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KILN DRYING HANDBOOK 85 
ADJUSTMENT OF RECORDING THERMOMETERS 
Calibration shows how the error of an instrument, if any, varies 
throughout its operative range. Errors are usually of two types, 
constant and cumulative. Sometimes, however, derangement of the 
mechanism may cause erratic errors. Constant errors, in which the 
pen reading of a recording thermometer is off the same amount 
throughout the entire range, can be corrected by adjustment of the 
en arm itself. For making this adjustment there is usually pro- 
_ vided a small screw at or near the pen-arm pivot, the turning of which 
moves the pen over the scale. Cumulative errors, in which the error 
increases or decreases progressively as the temperature rises, can be 
corrected in some makes of instrument by changing the leverage of 
the pen arm or the effective length of the hollow spring. Such ad- 
justment is rather delicate and should not be attempted by unskilled 
hands. 
Most recorders operate over a comparatively small range and it 
usually is possible to adjust them at the kiln so that they will be 
sufficiently accurate within this range. The adjustments should be 
made during the calibration in water, and should be checked for 
accuracy before remounting the instrument. If it has been possible 
to do the calibrating with the case and the bulb at the relative 
elevation they assume in the kiln, probably no further adjustment 
will have to be made, Once a thorough check of the instrument has 
been made, the usual calibrations in place should suffice as long as it 
remains in proper operating condition. 
SERVICE CHECK OF CALIBRATION AND ADJUSTMENT 
After a thorough calibration in the manner described the instru- 
ment should be remounted in the kiln and then again checked up at 
several points in its range by comparison with a standard thermometer 
hung close beside its bulb. Such comparison can well be made during 
the next run, if care is 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. Simple further adjust- 
ment of the instrument, if it is necessary, may be made after securing 
check readings sufficient to show what is required. 
PECULIARITIES OF DIFFERENT TYPES OF RECORDERS 
Jt must be remembered. during the calibration of recording ther- 
mometers that the three types, vapor-filled, gas-filled, and mercury- 
filled, do not behave alike. The elevation of the bulb, with respect to 
the case, has an important effect upon the reading of both the vapor- 
filled and the mercur y-filled types; raising or low ering the bulb will 
move the pen arm up or down. Hence final calibration of recorders 
of either of these two types must be made with the bulb and the case 
of each instrument at the relative elevation they will have in service. 
With the gas-filled type, on the other hand, such manipulation has no 
effect upon the reading. Again, variations in either the tube or the 
case temperatures or both may affect the reading of gas-filled and of 
mercury-filled recorders, and important changes in these temperatures 
