INCREASING THE ACCURACY 143 



matter is one worthy of discussion. The cost of a packed meter 

 would be much greater than the cost of the meters that are in 

 common use. 



Devices for Increasing the Accuracy of Large Meters. Large 

 meters are frequently required on services because of the occa- 

 sional requirement of large capacity, even though the ordinary 

 rate of use is small. For instance, a 6-inch meter may be put 

 on a 6-inch pipe line, and the 6-inch size may be required because 

 the amount of water represented by it must be available for fire 

 service, although the amount of water required for ordinary 

 use would be passed by a 2 -inch pipe and meter. In such a 

 case, the registration of the 6-inch meter will be much below the 

 truth, because its moving parts are too large to be moved by 

 the ordinary flows. 



Detector meters have come into common use for this service. 

 They consist of a small meter and a large by-pass connected 

 in such a way that when the difference in pressure on the two 

 sides of the meter exceeds a certain amount it will open a valve 

 and make the full capacity of the pipe available. The detector 

 meter is arranged to give a record of the fact that the by-pass 

 has been opened, although it does not record accurately the 

 quantity of water that has passed through it. 



The compound meter differs from the detector meter in that 

 an effort is made to record all quantities of water that pass. 

 The larger quantities are recorded by a larger meter, the smaller 

 quantities by a small meter, and an automatic arrangement shifts 

 the flow from one to the other, according to its amount. Com- 

 pound meters were in use in Europe many years ago. They have 

 been introduced in numbers in the United States only in the last 

 few years, but their use is rapidly increasing. 



There is a great difference in the quality and accuracy of 

 water meters. Meters can be manufactured at low prices that 

 will on test record large flows in a fairly satisfactory manner, 

 but which do not have either the durability to retain their accu- 

 racy or the delicacy to record the low flows. Under sharp 

 competition there is always a temptation for manufacturers 

 to cut the quality of the workmanship and to sell at lower 



