Pressure converter

How it works

Every conversion goes through absolute pressure (psia). The tool turns your value into psia, then writes it out in every unit.

• Gauge units (psig, kPa gauge, bar gauge) are measured from local air pressure. Absolute = gauge + local atmosphere.
• Absolute units (psia, kPa absolute, bar absolute, MPa absolute, microns) are measured from a perfect vacuum.
• Inches of mercury vacuum is how far below local air pressure you are. 0 in Hg means atmospheric pressure. At sea level, a perfect vacuum is 29.92 in Hg, which equals 14.696 psi.

The factors are 1 psi = 6.894757 kPa = 0.06894757 bar = 2.036 in Hg = 51,715 microns. A micron is a micrometer of mercury, one thousandth of a millimeter of mercury. Sea-level atmosphere, 14.696 psia, is 760 mm Hg or about 760,000 microns.

The elevation field sets the local air pressure from the standard atmosphere: 14.696 psia at sea level and about 12.23 psia at 5,000 ft. It changes the gauge and in Hg results, and the absolute value behind a gauge entry. It does not change one absolute unit converted to another.

Rows that do not apply show a dash. In Hg vacuum appears only below local air pressure, and microns only below 1 psia. A value below a perfect vacuum gets an error message.

Worked example

118 psig at sea level (the default):

At 5,000 ft the same 118 psig reading is 118 + 12.23 = 130.23 psia. That is why the PT calculator asks for elevation: manufacturer PT charts print psig on a sea-level basis, as the Chemours guide states.

500 microns:

• 500 ÷ 51,715 = 0.00967 psia.
• At sea level: (14.696 − 0.00967) × 2.036 = 29.90 in Hg vacuum.
• At 5,000 ft: (12.23 − 0.00967) × 2.036 = 24.88 in Hg vacuum.

Why evacuation is measured in microns

Microns are absolute, so 500 microns means the same thing at the coast and in the mountains. A compound gauge reads in Hg vacuum relative to local air. In the example above, the same 500 microns reads 29.90 in Hg at sea level and 24.88 in Hg at 5,000 ft. Near a full vacuum the in Hg scale is also far too coarse: the whole range from 500 microns to a perfect vacuum is 0.02 in Hg.

OEM evacuation targets are typically 350 to 500 microns, and decay criteria vary by manufacturer. Rheem, for example, requires holding 500 microns or lower for at least 15 minutes. See evacuation and microns.

Recovery uses the in Hg scale. 40 CFR 82.156 Table 1 sets required recovery levels for equipment made on or after November 15, 1993. High-pressure appliances under 200 lb of charge go to 0 in Hg and those of 200 lb or more to 10 in Hg. Medium-pressure appliances go to 10 in Hg under 200 lb and 15 in Hg at 200 lb or more. Very high-pressure appliances go to 0 in Hg. Low-pressure appliances go to 25 mm Hg absolute, which is 25,000 microns, 0.483 psia, or 28.94 in Hg vacuum at sea level.

Reading the result

• psig vs psia. Refrigerant gauges and PT charts use psig. Absolute pressure is used in property data and in the Section 608 pressure classes, which are defined by liquid saturation pressure at 104°F in psia.
• kPa and bar. Check whether a metric chart is gauge or absolute. The two differ by about 101 kPa (1.01 bar) at sea level.
• Nitrogen test pressures given in bar or kPa can be converted here. Check whether the figure is gauge or absolute. See the nitrogen pressure test guide.

Common mistakes

• Adding 14.7 at altitude. Local air pressure at elevation is lower. Enter the elevation.
• Treating in Hg as absolute. It is relative to local air, so the same vacuum reads differently at different elevations.
• Judging an evacuation on a compound gauge. Use a micron gauge.
• Mixing gauge and absolute metric units. “bar” alone is ambiguous.
• Trusting a gauge beyond its accuracy. CEC requires digital refrigerant gauges accurate to ±3.5 psi on the suction side and ±7.0 psi on the liquid line for charge verification (RA3.2).