KILN DRYING HANDBOOK 87 
will be small—perhaps about 2° on the chart. Now place the set- 
ting arm halfway between these two positions and adjust the pointer 
(there is usually a simple screw adjustment on or near the arm) 
until it corresponds to the temperature indicated by the recorder pen. 
THERMOSTATS. 
Thermostats as a rule require no calibration other than the deter- 
mination that, with a wet-bulb instrument, the circulation past the 
bulb is sufficient to insure proper depression. Such determination 
can be made by means of a wet-bulb thermometer placed right at the 
regulator bulb. The test must be made under actual operating con- 
ditions, since the air circulation past the bulbs may be quite different 
under other conditions. The thermometer should be read as soon’ 
as it has reached a constant indication and should then be vigorously 
fanned. If this fanning produces an additional drop in the reading 
of the thermometer, the normal circulation is inadequate. 
It is necessary, however, to give the thermostat regulators occa- 
sional attention. With a self-contained instrument, which has a stuf- 
fing box on the valve stem, a small quantity of oil and graphite ap- 
plied occasionally to the stem at the box will help to reduce friction, 
thus making the instrument more accurate. The stuffing box should 
be* tightened only enough to prevent leakage. In the air-operated 
type the small valves in the regulator head must be kept free from the 
_ oil that is apt to be carried by the air. An occasional washing of the 
head, by disconnecting the air lines and pouring gasoline through it, 
will keep the parts clean, thus preventing sticking. 
DRYING OVENS 
The drying oven needs no particular attention, except to make sure 
that it is maintaining an average temperature of 212° F. and that 
its maximum variation, between limits, is not more than 5° F. 
Noticeably different values of oven temperature give appreciably 
different results in the moisture-content determinations. Steam 
ovens are easily regulated by means of a reducing valve in the steam 
main, and electric ovens by an adjustable thermostat operating on 
the heating circuit. 
SCALES AND BALANCES 
REQUIREMENTS 
Scales for weighing samples and sections should be sensitive to the 
smallest quantity that they are intended to weigh; if they are not, 
they should either be repaired locally, returned to the factory for 
adjustment or replaced. The absolute accuracy of the scales, how- 
ever, 1s not of paramount importance, as long as all the readings are 
in proportion. Thus, supposing for instance that a certain scale is 
constantly 5 per cent in error, the error will apply just as much 
to the original and the current weights as to the oven-dry weight, and 
the moisture percentages will work out just the same as they would 
if the scale were accurate, provided, of course, that all the weighings 
are made on the same one. This single example is sufficient to show 
that it is not absolutely necessary to have a set of standard weights 
for calibration. It is necessary, however, to be certain that the indi- 
