The Heat is on – Measuring Hot Fluids

The purpose of this technical bulletin is to answer the question of how temperature affects the readings on the Palm Abbe digital refractometer and how to get the most accurate possible readings.

Refractive index is a physical property of a substance and is related to the speed at which light passes through it. The denser the substance the slower the light will travel through it and the higher its reading will be on the refractometer. However, as the temperature of a substance increases, its density decreases, and so does its refractive index.

The change in refractive index with temperature is rather small for solids, like glass, but is quite large for liquids, which are VERY temperature dependent. Temperature is one of the single most important factors influencing accurate refractometer readings and is one of the largest sources of error in measurement.

To make comparisons between refractive index measurements meaningful, the measurements must be referenced to a particular temperature. For aqueous (water-based) fluids this reference temperature is usually 20 °C.

However, there is not a simple linear (straight-line) relationship between temperature and refractive index. Not only is the temperature effect different for most solutions but it also differs within a specific solution based on both its concentration and temperature.

To make a refractometer measurement meaningful, some form of temperature compensation or control must be employed to adjust a raw index measurement back to the reference temperature.

With out compensation or control, it would be necessary to measure the temperature of the fluid and make a manual adjustment to the reading. Temperature compensation relieves the user of the responsibility for making manual corrections.

The chart below compares a temperature compensated MISCO Refractometer (red) to a non-compensated refractometer (blue). It is plain to see that at 20 °C (68 °F) they are both correct; however, as the temperature deviates from 20 °C, there is a marked error in the non-compensated instrument and the further the temperature is from 20 °C the larger the error.

 

Non-Linear Compensation

The MISCO Palm Abbe has a precision solid-state thermistor that is in contact with the measuring prism. This thermistor is able to accurately sense the exact temperature of the optics. An internal microprocessor interprets the signal from the thermistor and applies the appropriate correction factor for the specific fluid, concentration, and temperature.

The Palm Abbe can be programmed with multiple temperature correction tables, for individual scales, over a temperature range of 0 to 50°C (32 to 122 °F). One of the most amazing features of the Palm Abbe is that the compensation is non-linear, meaning that the Palm Abbe is able to discriminate and apply different corrections for different concentrations and temperatures.

However, no matter how advanced the technology, accuracy of measurement really comes down to common sense. In order for temperature compensation to work properly, the temperature of the instrument, the fluid, and the surrounding environment must all be in equilibrium within the temperature compensation range of the instrument.

This can not be overstated. If the ambient temperature is at one extreme, the fluid temperature at the other extreme, and the instrument somewhere in the middle, the instrument will not be able to accurately apply a correction.

Error in measurement is most likely to occur at or near the extremes of the measurement range or the temperature range of the instrument. The farther the temperature is from 20 °C, the larger the temperature correction must be and the more chance there is for error.

Measuring Hot Fluids

When a fluid sample in excess of 50 °C (122 °F) is applied to the sample well of the Palm Abbe, the instrument optics are shocked by the instant temperature change. However, the thermal mass of the instrument is larger than the thermal mass of the fluid so the temperature changes rapidly as the instrument and the fluid try to reach temperature equilibrium. You can see from Figure #1 that in the first 45-60 seconds after applying a hot fluid sample to the refractometer that there is a rapid drop in fluid temperature.

Depending on the temperature difference between instrument and the sample, the rate of change (temperature vs. time) can be very steep in the first seconds. Although the instrument has automatic temperature compensation, the rate of change may be so steep that the Palm Abbe may display a temperature error or it may not be possible to get a stable repeatable reading for as long as 60 seconds. Thin low viscosity fluids generally give up their heat and stabilize much quicker than thick viscous fluids.

Summary

The closer the fluid, the instrument, and the ambient temperature are to 20 °C, the more accurate the readings will be since a smaller temperature correction will have to be applied. A good rule of thumb is to wait at least thirty seconds for every 20 °F (10 °C) difference in temperature between the fluid and the instrument. You will have to allow more time for fluids above 50 °C and for thick viscous fluids.

Another suggestion is that you can press the <GO> button (<READ> on model PA201) repeatedly until the reading stabilizes. If the reading changes with each press of the button it more than likely means that the instrument has not reached equilibrium and the temperature is still changing. Once equilibrium is reached the readings should be very repeatable.