Could someone riddle me this, I added OPV and did initial tests with cold water (elements unplugged) and pressure was 10 bar. Then tested again when fully warmed, and pressure dropped to 9 bar. In my mind pressure would be greater under heat if anything, not less, so I did it again with same results, pressure was higher at room temp. The valve is thermally isolated from the boilers and stays cool to the touch and no leaks. Does that mean my pressure gauge reads differently with hot water or does machine pressure actually drop under heat?

Thanks if anyone knows.

Is this on your Mr. Coffee?


FIrst off...why was your pressure 10bar with cold water? How were you getting any pressure at all?

Secondly, all pressure gauges have error. The error is worse as you move away from the center of its reading. I am guessing you have a gauge with a high error. In addition, high temperatures will cause the error to increase, and eventually for the device to simply fail or, if not fail, become completely unreliable.

I assume you have a low quality cheap brass wetted parts pressure gauge? That is probably the reason.

My greater concern is 10bar with cold water... (just kidding, I assume you mean your pump.....)

Check out the Wika gauge company, they have some good explanations.

Wait...how did you know it was my Mr. Coffee? Man, you're like totally psychic...wait...is that you standing outside my window? Yes, I unplugged the heating elements and ran the pump. And yes, it is very low quality, I built it for $14, and I realize they have error and 10 bar may actually be 9 bar or whatever. I wasn't inquiring for purposes of achieving greater accuracy, merely curiosity because of the bassackwardness of it. For my purposes it's good enough that I have a consistent reference point, and so far I am getting consistency from test to test. How long will that will last??? I will take a look at the site, but I guess your point is it's the instrument, which does makes the most sense, thanks!