Millimeter Water (4 °C) to Inch Mercury

HVAC technicians originally measured duct pressure with a simple U-tube manometer filled with water — you literally read the height difference in millimeters. One mmH₂O ≈ 9.81 Pa, so a typical 25–250 mmH₂O duct pressure range corresponds to 245–2,450 Pa. The water column scale is still used because the instruments are cheap, intuitive, and field-rugged, even though digital gauges now display the same numbers electronically.

Water reaches maximum density at 3.98 °C (roughly 4 °C), where one cubic centimeter weighs exactly 1 gram. Specifying 4 °C ensures the pressure per millimeter of column height is reproducible and standardized. At 20 °C, water is about 0.2% less dense, introducing a tiny error. For most HVAC and lab work the difference is negligible, but calibration labs insist on the 4 °C reference for traceability.

Connect one side of a U-tube to the duct and leave the other open to atmosphere. The water level drops on the pressurized side and rises on the open side. The total height difference in millimeters is the gauge pressure in mmH₂O. Inclined (slant) manometers amplify small readings by tilting the tube — a 10:1 slope makes each millimeter of travel represent 0.1 mmH₂O, improving resolution for filter pressure-drop testing.

A clean residential furnace filter creates 12–50 mmH₂O of pressure drop. When the drop exceeds 125–250 mmH₂O (varies by manufacturer), the filter is restricting airflow enough to hurt efficiency and strain the blower motor. Commercial systems set alarms at specific mmH₂O thresholds — when the differential pressure sensor hits the limit, a "replace filter" indicator lights up on the building management system.

1 inch of water = 25.4 mmH₂O (since 1 inch = 25.4 mm). US HVAC specs use inches of water gauge (often written "in. w.g."); European and Asian specs use mmH₂O. If a US furnace manual says "maximum 0.5 in. w.g. static pressure," that is 12.7 mmH₂O. The conversion is just the familiar inch-to-millimeter factor applied to a column of water.

The US National Weather Service inherited the convention from early American meteorology, which used mercury barometers calibrated in inches. A typical sea-level reading of 29.92 inHg is easy to remember and fits weather maps without decimal clutter. Most other countries switched to millibars or hectopascals, but the US stuck with inHg for the same reason it kept Fahrenheit — familiarity and institutional inertia.

US air traffic controllers broadcast the local barometric pressure in inches of mercury — for example, "altimeter two niner niner two" means 29.92 inHg. Pilots dial this into their altimeter so the instrument reads correct altitude above sea level. If the setting is wrong by just 0.1 inHg, the altimeter reads roughly 100 feet off — enough to matter during instrument approaches in fog.

At sea level, 29.92 inHg is standard. Readings above 30.20 inHg are high-pressure (clear skies, calm winds). Below 29.50 inHg is considered low pressure and often signals approaching storms. The lowest sea-level pressure ever recorded was Typhoon Tip in 1979 at 25.69 inHg (870 mbar). A household barometer swinging from 30.50 down to 29.30 is a reliable sign that weather is deteriorating.

Refrigeration techs evacuate AC system lines to remove moisture before charging with refrigerant. They measure the vacuum in inHg below atmospheric pressure — a reading of 29 inHg (out of 29.92 max) means near-total vacuum. Industry best practice requires pulling to at least 29.92 inHg (or equivalently, below 500 microns on a micron gauge) to ensure all moisture has boiled off at room temperature.

1 inHg ≈ 33.86 mbar ≈ 0.491 psi. So standard atmosphere (29.92 inHg) is about 1013 mbar or 14.7 psi. For quick mental math: multiply inHg by 34 to get millibars, or divide by 2 to get a rough psi estimate. These conversions come up constantly when comparing US weather data with international sources or converting aviation altimeter settings for foreign aircraft.