphs 9> 10 and 11 are as follows

maximum response increased with the carrier gas flow rate, as illustrated in graph 9 '•

Carrier gas flow rates:

Heater voltage (Vjj) at which the T.G.S. 711

gives maximum response to hydrogen (volts DC) 10 cm min 20-30 cm^min 3 40-50 cm min 5.5 6.0 6.5

For all flow rates tested, carbon monoxide showed a progressive decrease in response as the heater voltage is increased from 6.0 volts to 7*5 volts DC (graph 10), whereas5

3.2.2 T.G.S. 812 response to methane, ethane, ethene and ethyne. The variation in response of the T.G.S. 812 sensor to

methane, ethane, ethene and ethyne with heater voltage is illustrated in graphs 12 to 15 inclusive. It is noted that

methane and ethane show increasing peak heights with increasing heater voltage for all carrier gas flow rates except that, at the lowest carrier gas flow rate ( 10 cm^ minute ^ ) ethane shows a maximum response at a heater voltage of 6 volts ( 0.86 watts ).

The two unsaturated compounds, ethene and ethyne-, exhibit the opposite trend i.e. both show increasing peak heights at lower values of the heater voltage, over the range 6.5 to 5-5 volts DC ( 1.01 to 0.73 watts ).

3.2.3 T.G.S. 812 response to hydrogen, carbon monoxide and methane The variation in the response of the T.G.S. 812 to hydrogen, carbon monoxide and methane resulting from changes in heater voltage over the range 5»0 to 6.0 volts ( 0.6 to 0.8b watts ) is illustrated in graph 16.

methane response increased with heater voltage (graph ll).

a continuous increase with heater voltage over the range tested. Hydrogen and carbon monoxide show much less variation in response with heater voltage. In this case, at a flow rate of 00 cm^minute,^

hydrogen response is greatest at a heater voltage of about 5»5 volts ( 0.73 watts ). Carbon monoxide shows a slight increase in response at lower heater voltages, down to 5 volts DC ( O.b watts). 3.2.4 Response of the Matsushita sensor to

hydrogen, carbon monoxide 'and methane.

The change in response of the Matsushita sensor to the :same three gases was determined by varying the heater voltage over the range 3*5 to 5 volts DC. Over this range, methane showed a

continuous increase in response with heater voltage, the gradient of the response curve increasing markedly for heater voltages in excess of 4»2 volts.Carbon monoxide shows a maximum peak height response in the region of 4*2 volts and hydrogen response

increases with heater voltage at least up to 5 volts but the slope of the curve decreases in the region 4*3 to- 5 volts.

It should be noted, particularly at higher heater voltages, that the actual peak heights for methane are considerably more than ten times those of hydrogen and carbon monoxide, which have teen plotted on a more sensitive scale to show up the relatively minor changes in response to these two gases.

3.2.5 Response of the Matsushita sensor to methane. ethane, ethene, propane and ethyne.

The change in response of the Matsushita sensor to methane, ethane, ethene, propane and ethyne when the heater voltage is varied over the range 3*6 to 5*2 volts DC is illustrated in graph

response at least up to 4*5 volts DC. Propane peak height appears to reach a maximum at a heater voltage of about 4*6 volts and the slopes of the ethane and ethene response curves are less steep above heater voltages of 4*8 and 4*6 respectively. Ethyne peak heights increase very slightly as the heater voltage is increased from 3*6 to about 4*5 volts; the slope of the response cum/e is noticeably steeper at .values of in excess of 4»8 volts,

3.2.6 Response of the I.S.T. sensor to hydroegn, carbon monoxide and methane.

The variation in the response of the I.S.T. sensor to hydrogen, carbon monoxide and methane resulting from changes in the heater voltage over the range 2.0 to 3*0 volts DC is

illustrated in graph 19. No response to carbon monoxide ( at the parts per million level ) could be detected over this range of heater voltage.

In contrast to the other sensors examined, methane response is more critically dependent upon heater voltage and exhibits, a maximum response in the region of 2.4 volts i.e. within the heater voltage range of 2.3 to 2.45 volts recommended by the suppliers. The methane response falls off sharply at heater voltages above and below these values.

Compared with Taguchi sensors, the response of the I.S.T. sensor to hydrogen is relatively small and varies rather more with the heater voltage. Maximum response to hydrogen is obtained at a heater voltage in the region of 2.2 volts DC. 3.2.7 Review of results relating variation in response

with heater voltage, for all sensors.

Attempting an overall review of the results relating peak (52)

comparison may be made, firstly, between the variation in response of the T.G.S. 711, the T.G.S. 812, the Matsushita and I.S.T. sensors to hydrogen, carbon monoxide and methane and, secondly, between the variation in response of the T.G.S. 812 and the Matsushita sensors to the lower hydrocarbon gases.

The T.G.S. 711 was tested using heater voltages which were varied between 5*5 and 7»5 volts (0.3 to 0.57 watts) and exhibited a maximum response to hydrogen, for.most carrier gas flow rates, within the range 6.0 to 6.5 volts (0.36 to 0.43 watts). The T.G.S. 812 was tested by varying the heater voltage between 5 and 6 volts (0.6 to 0.86 watts) and, at a carrier gas flow rate

3 -1

of 20 cm minute , a maximum response to hydrogen was obtained