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Title: Fundamental Principles Of Positive Displacement Meters
Author: Allen D. Maclean
Source: 1940 Southwestern Gas Measurement Short Course (Now called ISHM)
Year Published: 1940
Abstract: One of the most fundamental precepts of physics, or in fact of our existence in general, is that two bodies can not occupy the same space at the same time. If, by reason of greater force, one body moves in, the body previously in that space must move out. All we do in making positive displacement gas meters is to provide a way for a certain amount of gas to muscle in where a like amount existed previously, and this gas is in turn muscled out by a succeeding increment of gas. To be sure, this arrangement whereby we permit, or even encourage, a muscling in process, may be somewhat complex, may involve some rather intricate mechanical structures, yet all we do is control and count the number of muscling in cycles, and we have a Iositive displacement gas meter. In one of our meter schools a few years ago we approached the principle of operation of a gas meter by putting ourselves in the place of old Xerses, the Persian general, who, as explained, just missed, by about a hair, being the designer of the first positive displacement meter. While Xerxes was on one of his periodical campaigns against his traditional enemies, the Greeks, he was faced with the task of counting the number of his vast army as it was on the march. He built a stockade along the line of march such as shown at the left of Fig. 59 and allowed his soldiers to march into it until it was full. By actual count, he knew the number of troops required to fill it once, and by filling and emptying, filling and emptying as his army marched through, and counting the number of times it was filled, he could obtain the number of his troops.

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