90 BULLETIN 1136, U. 8S. DEPARTMENT OF AGRICULTURE 
where the recorder and the regulator bulbs must be hung in order 
to secure exposure to conditions corresponding to those of the enter- 
ing air. After the placing of the instrument bulbs correction can 
be made in the setting or the reading of the instruments proper. In 
cross-piled kilns and in the single-track end-piled type without cen- 
tral chimneys in the lumber piles, bulbs are often hung on the wall, 
since removing and replacing them in such kilns each time the charge 
is shifted is considered not worth while. Further, the circulation 
in these kilns is frequently such that no definite entering-air point 
can be determined. 
In compartment kilns, such as the one in Figure 11, the bulbs 
can be placed in the central chimney formed by the lumber, which 
is the proper place for them. They should be at least 15 feet from 
the end of the kiln and should also be fully 6 feet above the heating 
coils unless they are shielded from direct radiation. 
In external-blower kilns the bulbs are sometimes placed within 
the entering-air duct, but it is considered better practice to place 
them within the entering-air chimney in the lumber pile, 4 or 5 
feet above the duct. The air is more representative of entering 
air at this point than within the duct. If the heating units are in 
the form of pipe coils within the kiln, the bulbs must not be placed 
in the air duct. . 
In the internal-fan kiln the fans are usually operated so that the 
air circulation remains in its initial direction only during the first 
part of the drying; the bulbs should be located in what is entering 
air under these conditions. Most internal-fan kilns are of single- 
track design; in edge-stacked kilns of this design the proper place 
for the bulbs is in a side flue, and in flat-piled kilns it will usually 
be necessary or at least expedient to put them in this location also, 
even though the logical place in such kilns, in general, is in the 
central chimney within the lumber pile. In double-track flat-piled 
kilns of the internal-fan type the proper place for the bulbs is in 
the central chimney between the two rows of trucks. For all of 
the locations suggested, when the circulation is reversed, the bulbs 
will be in a cooler leaving-air current, and consequently the dry-bulb 
thermostat will probably have to be lowered a few degrees so that 
the entering-air conditions may remain constant. The wet-bulb 
thermostat will usually require no change, since the drop in wet- 
- bulb temperature during the passage of the air through the lumber 
is negligible. 
A special type of vapor-filled thermostat has been developed for 
temperature control in reversible-circulation kilns. The wet-bulb 
thermostat has the usual single bulb, which may be placed as already 
indicated. The dry-bulb thermostat, however, has two bulbs and 
two extension tubes, both connected to the same tube system. One 
dry bulb is placed near the wet bulb and the other in an opposing 
flue, that is, a flue that carries leaving air when the other carries 
entering air and conversely. The properties of a system of this kind 
are such that the instrument is automatically actuated by the bulb 
having the higher temperature, namely, the entering-air bulb. Thus 
as the air circulation is reversed, the control is automatically shifted 
from one dry bulb to the other. 
