8. Tolerance Allowed for Watthour Meters

No measuring device can be made which is absolutely accurate. The amount by which it may deviate from strict accuracy and still be considered "commercially correct" is called the tolerance. The usual tolerance for watthour meters is 4 per cent; that is, a meter which registers 96 to 104 per cent of the energy passed through it is considered correct. This does not mean that a meter 4 per cent in error would be left in that condition, as the practice of well-managed meter departments is to have their meter testers adjust meters found fast or slow until they are within 1 per cent either way.

VI. GAS

The gas used in the household is usually measured on the premises by a gas meter. In the United States and parts of Europe, the so-called dry gas meter is most commonly used for this purpose. There are a number of different types of dry gas meters in use, which, while differing considerably in external appearance and design of parts, operate on the same general principles. These general principles of operation would be understood from a description of any one of the types and therefore only one of the most commonly used will be described.

1. Description of a Gas Meter

The external appearance of the meter (see Fig. 34) is well known. The interior is shown in Fig. 35, with the front and top of the meter and the top of the valve chamber removed. Essentially the meter consists of four chambers, which are filled and emptied of gas by the action of the meter mechanism. The number of times this filling and emptying of the measuring chambers is repeated is indicated on the dial in cubic feet. Two of the measuring chambers are shown in the figure. One is the space between the disk (201) with attached leather diaphragm (203) and the middle partition (the plate just behind the diaphragm) of the meter. The other is the space between this same disk and diaphragm and the outside walls of the meter. The other two measuring chambers are like the two described, and are situated symmetrically to them on the opposite side of the middle partition. The filling and emptying of the measuring chambers is effected by the backward and forward movement of the disks. These disks operate in conjunction with the valves and recording mechanism (see Fig. 35) above the measuring chambers. Each set of two measuring chambers thus constitutes a kind of doubleacting bellows, the number of times these are filled and emptied being a measure of the amount of gas passed through them. The power for operating this very simple and effective instrument is

There are a number of different types of dry gas meters in use, which, while differing considerably in external appearance and design of parts, operate on the same general principles. The above figure illustrates the exterior appearance of one of the so-called "tin gas meters" which are extensively used in this country.

The meter, the external appearance of which is shown in Fig. 34, would appear as above if the front, top, and top of valve chamber had been removed. Essentially the meter consists of four chambers which are filled and emptied of gas by the action of the meter mechanism. The number of times this filling and emptying of the measuring chambers is repeated is indicated on the dial in cubic feet.

furnished by the pressure of the gas itself which acts upon the disks, pushing them back and forth just as the power to operate the steam engine is furnished by the steam which presses upon the sides of the piston. The index (101) upon which the volume of gas passed is recorded, is connected to the other mechanism by means of the shaft (102) and gear wheel (103).

2. The Gas Meter Index and How to Read It

Fig. 36 illustrates the index of an ordinary gas meter which is similar to that of an electric meter or a water meter. The smaller top dial, which is marked "Two feet" inside of the circle, is generally called the "testing circle" or "proving head," and is used

FIG. 36.-The index of a gas meter

Each dial is marked with the volume of gas passed per revolution. The smaller top dial, which is marked "Two Feet" inside of the circle, is generally called the "testing circle" or "proving head" and is used principally in testing the meter.

principally in testing the meter. One revolution of the hand of the testing circle indicates that 2 cubic feet of gas have passed through the meter. In some meters one revolution of the hand of the testing circle represents more or less than 2 cubic feet of gas and the testing circles are correspondingly marked. The indication of the hand of the testing circle is ignored in the ordinary reading of the meter.

Of the large dials the first one at the right is usually marked "I thousand." This means that during one complete revolution of the hand 1000 cubic feet of gas has passed through the meter. This dial is divided into 10 equal parts so that the passage of the hand over each part indicates the passage of one-tenth of 1000