86 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
must be guarded against during calibration and in so far as possible 
ae ing use, although this matter is not at all important with vapor- 
filled instruments. 
CALIBRATION OF WET-BULB INSTRUMENTS 
Wet-bulb recorders also should be calibrated regularly, preferably 
without the wick, as frequently as the dry-bulb recorder and in a sim- 
ilar manner; double-pen instruments should have-both bulbs cali- 
brated, dry, at the same time. An occasional check with a wet and 
dry bulb thermometer will show whether the wet bulb is really record- 
ing the wet-bulb temperature. When making such checks it must be - 
kept i in mind that a reasonable amount of circulation past the bulb 
is necessary to secure evaporation enough to bring the actual tem- 
perature of the bulb down to the value correct for the conditions 
existing. 
SERVICE CALIBRATIONS 
Recorders of all types should be calibrated in place at least once 
every two months, and oftener if they show a tendency to fluctuate 
abnormally. They should be handled carefully, in accordance with 
the manufacturer’s instructions, special pains being taken in changing 
charts not to bend the pen arm, and when filling the pen not to spill 
ink down the arm. Instruments should be returned to the manufac- 
turer when other than the clock mechanism néeds repair. Competent 
jewelers can keep the clocks in order. 
INSTALLATION OF RECORDERS 
Although recording thermometers can be obtained in weatherproof 
cases which need no special protection from the elements, it will be 
found advantageous to mount them in the operating room in some 
place that is readily accessible and as free from temperature changes 
as possible. 
RECORDER CONTROLLERS 
The manufacturer’s instructions should be followed with especial 
care in the calibration and adjustment of recorder controllers. The 
calibration of the recording mechanism can be made in general in 
the manner outlined for recording thermometers. In some types, 
however, the thermostat mechanism interferes with the movement of 
the pen arm when the position of the setting arm or pointer fails to 
correspond to that of the pen arm. Hence it is desirable that the 
setting arm be moved in unison with the pen arm during calibration 
so that any possible interference effect may be avoided. 
The adjustment of the setting arm must be made with the instru- 
ment in place and air pressure on. It also is desirable to have the 
temperature of the bulb within the usual operating range of the 
instrument. Starting with the setting arm well below the tempera- 
ture of the bulb, move it, by means of the key or other mechanism 
provided for that purpose, until the diaphragm valve on the steam. 
line opens, and note the position of the arm. Then move it further, 
until the valve closes, again noting its position. The last movement 
