﻿KILN DRYING HANDBOOK. 59 



an appreciable difference in the moisture-content determinations. 

 Steam ovens are easily regulated by means of a reducing valve in 

 the steam main, and electric ovens by an adjustable thermostat 

 operating on the heating circuit. 



SCALES AND BALANCES. 



Scales for weighing samples and sections should be sensitive to 

 the smallest quantity which they are intended to weigh ; if they are 

 not, they should be repaired or returned to the factory. The absolute 

 accuracy of the scales is not, however, of paramount importance, so 

 long as all the readings are in proportion. Thus, suppose that a 

 scale is 5 per cent in error; this error will apply just as much to the 

 original and the current weights as to the oven-dry weight, and the 

 moisture percentages will be just the same. This assumes, of course, 

 that all the weighings are made on the same scale. This illustration is 

 given simply to show that it is not absolutely necessary to have a 

 set of standard weights for calibrating the scales. It is necessary, 

 however, to be assured that the indicated weights are always in pro- 

 portion. If, for instance, one sample weighs twice as much as another, 

 the scales must show this. Specifically, this means that all of the 

 weights and the poise must be in proper proportion. This can be 

 readily determined on platform scales by any scheme which allows 

 the same piece or quantity of material to be weighed with the dif- 

 ferent loose weights and the poise. To illustrate : A 200-pound silk 

 scale has a single poise and beam graduated to 2 pounds by hun- 

 dredths of a pound and loose counterpoise weights of 100, 50, 20, 

 10. 10, 5. 3, and 2 pounds respectively. This scale can be checked 

 up as follows: Balance the beam accurately, set the poise at 2 

 pounds, and place just enough weight on the platform to balance. 

 Then return the poise to zero and put the 2-pound loose weight on 

 the counterpoise. The beam should balance again. If it does not, it 

 can be brought to balance by adding to or removing weight from the 

 platform and then weighing again with the poise. Having checked 

 the 2-pound weight against the poise, check the 3-pound weight by 

 putting enough additional weight on the platform to balance at 3 

 pounds with the poise set at 1 pound, and the 2-pound weight on the 

 counterpoise. Remove the 2-pound weight, return the poise to zero, 

 and place the 3-pound weight on the counterpoise. This scheme of 

 comparisons may be continued through the entire capacity range of 

 the scale. It is most convenient, in securing the final balance of the 

 beam at the different weights, to use a pan of shot, sand, or water. 



Balances using loose weights can be checked simply by interchang- 

 ing the contents of the two pans. If they were in balance in the first 

 place and remain so, the balance arms must be of equal length. If 

 the arms are not of equal length, the balance can still be used, by 

 always placing the weights on the same pan. The individual loose 

 weights may be checked against, each, other by placing the same 

 nominal weight on each pan. 



STEAM GAUGES. 



Steam-pressure gauges, if used, simply to give a general idea of 

 the amount of pressure available or to check the operation of a re- 

 ducing valve, need not be very accurate. However, the operator 



