84 BULLETIN 1136, U. 8S. DEPARTMENT OF AGRICULTURE 
THERMOMETERS 
STANDARD THERMOMETERS 
The easiest and most satisfactory way to calibrate indicating and 
recording thermometers is to compare them at different temperatures 
within their ranges with a standardized thermometer, the accuracy 
oi which has been determined by its manufacturer. Hence each 
operator should have at least one standard indicating thermometer, 
made by a reliable concern—even good glass shrinks slightly with 
age, so that thermometers manufactured too hurriedly become inac- 
curate in time. The type recommended is a 12-inch mercury-filed 
glass chemical thermometer, with graduations in degrees Fahrenheit 
etched on the stem, and correct for ordinary purposes over a range 
of 30° to 220° F. Such a thermometer can be purchased at a list 
price of about $3, and a brass protecting sleeve, desirable for kiln 
service, can be had for about $1.50 net. 
CALIBRATION OF INDICATING THERMOMETERS 
The first step in the usual laboratory method of calibration by 
comparison with a standard thermometer is the immersion of the 
standard and the thermometers to be calibrated in a vessel of water, 
which is constantly stirred to keep the temperature uniform through- 
out. ‘The water, which should be cold at the start, is gradually 
heated, and the thermometers are read at intervals of a few degrees 
throughout their working range. The differénce between the read- 
ings of the standard and of another thermometer at any temperature 
is the error of the other one at that temperature; a correction of this 
amount must be applied to the faulty reading to give the actual 
temperature. Jf the standard reads the higher, call the correction 
plus (+) and add it to the future readings of the other thermometer ; 
if not, subtract it from those readings. This method is apphcable to 
all portable thermometers used in kiln work, including wet-bulb, 
the wicks of which are removed during the process. Once every six 
months should be sufficient for the calibration of glass-stem ther- 
mometers. 
CALIBRATION OF RECORDING THERMOMETERS 
Recording thermometers require more attention than other types 
on account of the comparative ease with which they may become 
deranged; those used in dry kilns are almost invariably of the 
extension-tube type. They should be calibrated in water, as described 
for glass indicating thermometers, the bulb and about a foot of the 
tube being immersed. Sometimes this calibration can be accomplished 
by dismounting the bulb and tube alone, but it is often more 
convenient to dismount the entire instrument from its position in 
the kiln. Because of the size of each bulb and the construction of 
the instrument, recording thermometers respond less quickly to tem- 
perature changes than glass indicating thermometers. This natural 
lag requires that more time be given for the recording thermometer 
to adjust itself during calibration. 
