Bar, PSI and the Colored Zones on the Dial

Look at the printed units before you look at the needle. Gauges can show bar, psi or something else, and a dial with two scales makes it easy to read the wrong ring.

The two units

The bar is a metric unit of pressure¹. One bar is slightly less than the average air pressure at sea level¹. Psi stands for pounds per square inch. It's used mainly in the United States². If your machine was made for the US market, you may well see it on the dial.

Both describe the same thing, so a manual may quote a figure in either. As one example, a reference for lever machines gives a spring pressure of "usually 8 to 10 bar", or 116 to 145 psi³. The same range, written two ways.

The scale doesn't stop at the working range

A gauge scale usually runs well past the pressure you'd normally see. One maker describes a brew gauge scaled from 0 to 16 bar, and a boiler gauge scaled from 0.5 to 2.5 bar⁴. The boiler dial is zoomed in on a small range because the boiler pressure it watches is small.

This is why you can't compare two machines by where the needle sits on the face. Different dials, different scales. Always go back to the numbers.

The colored zones

Some gauges skip fine numbers and use colored bands instead. One maker's manual describes a dial with a low zone for pre-infusion and a middle zone it calls the ideal espresso zone. It adds a lower zone for too little pressure and an upper zone for too much⁵.

Take the wording seriously. The same manual says a needle in the middle zone "indicates the espresso will be extracted at the ideal" pressure⁵. That's the maker describing its own machine and its own dial. It isn't a rule for every machine.

The manual also says a needle in the lower zone can come from a grind that's too coarse, among other causes⁵. So the zone tells you something about the shot, not only about the pump.

Numbers and bands together

Some dials give you both. The colored band is the quick glance, and the figure is the check. If the two disagree with what your manual says, believe the manual.

One manual for a numbered brew gauge gives a range of about 9 to 9.5 bar while brewing⁴. Notice the condition: it holds only if other factors are right⁴. A figure like that belongs to that machine, set by its maker.

What to check on your own dial

• The unit printed on the face, and whether there are two scales.
• Where the scale starts. Some boiler gauges don't start at zero.
• Whether the dial has bands, numbers or both.
• What your manual says each band or mark means for your model.

Boiler dials and brew dials answer different questions. Which one you're looking at is covered in the page on boiler and brew gauges.