Checking a refrigerant charge without a scale — is that possible?

Assessing a refrigerant charge without an electronic charging scale at hand — it can be done, and most experienced installers use these techniques daily as an additional check alongside (not instead of) weighing. This article lists the three most commonly used measuring methods — pressure/temperature tables, superheat/subcooling and the sight glass — and explains where their limits lie.

Why you normally weigh anyway

For record-keeping, weighing remains the norm. Under BRL 100, refrigerant scales must have a minimum accuracy of 0.1 kg for cylinders under 30 kg and 1 kg for cylinders of 30 kg or more (source: brl100.nl (opent in nieuw tabblad)). You need those kg figures for the change entry in your refrigerant balance — the methods below tell you something about the quality of the charge, but do not deliver an exact weight that you can put straight on a work order.

Method 1: pressure/temperature tables (saturation curve)

Every refrigerant has a fixed relationship between pressure and saturation temperature — recorded in a pressure/temperature table or app per refrigerant type. By looking up the measured pressure, you know at what temperature the refrigerant should be evaporating or condensing at that point.

This table alone says nothing about the amount of refrigerant in the system — it only tells you whether the measured temperature matches the measured pressure. Charging purely "by pressure" is therefore insufficient, because pressure is strongly influenced by ambient temperature and the load on the system. The table is the basis you then use to calculate superheat and subcooling (see below) — only that combination says something about the charge itself.

Note with blends: refrigerants such as R-410A have a small temperature glide — an evaporation or condensation range instead of a single fixed point. So always use the table or app made specifically for the refrigerant type present, not a table for a different refrigerant.

Method 2: measuring superheat and subcooling

This is the most commonly used practical method for assessing a charge without weighing:

• Superheat — the difference between the actual suction gas temperature and the saturation temperature at the measured suction pressure. Prevents liquid from still reaching the compressor ("wet running").
• Subcooling — the difference between the saturation temperature at the measured condensing pressure and the actual liquid temperature after the condenser.

For systems with capillary injection or a DX system without a liquid receiver, the rule of thumb is: if the superheat is good and there is some subcooling, the system is properly charged. If there is a lot of subcooling, the system is overcharged (source: Koudecentraal — Subcooling and superheat (opent in nieuw tabblad)).

The exact target values for superheat and subcooling differ per system type, expansion device and manufacturer — a universal "good number" does not exist. If in doubt, check the type plate or the installation manual of the specific system instead of relying on a rule of thumb from a different project.

Method 3: the sight glass

A sight glass in the liquid line lets you watch the flow: a clear, bubble-free liquid column indicates an adequately charged system, while dense or persistent bubbles point to too little refrigerant or insufficient subcooling (source: Miracle Refrigeration — Refrigerant Sight Glass (opent in nieuw tabblad)).

Two caveats:

• Not every system has one. Many smaller split units and heat pumps are not fitted with a sight glass as standard.
• Less clear-cut with blends that have temperature glide. Because a gliding refrigerant evaporates or condenses over a temperature range instead of at a single fixed point, the "bubbles to clear" transition point is in practice less sharp than with a single-component refrigerant such as R-32. So use the sight glass with blends as a first indication, and confirm with the pressure/temperature and superheat/subcooling method.

What these methods do and do not replace

All three methods say something about the quality of the charge — whether the system is running within the expected margins, whether there is a risk of wet running, whether it is overcharged. None of them delivers the exact kg figure you need for your records. For that record-keeping, a calibrated scale remains the starting point; see calibration of measuring equipment BRL 100 for the requirements around that. In practice, most installers combine both: weighing for the exact weight on the work order, and pressure/temperature/superheat/subcooling as a functional check that the charge is actually correct.

Further reading

• Refrigerant balance: a practical guide — why the exact kg figure per change entry remains necessary
• Calibration of measuring equipment BRL 100 — requirements for scales, manifolds and thermometers
• R-32 vs. R-410A vs. R-454B — differences between single-component refrigerants and blends

Disclaimer: this article describes practical measuring methods and does not replace calibration or record-keeping obligations under BRL 100 or the F-gas Regulation. Target values for superheat and subcooling differ per system — if in doubt, consult the installation manual or get in touch via /contact.