How to convert the pressure of a manometer to temperature?

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.

The theory

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)
17.51 0.020
24.10 0.030
28.98 0.040
32.90 0.050
36.18 0.060
39.02 0.070
41.53 0.080
43.83 0.090
45.83 0.100
60.09 0.200
69.13 0.300
75.89 0.400
81.35 0.500
85.95 0.600
89.96 0.700
93.51 0.800
96.71 0.900
99.63 1.000
100.00 1.013
102.32 1.100
104.81 1.200
107.13 1.300
109.32 1.400
111.37 1.500
113.32 1.600
115.17 1.700
116.93 1.800
118.62 1.900
120.23 2.000

 

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.

“169”] Here’s how the application looks
  • 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.

Like what

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.

How to convert the pressure of a manometer to temperature?
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