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The disk (or displacement) meter is provided with a measuring chamber of a known size. As this chamber fills and empties, the mechanism of the meter records the quantity of water METER displaced each time the water METER leaves the chamber. The mechanism is operated by various means, including nutating disk, oscillating piston, and rotating gear. The disk type is costlier than the velocity type; it is made in %-inch to 6-inch sizes. While relatively accurate at low flows, the disk meter has a limited capacity range for a given size and high friction loss at high flow. If improperly installed, it tends to be noisy. For high capacities, a battery of disk meters, a velocity meter, or a compound meter will prove more practical.

In the fire-service meter, a certain proportion of the total flow passing through the meter is bypassed through a measuring device which is usually of the disk type, although sometimes of the velocity type. Where there is little or no frost or snow, it is advantageous to set the meter in a box in the sidewalk area, affording easy access for reading or changing the meter. In climates whe winters are severe, however, it is practii necessity to install a domestic meter in tht ment to prevent damage from freezing.

A velocity meter (also termed current, turbine, or torrent) is provided with a piston or impeller, the speed of the mechanism being calibrated to the quantity of water METER passing through it. This type is low in initial cost, inexpensive in repairs, and measures relatively large quantities with little friction loss. It comes in sizes 1% to 16 inches. Inasmuch as there is no obstruction to the free flow of water METER through this meter, it meets the requirements of the National Board of Fire Underwriters for use on fire-service lines. However, it is difficult to maintain at a high degree of accuracy, and to repair.