WEIGHTS AND MEASURES 17 



In this formula, feet are multiplied by a decimal of an inch. As 

 an example, the thickness of a half-inch pipe carrying a 50-foot 

 head of water would be : 



50 X .5 25 1 



- =__= = .033 in. 

 750 750 30 



20. Precision of Measurements. Mechanical problems 

 or operations usually consist of two parts: the collecting of 

 data, and the solving of the problem. Both of these opera- 

 tions require a basic knowledge of materials, considerable 

 judgment, and care for the accuracy of the work. One of 

 the most effective methods of checking measurements is to 

 take them twice, and then to arrange them in a systematic 

 and tabular form. To avoid errors, it is well to refrain from 

 using too many decimal places. It is generally a good plan 

 to carry all calculations to one place further than that in 

 which accuracy in the final result is desired. For instance, 

 if it is desired to have a final result accurate to a hundredth 

 of the whole, the calculations should be exact to the thou- 

 sandth of the whole. 



When one of a series of measurements has been taken 

 and the results recorded to three decimal places, the second 

 place of the decimals may be the same in all the measurements 

 but the third place may differ. In other words, the result 

 will be correct to two places, but the third place will be in 

 doubt. 



The following plan may be used to determine the place that is in 

 error in the final product: Place a circle around the last digit that 

 is nearest to the decimal place which is the least accurate. In the 

 case of 6.845 X 4.5 this is .5. 



