Thermometers in the Column: How to Use and Read Them

Thermometers in a rectification column are essential tools for obtaining the purest alcohol. Only by detecting temperature changes can we discard unwanted fractions and control the process.

What Type of Thermometers to Use: Analog or Digital?

Analog thermometers are unsuitable for distillation; they have too low a resolution and poor accuracy, making it impossible to detect critical temperature increases of 0.1°C. They can be installed as decorative elements only. For process control, only electronic thermometers are suitable.

Should You Use a Regular Thermometer or Precise Models?

Better thermometers provide better control. Thermometers with DS probes have a resolution to two decimal places. This allows for detecting every temperature change in the column and gives time to react. The repeatability of measurements allows you to draw conclusions from subsequent processes, better understand your column, and the phenomena occurring within it. This makes rectification of similar batches more predictable in terms of quantities, times, collection rates, and overall operation, requiring less attention from the operator.

Working with standard thermometers is also possible but requires more time to monitor the column during the process.

How Many Thermometers to Use?

When choosing two LCD thermometers or a precise thermometer with DS probes, one probe is installed at the so-called 10th plate height (the first place from the bottom of the column where the temperature is stable), and the other under the head (to determine the strength when distilling fruit spirits, whiskey, etc.). If choosing one thermometer, it is installed approximately two-thirds up the column's height. The more thermometers, the greater the control over the process. The higher the resolution, the earlier you are informed of changes in the column.

In summary, the better the thermometers and the more measurement points, the easier the process.

Should Two Thermometers in the Reflux Still Show the Same Reading?

No—and moreover, they almost never will. It is perfectly normal for loose thermometers to show different readings—each may have differently placed sensors in the sleeve, varying amounts of thermal paste, different solder connection resistance—dozens of factors affect the display. This is absolutely irrelevant to working with the column because what matters is not the reading but its change (delta)—i.e., resolution, not accuracy.

What Temperature to Monitor in the Reflux Still?

Never aim for a specific temperature by adjusting heating power or water flow. This goes against physics and will never succeed. The "textbook" 78.3°C is the boiling point of pure ethanol at sea level with normal pressure (1013 hPa). The stabilization temperature of the column depends on many factors (atmospheric pressure, altitude, thermometer sensor and probe accuracy, etc.), and it is perfectly natural for the thermometer to show a temperature different from 78.3°C. The important thing is that the temperature remains stable (when collecting spirits). The process's key aspect is the temperature change, not its exact value.

Will the Displayed Stabilization Temperature Always Be the Same?

No. The stabilization temperature, like the boiling point, depends on atmospheric pressure, so a different stabilization temperature each day is natural.

Can the Stabilization Temperature Change During Distillation?

Yes. A sudden change in stabilization temperature is related to atmospheric pressure. Sometimes pressure fluctuations are very rapid—if the process lasts 5-6 hours, there can be several such changes. It is helpful to refer to a meteogram with a pressure forecast (e.g., from IMGiW: meteo.pl) before the process.

Do Thermometers Need to Be Calibrated Before First Use?

No. Moreover, you do not have the tools for this at home. Putting the probe in boiling water is pointless because water boils at 100°C only under the normal conditions described above. Without a thermostatic bath, which compensates for factors like current pressure, water rarely reaches exactly 100°C when boiling. The vessel's condition and even the limescale in the kettle can affect this phenomenon.

Additionally, such "calibration" is meaningless because, in the column, what matters is the resolution (i.e., change) of the temperature, not its value.