techniques for approximating the international temperature ... - BIPM
techniques for approximating the international temperature ... - BIPM techniques for approximating the international temperature ... - BIPM
120 14. Diode Thermometers Diodes can be used above room temperature (e.g. in clinical thermometry) but not with sufficiently-high accuracy to be considered in this monograph. There is an extensive literature on semiconducting diodes with possible application as cryogenic thermometers [e.g. Swartz and Swartz (1974), Lengerer (1974), and Rubin and Brandt (1982)] but only two types intended for use as thermometers are commercially available: GaAs and Si. The temperature-indicating parameter is the forward-biased junction voltage, which decreases approximately linearly with increasing temperature when the current is kept constant. Because of their almost trivial cost, silicon diodes mass-produced for the electronics industry have been widely tested as thermometers. They would have particular appeal in large engineering projects requiring hundreds of sensors. It turns out, however, to be very costly to select the very small percentage that are adequate for thermometric use. The following discussion does not apply to these devices, but to diodes that are manufactured for specific use as thermometers. There are some specific drawbacks to diode thermometers: a) The typical I-V characteristic is such as to make the internal impedance of the device very high (easily greater than 100 kΩ) at small currents; or else - using a larger current - one encounters unacceptably high power dissipation at low temperatures. b) There is a transition region in the conduction mechanism around 20 K that makes fitting a V-T characteristic over the whole temperature range difficult for GaAs and impossible for Si (Figs. 14.1 and 14.2). For GaAs the least-squares fitted equation [Pavese (1974)] 7 i = A ( ln T') V ∑ = i 0 i (14.1) (where T' = (T/T1) + 1) fits to within about ± 0.1 K (~ ± 100 parts per million in voltage) from 4 K to 300 K. For higher accuracy, the range is subdivided into two sections with the junction near 90 K. An effective equation for the two-range fitting is [Swartz and Gaines (1972)]:
121 Fig. 14.1: Voltage and sensitivity of a gallium arsenide diode as a function of temperature and current (labels on curves) [after Pavese and Limbarinu (1972)]. Fig. 14.2: Forward biased voltage of a silicon diode as a function of temperature. The lower temperature region is shown to larger scale in the inset [Lanchester 1989)].
- Page 90 and 91: 70 Fig. 6.6: Use of an evacuated ja
- Page 92 and 93: 72 Following this, the connecting t
- Page 94 and 95: 74 Fig. 6.9: (c) N2, CO, Ar, O2, CH
- Page 96 and 97: 76 the Weber-Schmidt equation [Webe
- Page 98 and 99: 78 Table 6.1: Temperature values (K
- Page 100 and 101: 80 into the bulb (hydrous ferric ox
- Page 102 and 103: 82 Fig. 6.12: Effect on the vapour
- Page 104 and 105: 84 the remaining liquid increases.
- Page 106 and 107: 86 Curie constant (C⊥ is about 0.
- Page 108 and 109: 8.1 General Remarks 88 8. Platinum
- Page 110 and 111: 90 For a group of 45 thermometers h
- Page 112 and 113: 92 More recently, Seifert [(1980),
- Page 114 and 115: 94 For thermometers with W(4.2 K) <
- Page 116 and 117: 96 after 500 h at 1700 °C in air,
- Page 118 and 119: 98 normally extends about 50 cm bac
- Page 120 and 121: 100 inhomogeneities result from the
- Page 122 and 123: 9.5 Approximations to the ITS-90 10
- Page 124 and 125: 104 10. Infrared Radiation Thermome
- Page 126 and 127: 106 almost negligible. The final ac
- Page 128 and 129: 108 11. Carbon Resistance Thermomet
- Page 130 and 131: 110 Fig. 11.1: Resistance-temperatu
- Page 132 and 133: 112 Table 11.1: Typical Time Consta
- Page 134 and 135: 114 calibration after each cool-dow
- Page 136 and 137: 116 12. Carbon-Glass Resistance The
- Page 138 and 139: 118 Fig. 12.1: Resistance-temperatu
- Page 142 and 143: 122 2 BT V = E0 − − CT ln ( DT)
- Page 144 and 145: 124 Fig. 15.1: Principal features o
