﻿10 BULLETIN L136, V. 6. DEPARTMENT OF AGRICULTURE. 



CONTROL OF KILN TEMPERATURE. 



The proper measurement of the temperature in the kiln is essential 

 to proper- control and deserves much more time and attention than it 

 usually receives. Temperature-measuring instruments or thermom- 

 eters may be grouped in two classes, indicators and recorders. Indi- 

 cating glass-stem thermometers for kiln work are almost invariably 

 of the mercury-filled type, though sometimes alcohol-filled ones are 

 used. 



INDICATING THERMOMETERS. 



There are many kinds of mercury thermometers available, and 

 care must be used to select reliable instruments. The very cheap 

 ones, with separate scales stamped on metal and attached to the case, 

 are not accurate enough for kiln work and should be avoided. A 

 number of better grades also have separate scales, but the highest- 

 grade thermometers have the graduations etched on the glass stem. 

 These can be obtained with or without a metal protecting case. 

 Occasionally it is desirable to insert the thermometer through the 

 kiln wall, with the bulb inside and the scale outside. Industrial- 

 type thermometers are well adapted for this purpose. These have 

 a brass extension tube surrounding the bulb and part of the stem, 

 and a weatherproof brass casing with a glass face protecting the 

 scale. The extension tubes can be made 3 feet or longer, and the 

 stern fitted on at almost any desired angle. A right-angle stem is 

 desirable where the extension tube projects horizontally into the kiln, 

 because it permits the scale to be vertical and therefore most easily 

 read. 



An electrical-resistance thermometer has recently been developed 

 for dry-kiln use. This thermometer has a special panel and con- 

 necting wires, so that the temperatures at a number of places can 

 be read from the one instrument. The temperature is indicated by a 

 pointer moving over a graduated dial. 



RECORDING THERMOMETERS. 



Recording thermometers used in kiln work are almost invariably 

 of the extension-tube type provided with 1-day or 7-day charts. In 

 recorders of this type the sensitive element or bulb is connected to 

 the instrument by a capillary tube of suitable length. (See PL II, 

 fig. 2.) This tube is usually, protected by a flexible armor and ends 

 in a spring capsule in the case. This capsule may be any one of 

 several different types, all of which are flexibly constructed, so that 

 changes in internal pressure produce a movement of the capsule 

 which is usually transmitted through a series of levers to a pen arm, 

 which moves across a slowly revolving chart and produces a graphic 

 record of the temperature in the kiln. The chart is rotated by a 

 clock movement which is wound whenever the chart is changed. 



There are three types of recording thermometers, the difference 

 being in the material used for filling the bulbs. These three types 

 are commonly known as liquid-filled, gas- filled, and vapor-lilled. 

 The choice of type depends upon the accuracy desired and the con- 

 ditions under which the thermometer is to be used. 



In dry-kiln work the tube and the case of the thermometer are 

 liable to be subjected to wide variations in temperature, which influ- 



