Mastering water temperature in an espresso machine boiler is essential for achieving perfect extraction. Thanks to the Clapeyron formula, it is possible to convert the pressure measured by a gauge into water temperature, and thus optimize your machine's performance.

This article offers you a clear explanation of the principle, a precise conversion table, as well as a practical method for applying these calculations on a daily basis.

As I briefly explained in my previous articleMeasuring and Understanding Pressure on a La PavoniMaster water pressure management on your La Pavoni coffee machine to optimize your espresso extraction., it is possible to determine water temperature by measuring the boiler pressure.

Dual gauge
Example of a dual gauge: at the top the boiler pressure, at the bottom the extraction pressure.

Basic principle:

  • Water boils at 100 °C at an atmospheric pressure of 1.013 bar (sea level).
  • The higher the altitude, the lower the atmospheric pressure, and therefore the boiling temperature decreases (e.g. at 6000 m: 80 °C at 0.471 bar).

The theory​

In the case of a boiler (e.g. espresso machine), the water temperature can be determined by knowing the altitude and internal pressure.
This relationship is calculated using the Clapeyron formula:

$$ dp = \frac{L}{T (\Delta V)} \times \frac{dT}{T} $$

Legend:

  • T: Temperature
  • ∆V: Variation in molar volume during phase change
  • L: Latent heat (enthalpy variation at constant pressure)

Using this formula, I created a pressure/temperature conversion table for an altitude of 0 m (sea level).

tip

Barista tip: ask your roaster for the ideal extraction temperature for your coffee. Some coffees are better at 88 °C, others at 92 °C.

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Water Boiling Temperature According to Altitude​

Boiling table according to altitude
Reading: at 1800 m, water boils at 94 °C.
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Practice: simple method​

For everyday use, a free Android app, Nomograph, automatically calculates your boiler temperature.

Nomograph application
Nomograph application interface

Instructions:

  1. In "Normal Boiling Point in °C", enter your water's boiling temperature according to altitude (100°C at 0 m, 95°C at 1500 m).
  2. In "Pressure in mbar/hPa", enter the pressure reading from the gauge (e.g. 0.750 bar → 750 mbar).
  3. Click on "Temperature Estimation" to display the actual temperature.

Calculation Example​

Calculation at 0m altitude
750 mbar → 89.6°C at 0 m
Calculation at 1500m altitude
750 mbar → 84.74°C at 1500 m

You can also do the reverse: enter a desired temperature to get the corresponding pressure.

tip

For precise control, note your reference temperatures on a sheet next to your machine. This will allow you to quickly adjust your pressure for each espresso.