techniques for approximating the international temperature ... - BIPM
techniques for approximating the international temperature ... - BIPM techniques for approximating the international temperature ... - BIPM
68 Fig. 6.4: Schematic construction of a saturated vapour pressure thermometer: 1, bulb containing two phases of a substance; 2, pressure sensor; 3, connecting tube; 4, filling system. Fig. 6.5: Bulb of a helium vapour pressure thermometer [Moser and Bonnier (1972)]: 1, 4 He; 2, connecting tube; 3, radiation trap; 4, jacket; 5, thermometer well; 6, thermal leak; 7, differential thermocouple; 8, resistance heating; 9, conduction cooling.
6.2.2 Connecting Tube 69 Because the pressure sensor is usually at room temperature, the connecting tube must bea poor heat conductor; long enough to limit the contribution of heat due to the temperature gradient; equipped with a radiation trap to avoid direct radiation heating of the separation surface between the two phases; small enough that its volume is small compared to the volume of vapour in the bulb to prevent a large change in the liquid/vapour ratio; of cross section sufficient to limit thermomolecular effects and to avoid blocking. It is necessary to avoid creating a cold spot in the connecting tube where the enclosed substance can condense to form a drop of liquid that blocks the tube (although with helium such a cold spot does not form (it is self-quenching) [Ambler and Hudson (1956)] (see also Sec. 6.3.7)). Then the pressure measured would correspond to the temperature of the cold spot. This latter difficulty can be avoided by placing the connecting tube inside an evacuated tube (Fig. 6.6) and/or imposing a small heat flow along the connecting tube. The temperature gradient so created must be controlled (Fig. 6.5) and accounted for as a correction to the measurement results. For example, for a stainless steel connecting tube of 1 m length, internal diameter 2 mm, and external diameter 2.5 mm between 300 K and the helium temperature, the heat flux is about 6 mW (0.8 mW with glass), a heat leak which could produce a temperature difference of 3 mK between the bulb and the liquid helium. The inner diameters used are generally 0.5 to 3 mm unless thermomolecular effects are likely to arise. 6.2.3 Pressure Sensor The sensor is chosen according to the temperature range and the precision required. For an industrial thermometer it is most often a Bourdon gauge, or a mercury or oil (or any substance having small vapour pressure) manometer, used with a cathetometer. For high precision measurements, one can also use a Bourdon quartz spiral gauge periodically calibrated against a pressure balance. A diaphragm pressure transducer is also used, giving a signal that can be amplified and handled by a computer. It can be placed very close to the evaporation surface, which reduces many problems related to the connecting tube (Fig. 6.7), but then there are problems in calibrating it. Whatever pressure sensor is chosen, its internal volume must be small (see Appendix D) and the metrological characteristics of the thermometer will be determined by the manometer. If the substance used in the thermometer is corrosive, the pressure can be transmitted across a membrane or incompressible liquid.
- Page 37 and 38: 17 contained within a cylindrical c
- Page 39 and 40: 19 Fig. 2.5: Schematic drawing of a
- Page 41 and 42: 21 device SRM 767 [Schooley et al.
- Page 43 and 44: 23 Table 3.1 : Current Best Estimat
- Page 45: 25 The widely-used, but not very re
- Page 48 and 49: 28 fraction of sample melted) can g
- Page 50 and 51: 30 Fig. 3.2a: Apparatus for the cal
- Page 52 and 53: 32 Fig. 3.3: Sealed cell for realiz
- Page 54 and 55: 34 Final readings of the thermomete
- Page 56 and 57: 36 temperatures differ (usually) sy
- Page 58 and 59: 38 Fig. 3.4: Cross sectional drawin
- Page 60 and 61: 40 Fig. 3.5: Miniature graphite bla
- Page 62 and 63: 42 4. Germanium Resistance Thermome
- Page 64 and 65: 44 Fig. 4.3: Example of the Π-type
- Page 66 and 67: 46 Fig. 4.4: Differences between dc
- Page 68 and 69: 48 - conversely, p-doped thermomete
- Page 70 and 71: 50 Fig. 4.6: Effect of a radio-freq
- Page 72 and 73: 52 that it will not be subject to m
- Page 74 and 75: ln R n = ∑ i= 0 54 ⎛ ln T - P
- Page 76 and 77: 56 Fig. 5.1: Resistance (Ω) and s
- Page 78 and 79: 58 thermometer wires) caused a more
- Page 80 and 81: 60 6. Vapour Pressure Thermometry*
- Page 82 and 83: 62 transitions, but it could be app
- Page 84 and 85: n ∑ i= 2 64 L x [ Π − k ] P =
- Page 86 and 87: 66 Fig. 6.3: Diagram at constant pr
- 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
68<br />
Fig. 6.4: Schematic construction of a saturated vapour pressure <strong>the</strong>rmometer: 1, bulb<br />
containing two phases of a substance; 2, pressure sensor; 3, connecting<br />
tube; 4, filling system.<br />
Fig. 6.5: Bulb of a helium vapour pressure <strong>the</strong>rmometer [Moser and Bonnier (1972)]:<br />
1, 4 He; 2, connecting tube; 3, radiation trap; 4, jacket; 5, <strong>the</strong>rmometer well;<br />
6, <strong>the</strong>rmal leak; 7, differential <strong>the</strong>rmocouple; 8, resistance heating;<br />
9, conduction cooling.
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