Most of the service work performed in air conditioning requires the use of a set of two gauges, one for the high (head) pressure side of the system, the other for the low (suction) side.
The low side gauge records both pressure and vacuum. Vacuum readings are calibrated from 0–30 in. Hg (0–101 kPa) and the pressure graduations read from 0 to no less than 60 psi (413 kPa).
The high side gauge measures pressure from 0 to at least 600 psi (4134 kPa).
Both gauges are threaded into a manifold that contains two hand shut-off valves. Proper manipulation of these valves and the use of the attached test hoses allow the user to test high and low side pressures.
The manifold valves are designed so that they have no direct effect on gauge readings, but serve only to provide for, or cut off, flow of refrigerant through the manifold. During all testing and hook-up operations, the valves are kept in a closed position to avoid disturbing the refrigeration system.