The blue (low-pressure) needle of the manifold gauge shows the suction pressure. This pressure corresponds to the temperature at which the refrigerant is evaporating in the evaporator; the technician reads it as a temperature with the help of the chart. Examples: with R404A, −10 °C evaporating is about 3.3 bar g and −30 °C about 1 bar g; with R134a, −10 °C is about 1 bar g. Because of the pressure loss in the suction line, the value read at the compressor is slightly lower than that at the evaporator; in a good design the difference does not exceed 1–2 K.
Suction pressure is not an arbitrary number; it depends on the temperature of the room and the temperature difference (TD) of the evaporator. In a 0 °C room evaporation is at −6 to −10 °C, and in −18 °C frozen storage at around −25 to −28 °C. As the suction pressure falls the gas becomes thinner and the compressor moves less refrigerant with every revolution: every 1 K fall in evaporating temperature reduces capacity by about 3–4% and efficiency by 2–3%. Unnecessarily low suction pressure means, quite directly, excess electricity.
The causes of suction pressure lower than normal: low charge, a blocked filter drier, a throttled or iced-up expansion valve, an evaporator covered in frost, a stopped evaporator fan, an undersized evaporator. The causes of suction pressure higher than normal: warm product being loaded into the room (this is temporary), an expansion valve open too far, leaking compressor valves, a compressor that cannot keep up. Suction pressure alone is not enough to tell them apart; it is interpreted together with the superheat and the discharge pressure.
Points to watch when measuring: the gauge mostly reads gauge pressure (bar g), whereas charts may give absolute or gauge values; the difference between them is 1.013 bar. For zeotropic refrigerants the dew column is used for the evaporating temperature. The low-pressure switch is set according to the suction pressure and protects the system from falling into vacuum. You can find the temperature corresponding to the pressure you measure in our refrigerant pressure–temperature chart, and the bar–psi conversion in our unit converter. Always measure after the system has stabilised.