Technical definition
Refractive Index is a measurement of how much light changes direction, or refracts, as it passes through the heating-system fluid. It is expressed as a ratio and is therefore normally dimensionless, rather than being reported in units such as mg/L.
The refractive index of a fluid changes according to its composition and the concentration of dissolved substances within it. In heating and cooling systems, it is particularly useful for assessing fluids containing glycol-based antifreeze or heat-transfer products, because increasing glycol concentration produces a predictable change in refractive index when measured under controlled conditions.
Fernox uses refractometry specifically for glycol-containing systems. Its Refractometer measures refracted light from glycol-based antifreeze solutions, while Fernox heat-transfer fluids use refractometer readings to check the concentration and corresponding level of frost protection. (Fernox)
Simplified definition
Refractive Index measures how light bends when it passes through the system fluid.
For heating systems containing glycol or antifreeze, this measurement can be used to help determine how concentrated the glycol is and therefore whether the system has the expected level of frost protection.
Refractive Index is particularly important in systems containing glycol-based heat-transfer fluids, such as heat pumps, underfloor heating, solar thermal systems or other installations requiring frost protection.
Fernox states that frost-protection levels in its glycol-based heat-transfer fluids can be checked using a refractometer. For example, Fernox HP-5c is designed to operate at defined concentrations, with increasing product concentration providing progressively greater frost protection.
An abnormal or unexpected Refractive Index result may indicate:
Insufficient glycol concentration, resulting in inadequate frost protection.
Excessive glycol concentration compared with the intended system specification.
Dilution caused by topping-up the system with water.
Loss of glycol-containing fluid followed by replacement with untreated water.
Incorrect initial mixing or dosing of concentrated heat-transfer fluid.
Introduction of another fluid or chemical that has changed the composition of the system water.
Changes in the heat-transfer fluid since a previous test.
A concentration that does not match the intended product specification.
Maintaining the correct glycol concentration is important because an insufficient concentration can leave a system vulnerable to freezing during low-temperature conditions. This is particularly important for external or partially external equipment such as air-source heat pumps and solar thermal installations.
Inadequate frost protection can potentially contribute to:
Freezing of water within exposed pipework or components.
Expansion-related damage to heat exchangers and pipework.
Cracking or leakage.
Loss of circulation.
Damage to pumps and other components.
System shutdown or failure during cold weather.
However, more glycol is not automatically better. Heat-transfer products are designed to operate within specified concentration ranges. Fernox HP-5c, for example, specifies an in-use concentration range and corresponding frost-protection levels, demonstrating why the refractive-index result needs to be interpreted against the particular product being used.
Refractive Index can therefore provide a quick and valuable means of checking whether the circulating fluid remains consistent with the expected glycol concentration and frost-protection requirement.
Refractive Index should not normally be assessed in isolation, and there is no single acceptable refractive-index value applicable to every heating system.
The correct value depends heavily on:
The type of glycol or heat-transfer fluid used.
The intended product concentration.
The required frost-protection temperature.
The manufacturer's specification.
The temperature at which the measurement is taken.
Other dissolved chemicals present within the fluid.
Different fluids can have different refractive-index characteristics, meaning a particular reading cannot simply be converted into a universal glycol percentage without knowing the fluid type and appropriate calibration relationship.
For example, Fernox Solar S1 is a premixed monopropylene-glycol heat-transfer fluid with a specified refractive index of 1.381–1.385, while other Fernox heat-transfer fluids are supplied at different concentrations and specifications.
It is also important to distinguish between Refractive Index and Glycol Concentration:
Refractive Index is the physical property actually measured.
Glycol Concentration can be calculated or estimated from that measurement when the type of fluid and its calibration characteristics are known.
Frost Protection can then be determined from the concentration according to the specific heat-transfer fluid manufacturer's data.
Refractive Index should therefore be interpreted alongside the known heat-transfer fluid, intended concentration, frost-protection requirement, product specification and previous test results.
