Why do HX machines use a pressurestat instead of a thermostat?

[maximatica]

Just saw a cutaway of a low end machine and it showed 2 thermostats (brew and steam). So I wondered why higher end machines use pressurestats? Adjustability? Reliability? Tighter deadband? All of the above? Is there any drawback to a pressurestat vs. a thermostat?

Thanks for any info.

M./

[oofnik]

From what I understand, tighter deadband and adjustability are the major two reasons why pressurestats are used instead of thermostats. They are not really any more reliable than thermostats, however; it's just become the standard method of regulating boiler temperature. I'm sure thermostats could be made with tighter deadbands, but it's just inherently easier to do with pressure.

[2xlp]

i believe the pressurestat is a more uniform measure too -- pressure is consistent throughout the interior of the boiler.

with a thermostat, you have currents, hot areas, and the heat of the boiler itself affects the temp of an internal reading.

[Genesis]

Deadband and uniformity of measurement are the two primary reasons.

[Kristi]

Infinitely faster reaction time with the pressurestat, especially when steaming.

[keno]

HX machines keep the water superheated well beyond normal brew temps so that a single boiler can serve two purposes: steaming and brewing. For brewing water is drawn from the tank through the heat exchanger in the boiler, where it is instantaneously heated to brew temps. For steaming you open the valve to the steam wand and steam immediately escapes from the boiler. If the machine used a thermostat think of what would happen next - the pressure would drop big time and the heating element wouldn't kick on until the temperature dropped enough. Your steaming performance would be terrible. A pressurestat ensures that when steaming the element kicks on immediately and vastly improves steaming performance.

Cheers,
Ken