KILN DRYING HANDBOOK 13 
filling the tube system, and accordingly the three are known, re- 
spectively, as mercury-filled, gas-filled, and vapor-filled thermometers. 
In dry-kiln work, both the tube and the case of the recording 
thermometer are subject to variable temperatures, which during op- 
eration of the kiln differ from that of the bulb; the thermometer is 
intended to record bulb temptrature alone. Fluctuations in the tube 
and in case temperatures affect the accuracy of the instrument, espe- 
cially with the mercury-filled and the gas-filled types. In these 
types, ordinary variations in any one of three temperatures, bulb, 
tube, and case, will change appreciably the reading of the thermom- 
eter, except when compensation is made for variations in case tem- 
perature. The vapor-filled instrument, on the other hand, is nearly 
free from errors caused by tube and case temperatures, provided 
that its bulb is large enough and contains the proper amount of 
liquid, since the pressure of vapor in the tube and in the hollow 
spring then is virtually the vapor pressure of the volatile liquid 
in the bulb, at bulb temperature. 
Practically all extension-tube instruments now sold for dry-kiln 
work are of the vapor-filled type. This includes recorders, air- 
operated controllers, and recorder controllers. 
Charts recording temperature for 1-week periods are satisfactory 
for most purposes; those at least 10 inches in diameter are preferable. 
The divisions on the charts of mercury-filled and of gas-filled instru- 
. ments are uniform throughout the working range. This is not true 
of most vapor-filled instruments, because the vapor pressure does not 
vary in direct proportion to the temperature. One manufacturer, 
however, has produced a vapor-filled recording thermometer with 
uniform chart, by introducing a cam into the pen-arm movement. 
REDUCING VALVES 
The temperature in the kiln is controlled by means of auxiliary 
apparatus, such as valves and thermostats. The pipe leading from 
the steam main to the kiln is almost always provided with a globe 
or a gate valve, by which the entire steam supply to the luiln can be 
turned on or shut off. This valve also permits hand control of the 
temperature in case no other means is available. 
When boiler steam is used for heating dry kilns, it often happens, 
especially with low-temperature schedules, that the pressure in the 
steam mains is higher than is necessary for the proper temperature 
in the kiln; this pressure commonly fluctuates materially over the 
94 hours-of the day. A pressure-reducing valve (pl. 5) between the 
_ steam main and the kiln is likely to be desirable in such a situation. 
If a battery of kilns operates at high steam pressures, a single 
reducing valve may be made to serve the entire battery, but where 
steam at a customary boiler pressure is used either to augment the 
supply of exhaust steam or as the entire supply for low-pressure 
systems, it will be necessary to install two reducing valves in tandem, 
the first one reducing to perhaps 10 pounds and the second making 
the final step. The first valve will be a heavy, rugged type and the 
second a more sensitive one, capable of close adjustment. In an 
installation of this kind a steam receiver or a length or two of pipe 
should be placed between the two reducing valves to provide a 
cushion, thus preventing the first one from chattering. The varia- 
