Why the Gauge Reads High or Low

A needle outside the marked zone isn't always a broken gauge. Often it's telling you something true about the coffee, the water supply or the machine. Sometimes it's the gauge. Work out which before you decide anything.

The coffee in the basket

The simplest cause sits inside the portafilter. One maker's manual says a needle in the lower zone means the espresso was extracted with too little pressure. It lists causes that include a grind that is too coarse, and too little coffee in the filter or too light a tamp¹.

The same manual says a needle in the upper zone means too much pressure, and it ties that to water flowing too slowly through the coffee¹. So a high or low reading can come from the puck, not the pump.

That's one reason to look at the needle during the middle of a shot and to change one thing at a time. When to look is covered in the notes on a shot's stages.

The pump

A pump can drift. One retailer's guide says a worn pump may be unable to regulate the pressure it was set to, and that this needs servicing or replacing². If the pump is the cause, a different grind won't fix it for long.

The setting itself can also differ. The same guide notes the brew pressure may have been deliberately set slightly higher or lower, and that some baristas adjust it on purpose². A machine that's been adjusted before you owned it may sit away from the textbook figure.

The water coming in

Some machines take their feed from the mains. In that case, the guide says, if the mains pressure has changed since the pump regulator was adjusted, the brew pressure will change too².

The boiler side

A brew needle can also climb above its working number after water has run through the group. The same guide explains that cold water entering the boiler warms and expands, raising boiler pressure, until a thermal expansion valve opens, usually set at around 12 bar². The page on the over-pressure valve goes further into that part.

The gauge itself

Gauges aren't perfect. A general reference says typical high-quality modern gauges provide an accuracy of ±1% of span for a 100 mm class 1 gauge³. That's a large dial built to a standard. Nothing in these sources says how close the small gauge on a home machine is.

A manual's own figure comes with a condition too. One maker gives about 9 to 9.5 bar for brewing. It adds that the statement is only accurate if all other factors are right⁴. A reading is a clue, not a verdict.

What to do about it

• Note when the reading is off: at idle, at the start of a shot, mid-shot, or all the time.
• Check what you can control from outside the machine, like grind and dose, one change at a time.
• Read your own manual for the right range and for what it says about adjustments.
• If the needle sits wrong in every shot, or won't move, stop there. Adjusting pump or valve settings means opening the machine, and that's for the maker or a qualified technician.

Pressure figures from other people's machines are poor guides to yours. Use your own manual's range.