Aircon Pressure Readings: What a Number Needs Beside It

A manifold senses pressure and nothing else. Every other figure on the display is worked out from that measurement, using stored properties of the refrigerant. The instrument never sees inside the pipework; it reads a port and does arithmetic.

Pressure also has to be counted from somewhere, and two baselines are common. A gauge figure is counted from the air in the room; an absolute figure from a vacuum. Digital manifolds carry both as a display setting, so one condition can be written down as two different numbers.

Units multiply the problem. A manifold can show the same reading in psi, bar, or kilopascals, and the three look nothing like each other on a page. An owner holding reports from two firms may be looking at one condition twice, described in two units against two baselines, with neither convention written down.

None of that makes the instrument unreliable; it makes a bare figure hard to move. The number is true where it was taken, and it loses most of its meaning once copied onto a page with nothing around it.

Pressure and temperature move together whenever liquid and vapour sit side by side, which is the ordinary condition inside a working coil. Set the pressure and the temperature follows, because the refrigerant's own properties decide the pairing.

Every refrigerant has a different pairing. The same pressure therefore describes a different temperature depending on what is in the circuit, which is why an analogue gauge face carries several temperature scales around the dial, one per refrigerant. The dial is a property table wrapped into a circle.

Digital instruments make the link impossible to miss. A manifold has to be told which refrigerant it sits on before it shows any temperature, until then reporting no refrigerant selected. The testo digital manifold manual lists sixty to pick from, numbered under ISO 817.

The refrigerant identity is therefore part of the reading itself. A pressure quoted without it cannot be converted into a temperature by anyone who reads the report later; it stays a number about a pipe.

This carries a blunt commercial edge. A supplier unable to name the refrigerant behind a figure has performed a translation without naming the language. The figure may still be correct, but nobody downstream can prove it either way.

Some refrigerants are one compound and some are a blend. The difference shows up at the exact moment a pressure gets turned into a temperature.

A single compound boils and condenses at one temperature for a given pressure; for a single-component refrigerant, the temperature where boiling begins and where the last liquid disappears is the same.

Blends behave differently. Their components boil at different temperatures, so the liquid changes composition while it boils and the temperature climbs as the change of state proceeds. The gap between start and finish is called temperature glide.

Producer documentation names both ends of that gap. The bubble point is where liquid first begins to boil, and the dew point is where the last liquid has boiled away, which is also where vapour first condenses on the return journey.

A blend's published chart prints two temperature columns against a single pressure. Honeywell's chart for R-407C sets them side by side, headed Bubble and Dew. Which column belongs to which calculation is a diagnostic question, answered in the superheat and subcooling guide.

For a report, the consequence is narrow and easy to check. A saturation temperature taken off a blend, with no note of which end it came from, is ambiguous by the width of that glide. On wider blends the ambiguity exceeds the difference anyone is trying to measure, turning a precise-looking figure into a soft one.

Most systems sold in Singapore today sit on the easy side of this: their refrigerant gives one workable temperature for a given pressure, so the question rarely surfaces. It surfaces on older equipment and systems retrofitted at some point, where the paperwork tends to be thinnest already.

Manufacturers do not publish a correct pressure for a system. They publish curves, and the curve's shape is the whole point.

Mitsubishi Electric's service manual for its split systems plots condensing and suction pressure against outdoor air temperature. Each indoor dry bulb temperature gets its own line. The chart then states its conditions underneath: fixed indoor and outdoor humidity, and the fan set to high speed.

The same manual's standard operation data repeats the job in table form. Both pressures sit in one column of a wider table, beside the mode, the indoor and outdoor intake air temperatures in dry and wet bulb, the airflow, the pipe run length, and the refrigerant charge weight.

Read that list as a requirements list, because that is what it is: the maker will not state a pressure without stating all of it. A report that keeps the figure and drops the surroundings has kept the part that travels worst.

The curve also moves through the day, which is the practical half of this. A value correct at one outdoor temperature is out of range at another a few hours later, because the only thing that changed is the air the outdoor coil works against.

Singapore adds a wrinkle that is easy to overlook. A ledge in direct sun and a shaded corridor on the same block are different operating conditions, and a figure from one cannot be set against a figure from the other. Nothing on the page warns the next reader.

One more omission is almost as common. Plenty of reports quote the pressure without saying which side of the circuit it came from, as though the system carried a single value. Two figures exist while the unit runs, and they answer different questions; a report naming neither has described half of something.

Two sentences do most of the damage on real reports: pressure was normal, and pressure was low. Both are verdicts wearing the clothes of a measurement.

Five questions turn a quoted pressure into something checkable, and none need technical knowledge to ask. Each asks for something the technician already had that day.

Start with the refrigerant. Ask which one the manifold was reading against, and whether it matches the outdoor unit's own label. Then ask whether the figures are gauge or absolute, and in which unit they were written.

Ask what the air was doing at both ends while the readings were taken. Outdoor air at the condensing unit and return air at the room unit matter most, and both cost nothing to record.

The last question decides the value of the rest: what was the figure compared against, and where did that reference come from. A maker's document for that model is a real answer; a remembered figure from another job deserves to be labelled as what it is.

Push back when the only refrigerant-circuit evidence on a report is a pressure and a verdict. Nothing on that page can be checked afterwards, by a second opinion or by the same firm next visit. Approving refrigerant work against a number nobody can rebuild is how one fault gets paid for twice.

Answers to those five have a use beyond the argument at hand. A figure with its conditions attached can be handed to another firm without that firm attending first, making a second opinion cheap; a verdict on its own forces the whole visit to be repeated.

• •The refrigerant the instrument was set to, and the plate checked against
• •Whether each figure is gauge or absolute, and the unit used
• •Air temperature at both the outdoor unit and the indoor return
• •The mode the unit was in when the port was read
• •The reference figure used, and its document