Pressure, vacuum and gauges

Theo's first real lesson with the gauges came on a start-up. A new ductless heat pump had been installed in the attic of an old brick house, and the line set had been brazed and pressure tested with dry nitrogen. Dana connected the manifold, opened the nitrogen and stopped at a reading on the dial. "What is the pressure in that pipe?" Theo read the needle. Dana shook her head. "That is what the gauge says. The pressure in the pipe is more than that, because the gauge does not count the atmosphere. If you do not know which pressure you are looking at, you will make mistakes with every table and every chart in this trade."

This lesson covers what pressure is, the units used for it, the difference between gauge and absolute pressure, vacuum, and the gauges technicians use to read all of it.

What pressure is

Pressure is force spread over an area. A gas exerts pressure because its molecules are constantly striking the walls of its container; each impact pushes a little, and billions of impacts each second add up to a steady force. Heat the gas and the molecules move faster and hit harder, so the pressure rises. Squeeze the gas into a smaller space and more molecules hit each square centimetre, so the pressure rises again. A liquid exerts pressure because of its weight and because it transmits any pressure applied to it equally in all directions.

In SI units, pressure is measured in pascals (Pa), one newton per square metre. A pascal is very small, so the trade uses kilopascals (kPa), and sometimes bar (1 bar = 100 kPa) or megapascals (1 MPa = 1,000 kPa). In imperial units, pressure is measured in pounds per square inch (psi). One psi is about 6.895 kPa. Small pressures, such as the static pressure in a duct, are measured in inches or pascals of water column, and vacuums in inches of mercury or in microns.

Pressure can store a lot of energy, and the force it produces on a surface grows with the area. That is why a small leak at a gauge port is a hiss, but a failed end cap on a receiver or a cylinder valve knocked off can be lethal.

Atmospheric pressure

The air above us has weight, and at sea level it presses down with a standard atmospheric pressure of 101.325 kPa, which is 14.696 psi, or 29.92 inches of mercury. Atmospheric pressure falls with altitude, because there is less air above, and it varies a few per cent with the weather. Kitchener sits about 330 m above sea level, where the normal pressure is about 97 kPa (14.1 psi); in a mountain city at 1,600 m, it is about 84 kPa (12.2 psi).

A barometer measures atmospheric pressure. The classic one is a glass tube of mercury, closed at the top and standing in a dish: the atmosphere holds the mercury up to a height of about 760 mm (29.92 in) at sea level. That is where the units of inches and millimetres of mercury come from, and why vacuum is still measured in them.

Gauge pressure and absolute pressure

Most pressure gauges read zero when they are open to the atmosphere. They measure gauge pressure: the pressure above atmospheric. A tire gauge reading 220 kPa (32 psi) means the air in the tire is 220 kPa above the air outside. Absolute pressure is measured from a perfect vacuum, zero pressure. The two are related by the atmospheric pressure.

In imperial units, the letter after psi tells you which: psig is gauge, psia is absolute. In SI, the trade writes kPa (gauge) or kPa (abs), or kPa(g) and kPa(a). In this course, pressures marked gauge are gauge pressures, and pressures marked abs are absolute. Refrigerant property tables and the pressure-enthalpy diagrams in Lesson 9 use absolute pressure; most gauges and pressure-temperature charts printed for technicians use gauge pressure. Always check which one a table uses.

Compression ratio is one example of a calculation that must use absolute pressure. The gas laws, refrigerant tables and compressor data all do too. The rule is simple: whenever pressures are multiplied, divided or used in a formula with other properties, convert them to absolute first.

Vacuum

A vacuum is any pressure below atmospheric. In the trade, vacuums matter in two places: some low-temperature systems run with their suction side below atmospheric pressure, and every system must be evacuated, pumped down to a deep vacuum, before it is charged with refrigerant. Evacuation removes air, which would raise head pressure and carry oxygen into the system, and moisture, which would react with refrigerant oil to form acids, freeze in metering devices and corrode parts.

Vacuum is measured in several ways. Compound gauges, the low-side gauges on a manifold, read below zero in inches of mercury vacuum, from 0 at atmospheric pressure to about 30 at a near-perfect vacuum. That scale is too coarse for evacuation: the last few thousandths of an inch of mercury are where all the moisture is. A micron gauge measures absolute pressure in microns of mercury, where one micron is a thousandth of a millimetre of mercury. Atmospheric pressure is 760,000 microns. A common target for a finished evacuation is 500 microns or lower, held steady when the pump is isolated, but always follow the equipment manufacturer's instructions.

The table shows why a micron gauge is essential. Water boils at room temperature when the pressure falls below about 17,500 microns, and the vacuum pump removes moisture by boiling it off. At 500 microns, water boils far below its freezing point, so even moisture in cold corners of the system is pulled out as vapour. A compound gauge cannot tell 17,500 microns from 500: both read nearly 30 inches of vacuum.

