﻿58 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



comparisons can well be made during the run. if care be taken to 

 read the instruments only after the temperature has been practically 

 constant for 10 minutes, to allow the recorder bulb to overcome its 

 natural lag. A full correction curve can then be plotted for the 

 entire range, and the pen arm of the recorder adjusted to reduce this 

 correction to the minimum. For making this adjustment there is 

 usually provided a small screw at or near the pen-arm pivot, the 

 turning of which moves the pen over the scale. 



Wet-bulb recorders should be calibrated similarly, preferably with- 

 out the wick, and double-pen instruments should have both bulbs 

 calibrated, dry, at the same time. An occasional check with a wet 

 and dry bulb thermometer will show whether the wet bulb is really 

 recording the wet-bulb temperature. It must be kept in mind that 

 a reasonable amount of circulation past the bulb is necessary to 

 secure enough evaporation to bring the bulb temperature clear down 

 to the actual wet-bulb temperature. 



Recorders should be calibrated in place at least once every two 

 months, and oftener if they show a tendencj 7 to fluctuate abnormally. 

 They should be handled carefully, in accordance with the manufac- 

 turer's instructions, special care being taken in changing charts not 

 to bend the pen arm, and when filling the pen not to spill ink down 

 the pen arms. Instruments should be returned to the manufacturer 

 when other than the clock mechanism needs repair. Competent 

 jewelers can keep the clocks in order. 



Although recorders can be obtained in weatherproof cases which 

 need no special protection from the elements, it will be found advan- 

 tageous to mount them in the operating room in some place which 

 is readily accessible and as free from temperature changes as 

 possible. 



THERMOSTATS. 



Thermostats do not as a rule require any calibration except to 

 determine whether in the case of the wet bulb there is enough circu- 

 lation past it to insure proper depression. This can be done by 

 comparison with a wet-bulb thermometer placed right at the regu- 

 lator bulb. It should be read first without fanning and again after 

 vigorous fanning. The drop in temperature will indicate the extent 

 to which the regulator bulb represents the actual wet-bulb temper- 

 ature. 



It is necessary, however, to give the thermostat regulators occa- 

 sional attention. In self-contained instruments which have a stuff- 

 ing box on the valve stem a small quantity of oil and graphite ap- 

 plied occasionally on the stem at the box will help to reduce fric- 

 tion and make 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 which is apt to be carried by the air. An occa- 

 sional washing of the head, by disconnecting the air lines and pour- 

 ing gasoline through it, will keep the parts clean and prevent 

 sticking. 



DRYING OVEN. 



The drying oven needs no particular attention, except to make 

 sure that it is maintaining a temperature of 212° F. and not vary- 

 ing over 2° or 3°. Variations in the oven temperature will make 



