Methods of Measuring Temperature

Methods of Measuring Temperature Points : methods of measuring temperature, vapour pressure thermometers, resistance thermometers, phosphor bronze, carbon resistor, germanium resistance, magnetic thermometer, thermocouples, iron-construction thermocouples, industry thermometry Gas Thermometer Gas thermometry is the only present method of measuring temperature on the Absolute Temperature Scale but it is an exacting and time consuming technique, and is usually reserved to those programmes in which thermometry is the subject of the research.

A schematic diagram of a constant volume gas thermometer is given in figure. A small amount of gas is filled in bulb which is in communication via the capillary tube with one limb of the mercury manometer. The other limb of manometer is open to atmosphere. To this is attached a device for adjusting the amount of mercury so that the gas may be confined to a fixed volume defined by the meniscus in contact with a fixed steel Index. The contact between these may be determined visually or electrically. The pressure in the bulb is used as a thermometric property.

In a constant pressure gas thermometer the mercury levels have to be adjusted to keep ‘Z’ constant and the volume of the gas V, which would vary with the temperature of the system, becomes the thermometric property. The constant volume gas thermometer is, however, mostly in use, since it is simpler in construction and easier to operate. Vapour Pressure Thermometers If it is required to make measurements over only a limited temperature range but with high degree of accuracy vapour pressure thermometers may be used. The vapour pressure over a liquid at temperature T° K is given to a reasonable accuracy by the formula

Log, p = A = B/T

In which A and B are constants. Near the normal boiling point the variation of pressure with temperature, dp/dt, is for many liquids quite large. Thus, at this temperature an accurate measurement of pressure gives a very accurate measurement of the temperature on the International scale.

The whole temperature range between 90 K and 300 K can be covered by using a sufficient variety of liquids. Below 90 K only rather short parts of the range can be covered because of the Lack of suitable liquids.

Resistance Thermometers 1. Phosphor Bronze Resistance thermometers made from wires of phosphor bronze of brass alloys containing small amounts of lead in the unannealed, springy condition can be used at temperatures below 7°K. The temperature co-efficient of resistance is about 0.03/K, but varies with the diameter of the wire. The resistance of the thermometer varies both with current and magnetic field so that a calibration in the apparatus to be used is essential. 2. Carbon Resistor Carbon resistors of the type commonly used in radio circuits can also be used as thermometers at temperature between boiling points of helium d nitrogen i.e. 4.2°K and 77.3°K respectively.

In the form of a coat of colloidal graphite on porous paper a carbon thermometer is insensitive to magnetic fields, and has a high temperature sensitivity which increases as the size of the carbon particles is reduced.

3. Germanium Resistance It has been suggested that resistance thermometers of germanium would be particularly useful at liquid helium temperatures but these have shown poor reproducibility variable contact resistance and deterioration caused by surface cracking. Contact resistance can, be stabilized by gold plating small areas to which the connecting wires are soldered and the crystal is suspended on these wires, so reducing strains due to thermal contraction. Magnetic thermometer It has been suggested that resistance thermometers of germanium would be particularly useful at liquid helium temperatures but these have shown poor reproducibility variable contact resistance and deterioration caused by surface cracking. Contact resistance can, be stabilized by gold plating small areas to which the connecting wires are soldered and the crystal is suspended on these wires, so reducing strains due to thermal contraction. Magnetic thermometer Temperatures attained by adiabatic demagnetization of a paramagnetic salt are measured in terms of the magnetic properties of the salt used. The salts most frequently studied are those which obey Curie’s Law

M/H = C/T = x

down to the lowest temperatures. In this expression M is the magnetic moment of the salt in a field H, T the temperature, C a constant and x the susceptibility. The constant C can be determined by measurement of the susceptibility at temperatures measurable by gas thermometers or the helium vapour pressure thermometer, about 1 °. A further measurement of susceptibility at the temperature gives the latter directly from equation. However, no salt obeys Curie’s Law exactly down to the absolute zero and so% as with gas thermometers, it is necessary to apply corrections to compensate for the departure of the real salt from the ideal state. For some cases, theoretical corrections can the calculated but for other, empirical ones are necessary. It is customary, to use equation to calculate the ‘magnetic temperature’ T defined by the equation.

T* = C/x

Experimental results may then be quoted in terms of T or of the absolute temperature T if the relationship between T and T* is known Thermocouples Although thermocouples have a lower degree of accuracy as compared to platinum resistance thermometers, still they may be pretend for ease of construction and of reading. Copper-construction thermocouple It has a thermo-electric power of about 50 microvolts/°C near 0°C, but only about 20 microvolts/°C at -180°C. It is thus less sensitive. Chromel-alumel thermocouple Thermal e.m.f. are reproducible to within 1%. so that for measurement to this accuracy individual calibration is not required. Iron-construction thermocouples Higher Thermoelectric powers. Iron is a very variable material thermo-electrically and hence calibration is necessary every time. Corrosion promoted by the presence of’ moisture is a serious problem. Measurement with thermocouples can often be facilitated by using a reference junction at a temperature near to be measured. For example, near the boiling point of liquid oxygen, one junction is much reduced rind any change, therefore, represents a much bigger fraction of the total quantity measured. Industry Thermometry As an introduction to thermometry more nearly approaching industrial techniques mention may be made of a suggested thermometer designed to cover the range from about 3°K to 300°K or aver. It consists of a Bourdon gauge connected by capillary tubing to a thermometer bulb, the whole being filled with Helium. Below the critical temperature of Helium, 5.25°K, it acts as a vapour pressure thermometer, the maximum pressure being 2.26 bar, and at higher temperatures as a gas Thermometer, the pressure then depending upon the relative volumes of the bulb and the Bourdon tube and the mass of gas contained. Liquid in glass thermometers are used industrially for routine inspection of the temperature distribution in large refrigerated warehouses and food stores, where close maintenance of the required temperatures is an important means of ensuring the quality of contents. Temperatures between —10° and +10°C are of chief interest and either mercury or alcohol thermometers are suitable. While mercury thermometers are restricted - to temperature above —39°C alcohol thermometers can he used down to —100°C.