# Gauge, absolute and vacuum conversions (SI with psi and inHg alongside)
KPA_PER_PSI = 6.895
KPA_PER_INHG = 3.386
MICRONS_PER_KPA = 7500.6     # 1 micron of mercury = 0.13332 Pa

def atm_kpa(elevation_m):
    # standard atmosphere model, good to a fraction of a per cent below 3000 m
    return 101.325 * (1 - 2.25577e-5 * elevation_m) ** 5.25588

p_atm = atm_kpa(330)          # about the elevation of Kitchener
print(f"atmospheric pressure at 330 m: {p_atm:.1f} kPa abs = {p_atm / KPA_PER_PSI:.2f} psia")

for gauge_kpa in [2600, 820, 0]:
    absolute = gauge_kpa + p_atm
    print(f"{gauge_kpa:5d} kPa gauge -> {absolute:7.1f} kPa abs ({absolute / KPA_PER_PSI:6.1f} psia)")

for inhg_vac in [15, 25, 28.5]:
    absolute = p_atm - inhg_vac * KPA_PER_INHG
    print(f"{inhg_vac:5.1f} inHg vacuum -> {absolute:6.2f} kPa abs = {absolute * MICRONS_PER_KPA:8.0f} microns")

atmospheric pressure at 330 m: 97.4 kPa abs = 14.13 psia
 2600 kPa gauge ->  2697.4 kPa abs ( 391.2 psia)
  820 kPa gauge ->   917.4 kPa abs ( 133.1 psia)
    0 kPa gauge ->    97.4 kPa abs (  14.1 psia)
 15.0 inHg vacuum ->  46.63 kPa abs =   349775 microns
 25.0 inHg vacuum ->  12.77 kPa abs =    95805 microns
 28.5 inHg vacuum ->   0.92 kPa abs =     6915 microns

The program uses the local atmospheric pressure rather than the sea-level standard, which matters at altitude: a compound gauge reads vacuum relative to the local atmosphere, so at Kitchener's elevation it can never read more than about 28.8 inches of vacuum. On the vacuum side, the deepest reading in the list shows the problem again: 28.5 inches of vacuum on a compound gauge is still several thousand microns, far from a finished evacuation.

Pressure in liquids

A column of liquid exerts pressure because of its weight: the pressure at the bottom is the density times the acceleration due to gravity times the height. This matters in refrigeration when liquid refrigerant must rise from a condenser on the ground to an evaporator on an upper floor. Liquid refrigerant has a density of roughly 1,000 to 1,200 kg/m³, so a lift of 6 m costs roughly 60 to 70 kPa of liquid line pressure, which can be enough to start the liquid flashing into vapour before it reaches the metering device.

Lesson 7 shows why this matters: the liquid must stay subcooled all the way to the metering device, and a lift, a long run or a restriction can use up the subcooling.

Gauges and manifolds

The traditional analog gauge uses a Bourdon tube: a curved, flattened metal tube that straightens slightly as pressure inside it rises, moving a needle through a linkage. It reads gauge pressure, and many refrigeration gauges also carry printed temperature scales for common refrigerants, which Lesson 5 explains. Digital gauges use electronic pressure sensors, read to finer resolution, carry pressure-temperature tables for dozens of refrigerants and often calculate superheat and subcooling with clamp-on temperature probes.

A gauge manifold connects the gauges to the system. The low-side gauge, traditionally blue, is a compound gauge that reads both vacuum and pressure and connects to the suction side. The high-side gauge, traditionally red, reads higher pressures and connects to the discharge or liquid side. The centre hose, traditionally yellow, connects to a vacuum pump, recovery machine or refrigerant cylinder. Hand valves on the manifold open each side to the centre hose; with both closed, each gauge simply reads its own side of the system. Hoses carry low-loss fittings or ball valves so that only a small amount of refrigerant escapes when they are disconnected.

Gauges have accuracy limits, and analog gauges drift. A gauge should read zero when open to the atmosphere; if it does not, it needs recalibrating, which on analog gauges is done with an adjusting screw. Hoses and gauges must be rated for the pressures of the refrigerant: gauges and hoses for older, lower-pressure refrigerants are not safe on R-410A or carbon dioxide systems. Rules for connecting and disconnecting without releasing refrigerant are part of RAC-120.

Pressure safety

Pressure is the hazard a refrigeration technician lives with every day. Cylinders must never be heated with a torch or left in a hot vehicle, never be filled beyond their rated fill, and always be secured upright. Pressure testing new piping is done with dry nitrogen through a regulator, never with oxygen, which explodes in contact with oil, and never with compressed air or refrigerant. Systems have pressure relief devices, and a technician never defeats a high-pressure control or relief valve to get a system running. Eye protection and gloves are worn whenever hoses are connected or disconnected, because liquid refrigerant flashing to vapour at atmospheric pressure can be cold enough to freeze skin and eyes instantly.