Modern Gasworks Practice

240 MODERN GASWORKS PRACTICE factor ; so that, put briefly, it may be said that 1 per cent, increase in the temperature of the radiating body is followed by an increase of 4 per cent, in the energy receivecL by the pyrometer. The rapid increase in energy at the higher temperatures accounts for the open nature of the scale calibrations at these temperatures as compared with. the smaller divisions found at lower temperatures. An advantage which has also-been pointed out is tliat a change in sensibility of 1 per cent, in the apparatus would only result in a change of 0’25 per cent, in the temperature reading. Within fairly wide limits, the distance at which a radiation instrument is set up from the source of heat is immaterial. This consideration renders the radiation pyrometer particularly convenient for making quick observations during the brief peri od for which the retort doors are open. For the purpose of recording the temperatures prevailing in water-gas apparatus the radiation pyrometer is unsuitable owing to the impossibility of focusing any portion of the enclosed vessels. In such cases it is customary to employ the thermo-electric instrument having a thermo-couple enclosed in a protecting tube. The tube may then be built in through the sides of the vessel and left permanently in position. DIRECT INSERTION INSTRUMENTS Direct insertion thermo-couple instruments for supplying intermittent readings or a continuous record are made in several forms, and the decision as to which typ& should be employed depends uponthe conditions to which it Fig. 141.—Dikbct Insertion Instrument. will be subjected and the accuracy required. In general, the two main types as employed in this country may be classified as follows:— (®) Thermo-couples consisting of platinum and platinum-rhodium for temperatures not exceeding 1,400° C. (b) Base metal thermo-couples for temperatures not exceeding 1,000° C. It is essential that rare metal couples should be pro-tected by either quartz or porcelain tubes, and for tempera-taras exceeding 800° C. it is advisable that base metal couples should be similarly protected. A simple form of rare metal thermometer is shown in Fig. 141. in which the protecting porcelain tube is inserted in a socket supporting the porcelain head on which the terminal screws are mounted. If desired, a steel protecting slieath, not shown in the illustration, can be fitted. A pyrometer of this kind is useful for general testing work when it is not desired to instal it permanently. In oxidizing atmospheres, or where the couple is likely to come in contact with flame, a fireclay protecting tube and steel socket is used instead of the steel sheath. The Cambridge Scientific Instrument Company have in-troduced three types of base metal thermo-couples, namely : —