How is compression ratio calculated?

Compression ratio is the absolute discharge pressure divided by the absolute suction pressure. Both gauge readings must first be converted to absolute by adding atmospheric pressure, since the ratio is meaningless using gauge pressure that references zero at atmosphere.

What is a high compression ratio?

For typical reciprocating and scroll compressors, a ratio under about 10 to 1 is normal, 10 to 12 is elevated, and above 12 is high. High ratios raise discharge temperature, break down oil, and reduce volumetric efficiency, shortening compressor life.

What causes a high compression ratio?

Either low suction pressure or high discharge pressure. Low suction comes from a restriction, low charge, or a dirty evaporator; high discharge comes from overcharge, a dirty or undersized condenser, non-condensables, or high ambient temperature.

Does altitude change the calculation?

Yes. At higher altitude atmospheric pressure is lower, so the same gauge readings give different absolute pressures and a slightly different ratio. Set the atmospheric field to your local value for an accurate result.

Refrigeration System Compression Ratio Calculator

Q: Why convert gauge pressure to absolute?

Gauge pressure reads zero at atmospheric, but compressors work against true absolute pressure. Adding atmospheric pressure, about 14.7 psi at sea level, converts psig to psia so the ratio reflects the real pressure rise across the compressor.

Compression ratio is one of the fastest single numbers for judging how hard a compressor is working and whether something is wrong. This tool takes your gauge readings, converts them to absolute pressure, and reports the ratio with a plain diagnosis of whether it is safe.

How it works

The ratio is built from absolute pressures, not gauge pressures:

suction absolute   = suction psig + atmospheric psi
discharge absolute = discharge psig + atmospheric psi
compression ratio  = discharge absolute ÷ suction absolute

Adding atmospheric pressure (about 14.7 psi at sea level) is essential. A suction of 30 psig and discharge of 250 psig is not 8.3 to 1 on gauges but rather 264.7 divided by 44.7, about 5.9 to 1 in absolute terms.

Reading the result and tips

A normal system usually runs under about 10 to 1. Above 12 to 1 the compressor runs hot, oil starts to break down, and the pump moves less refrigerant per stroke. If the ratio is high, decide whether suction is too low (restriction, undercharge, airflow over the evaporator) or discharge is too high (overcharge, dirty condenser, non-condensables, high ambient) and correct that side. At high altitude, lower the atmospheric value so the ratio stays accurate.