- Page 146 and 147: 126 For a thermometer graduated abo
- Page 148 and 149: 128 rise decreases with time but in
- Page 150 and 151: 130 Fig. 16.1: (a) Fabrication of I
- Page 152 and 153: 132 capillaries of a twin- (or four
- Page 154 and 155: 134 advised to choose the thermomet
- Page 156 and 157: 136 Fig. 16.6: Distribution of the
- Page 158 and 159: 138 and between different thermomet
- Page 160 and 161: 140 therefrom) can then be used wit
- Page 162 and 163: 142
- Page 164 and 165: 144 Fig. 16.8: Tolerances for indus
- Page 166 and 167: 146 Stability on thermal cycling is
- Page 168 and 169: 18.2.1 Type T Thermocouple 148 With
- Page 170 and 171: 150 within ± 0.5 to 0.7 percent. T
- Page 172 and 173: 152 insulation even though it be af
- Page 174 and 175: 154 Table 18.4: Recommended Sheath
- Page 176 and 177: 156 In the case of the Types K and
- Page 178 and 179: 158 temperature is measured with th
- Page 180 and 181: 160 19. Thermometry in Magnetic Fie
- Page 182 and 183: 162 Type of Sensor T(K) Magnetic Fl
- Page 184 and 185: 164 be noted that these lower tempe
- Page 186 and 187: 166 Fig. 19.2: The change in calibr
- Page 188 and 189: 168 Carbon-glass thermometers (Chap
120<br />
14. Diode Thermometers<br />
Diodes can be used above room <strong>temperature</strong> (e.g. in clinical <strong>the</strong>rmometry) but not<br />
with sufficiently-high accuracy to be considered in this monograph. There is an extensive<br />
literature on semiconducting diodes with possible application as cryogenic <strong>the</strong>rmometers<br />
[e.g. Swartz and Swartz (1974), Lengerer (1974), and Rubin and Brandt (1982)] but only two<br />
types intended <strong>for</strong> use as <strong>the</strong>rmometers are commercially available: GaAs and Si. The<br />
<strong>temperature</strong>-indicating parameter is <strong>the</strong> <strong>for</strong>ward-biased junction voltage, which decreases<br />
approximately linearly with increasing <strong>temperature</strong> when <strong>the</strong> current is kept constant.<br />
Because of <strong>the</strong>ir almost trivial cost, silicon diodes mass-produced <strong>for</strong> <strong>the</strong> electronics<br />
industry have been widely tested as <strong>the</strong>rmometers. They would have particular appeal in<br />
large engineering projects requiring hundreds of sensors. It turns out, however, to be very<br />
costly to select <strong>the</strong> very small percentage that are adequate <strong>for</strong> <strong>the</strong>rmometric use. The<br />
following discussion does not apply to <strong>the</strong>se devices, but to diodes that are manufactured <strong>for</strong><br />
specific use as <strong>the</strong>rmometers. There are some specific drawbacks to diode <strong>the</strong>rmometers:<br />
a) The typical I-V characteristic is such as to make <strong>the</strong> internal impedance of <strong>the</strong><br />
device very high (easily greater than 100 kΩ) at small currents; or else - using a<br />
larger current - one encounters unacceptably high power dissipation at low<br />
<strong>temperature</strong>s.<br />
b) There is a transition region in <strong>the</strong> conduction mechanism around 20 K that makes<br />
fitting a V-T characteristic over <strong>the</strong> whole <strong>temperature</strong> range difficult <strong>for</strong> GaAs and<br />
impossible <strong>for</strong> Si (Figs. 14.1 and 14.2).<br />
For GaAs <strong>the</strong> least-squares fitted equation [Pavese (1974)]<br />
7<br />
i<br />
= A ( ln T')<br />
V ∑ =<br />
i 0<br />
i<br />
(14.1)<br />
(where T' = (T/T1) + 1) fits to within about ± 0.1 K (~ ± 100 parts per million in voltage) from<br />
4 K to 300 K. For higher accuracy, <strong>the</strong> range is subdivided into two sections with <strong>the</strong> junction<br />
near 90 K. An effective equation <strong>for</strong> <strong>the</strong> two-range fitting is [Swartz and Gaines (1972)]:
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