60 BULLETIN 1136, IT. S. DEPARTMENT OF AGRICULTURE. 
should know how to calibrate them so that he may do so when 
necessary. There are two general methods in common use; in one 
the gauge is compared directly with a standard test gauge and in 
the other the pressure to which the gauge is subjected is actually 
weighed by means of standard weights placed on a piston of known 
diameter. 
In both cases the test pressure is produced by means of a small, 
hand pump filled with oil, and provided with connections for the 
gauges. Pressures throughout the range of the gauge being tested 
are produced and the errors noted. Adjustment can be made by 
pulling the gauge hand from its pivot and putting it back again in 
the proper position, Testing equipment of this sort is carried by all 
boiler inspectors. If none is available, comparisons can often be 
made with other gauges, such as those on the boilers, and a fair 
idea of the accuracy obtained. 
LOCATION OF INSTRUMENT BULBS. 
The drying schedules in this bulletin are based on the assumption 
that the temperature and humidity are measured and controlled at 
some point where the air enters the lumber pile. The conditions at 
such points are the most severe, since the air becomes cooler and more 
moist as it travels through the lumber. 
Only a thorough examination by means of the smoke test and 
thermometers hung in different parts of the kiln will determine 
where the recorders and regulator bulbs must be hung to be exposed 
to conditions which correspond to those of the entering air. Cor- 
rections can then be made in the setting or reading of the instru- 
ments. It is usually necessary, in single-width progressive kilns and 
in other single-width cross-piled kilns, to hang the bulbs on the wall, 
since it is not considered worth while to remove and replace them 
each time the kiln charge is moved. Further, the circulation is 
frequently such that no definite " entering air " side can be deter- 
mined. 
In compartment kilns, such as the one in Figure 7, the bulbs can 
be placed in the central flue, and this is the proper place for them. 
They should be at least 15 feet from the end of the kiln and 6 feet 
above the heating coils, unless they are shielded from direct radia- 
tion. 
In the water-spray kiln illustrated in Figure 8, the thermostat 
and dry-bulb recorder bulbs should be located in the entering air 
flue and the dew-point thermometer in one of the baffle boxes. In 
the external blower kiln, the bulbs can be located on the side of 
the pile at which the air enters. When the heating coils are in the 
kiln, the bulbs must also be located in the kiln and on the entering- 
air side of the pile. 
In the superheated-steam kiln illustrated in Figure 11 the bulbs 
can be located in the center of the top passage, as indicated, or in 
the center of the lower passage. These locations are best, because 
they are free from direct radiation, out of the way, and subject to 
the same conditions, no matter which pair of jet lines happens to be 
open. The instruments, however, if so placed should be carefully 
checked against standard thermometers located on the entering-air 
side and properly shielded from direct radiation, since the tern- 
