The VM Hydrant Flow Gauge is universally used by Water Engineers and Fire Service Authorities in the appraisal of piped water supply for fire fighting, development planning, and as a probe for investigating the ageing conditions of water mains and the consequence of overloading. In all of these applications, there is no better cost effective substitute
The flow rate indicator is of the manometer compressed-air type, incorporating a precision-bore glass tube. Used as an open water column, this measures all flows between 2.6 and 7.2 litres per second, purely by the gravimetric head, which measures velocity. Over the middle range (7.2 to 34 lps) the capacity of residual air within the glass tube and compression chamber is accurately determined at the top end by the entrapped water entering from the pitot tube. This tends to resist in accordance with Boyle’s Law of the compressibility of gases.
A third range is provided, reading from 34 up to 60 lps. This is obtained by inserting a compensator plug into the chamber at the top of the manometer tube as well as fitting the compensation chamber between the manometer and pitot tube body. This increases the volumetric capacity.
One of the advantages of this device is that the calibration is permanent and cannot be affected by severe mechanical or hydraulic shock. Another advantage of the compressed-air manometer in this application is its virtually straight-line response to the square-law increment of velocity head. This gives an almost evenly spaced scale of calibration, and much greater clarity in reading the value of poor to indifferent flows at the lower end of the scale.