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Explain compound gauge
Compound gauges are usually diaphragm-type gauges. A diaphragm gauge has a flexible diaphragm connected by a rocker bar and associated mechanisms which magnify the movement to a pointer (see Figure below). As you look at the face of the diaphragm gauge, you will notice it has a long vacuum scale and a short pressure scale. The scale on the vacuum side typically measures zero to negative 100 kPa (1.00 x 100 kPa). This allows for more accuracy over a small range of vacuum readings. (See Figure) When a positive pressure is applied, (from water supplied from a hydrant or another appliance) a small area of the diaphragm moves towards the small cavity in the front housing, thus giving a reading on the smaller area of the gauge. On the other hand, if a negative (below atmospheric) pressure is applied, a large area of the diaphragm shifts in the opposite direction within the larger cavity of the rear housing, which magnifies the negative pressure reading.
Notes
Compound gauge measures both positive and negative pressure.
Measures the pressures at the inlet side of pump.
Negative pressure is shown when drawing from static water supply
1 metre of vertical lift is - 0.1 bar
Positive pressure is shown when water supplied from a hydrant or another pump
For example if 5 bar is supplied from hydrant , 5 bar will be shown on positive pressure side when valves are closed.
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