As I explained briefly in my previous article (which explains how to recover the pressure of extraction on a lever machine), it is possible to know the water temperature of a boiler by measuring the water pressure.
Table of contents (Table des matières)
So in the case of the use of a boiler (and in my case, the boiler of a coffee machine), it is to find precisely the temperature of the water knowing its altitude and its pressure. This relationship can be calculated using the Clapeyron formula.
T: Temperature ∆ V: Variation of the molar volume during phase L change: latent heat equal to the change of enthalpy to the changing state, at constant pressure
Using this formula, I was able to build a water temperature conversion chart compared to the bar pressure observed in the boiler of the Pavoni coffee machine. This table was made for an altitude of 40m (at sea level).
|Temperature (°c)||Pressure (bar)|
When you buy coffee from your roaster you can ask at what ideal temperature can be extracted such or such coffee. For some coffee it will be rather 88 °c while for others it will be rather 92 °c. Using the table and graph below, you can easily find the ideal temperature on your machine.
- In the menu “normal boiling Point in °c” The boiling temperature of the water must be indicated according to your altitude (e.g. 100 °c if you are at sea level or 95 °c if you are at 1500m).
- In the “mbar/hPa Pressure” section, the pressure reading on your manometer must be indicated. For example if the manometer indicates 0.750 bar It is necessary to enter the application 750 mbar.
- In the last menu: “Temperature in °c“, the temperature of your boiler will be displayed when you click on the “Temperature estimation” button.
In the two images above you can see the difference:
- In the first image, I calculated the water temperature in a boiler at 750 mbar of pressure. In this case I indicated a boiling reference value of 100 °c for an altitude of 40m. The boiler temperature is 89,6 °c
- On the first image, I calculated the water temperature in a boiler at 750 mbar of pressure. In this case I indicated a boiling reference value of 100 °c for an altitude of 1500m. The boiler temperature is 84,74 °c
It is also possible to search for the value in mbar using a desired temperature. In short, for people looking to control the temperature of a coffee machine boiler using a manometer, this application is really convenient. I made myself a table of certain temperatures used and I noticed them on a sheet that I leave next to my coffee machine.