Carbon resistance thermometer

In order to obtain actual performance data a carbon resistance heater was mounted in the sample chamber instead of the microwave cavity and the pressure in the evaporator was monitored with a McLeod gauge. The heat input at any given evaporator pressure is then the total available refrigeration. This is shown in Fig. 8. A carbon resistance thermometer was simultaneously calibrated in the evaporator. The ultimate temperature attained was 0.46°K. 

Comment by C. E. Chase, MIT Lincoln Laboratoiry Professor Millett of the University of Texas and I carried out some similar experiments last summer, using an array of 12 vertically spaced carbon resistance thermometers along the axis of a de-war. Our preliminary results agreed very well with the present ones, but we were unable to convince ourselves that the effects of radiation falling upon the thermometer could be neglected, and the work was discontinued. 

Carbon. The carbon resistance thermometer (CRT) has maintained its popularity in the field because it continues to provide high sensitivity at low temperatures in a low-cost, very small package. Carbon in one form or another is perhaps the most widely used of the high-resistivity materials for low-temperature thermometry, especially in the presence of magnetic fields. 

The bulk of the use of carbon resistance thermometers is for temperatures at and below 20 K. Their use at higher temperatures is limited because of decreasing sensitivity, reported stability problems, and competition with metallic resistors. By far the most-used form of the carbon thermometer is the standard radio type. In general, the 0.1-1 W resistors with room temperature resistances of 10-500 Q are most widely used. The advantages of these resistors are that they are physically small, rugged, cheap, and are the least sensitive of the resistance thermometers to magnetic fields. 

Preferences vary as to the exact type, size, and resistance of CRTs. Historically, the important brands have been Allen-Bradley, Ohmite, and Speer (more properly Airco Speer). Manufacture of the Airco Speer grade 1002-1/2 W resistors was discontinued in 1979. The Allen-Bradley radio resistors in sizes from 0.1 to 1 W, and resistance values from 10 to 500 Q, remain the most popular choices above 1 K, and are used occasionally below 1 K. The Speer radio resistors in various sizes and resistances are usually selected for the region below 1 K and may prove useful to as low as 0.02 K. A typical 0.5-W, 220-Q resistor will measure roughly 1 kQ at 1 K, 20 kQ at 0.1 K, and 300 kQ at 0.015 K, the last with about 10 W dissipation. Usually 0.1-1-W resistors with room-temperature resistances of 10-500 Q are used. The temperature dependence of the resistance for three types of carbon resistance thermometers is compared with that of two types of germanium resistance thermometers in Fig. 8.27. 

Clement and QuinneP have investigated carbon resistance thermometers... 

Borcherds (1969) Sensitivity of Carbon Resistance Thermometers by R H. Borcherds, Cryogenics 9, 138. 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

Special applications, such as in high-magnetic fields, require special thermometers. The carbon-glass and strontium-titinate resistance thermometers have the least magnetoresistance effects. 

Ge resistors are specifically produced for low-temperature thermometry carbon and Ru02 resistors are commercial products for electronics. Pure carbon is not a semiconductor. The negative slope R(T) is due to the production process which consists in pressing and sinterization of carbon particles with glue. The resulting resistance is probably determined by the contact resistance between the particles. The cost of the carbon resistor thermometer is very low. Manufacturers such as Speer, Allen-Bradley and Matsushita have produced in the past carbon resistors for many years. Most of firms have now ceased manufacture, although their products may still be found in the storerooms of research laboratories. 

Resistance thermometers are useful over a wide range. For example, platinum is used from 273 to 15 K. and carbon covers the range 20 to 2 K and has the advantage of being quite insensitive to magnetic fields. The foregoing arc examples of secondary standards where the scales are interpolated between fixed points. 

Thermostat bath with a heater, adjustable thermoregulator, and stirrer thermometer with a resolution of at least 0.01 K (a platinum resistance thermometer, thermocouple or quartz thermometer could be used see Chapter XVII) illuminating light and cathetometer for reading the vertical position of the menisci number of sealed capillaries of uniform bore containing varying amounts of carbon dioxide, prepared in advance. 

The bolometer is another type of thermal detector that can offer extreme sensitivity for specialized applications, t This is essentially a resistance thermometer, usually with a platinum, nickel, carbon, or germanium element, although a semiconductor thermistor can also be used. Typically, two elements are used in a bridge circuit with one exposed to radiation and the other kept dark as a reference. The germanium bolometer provides exceptional... 

Resistance-Temperature Scaling of Carbon-Composition Thermometers (2) 104... 

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