# Would single tube from water drain work as "open end manometer" to read boiler tank steam pressure?

Home heating steam pressure (a couple of psi maximum) would normally be measured from piping open to the boiler tank above the water line, with perhaps a (rarely installed) low-pressure dial-gauge, or a device (even a small U-tube manometer) to be connected during professional maintenance.

Here asking about an anecdotal "quick-and-dirty" rough-estimate procedure: Cool the boiler, attach a clear tube (say 1/2" ID) to the drain outlet, stabilize it vertically, open the drain-valve, mark the water level in the tube, re-start the boiler, and measure the difference in the height of the water column in the tube to roughly "read" the steam pressure (pushing down on the water in the tank) as per a change in the tube of "~27 inches water height = 1 psi".

Assuming this proposed mechanism can even work, for such a rough idea of pressure suspect things like water temperature density differences, surface tension, or that the boiler may not be a sealed vessel would not be of concern, but am wondering not only about the distance-to-pressure scale cited, but about the effect of surface area of the tank water vs the water in the open-end tube. Unless the >1000 times difference in area cancels out some factor, rather than magnifies it.

Thanks. A more complete representation of a steam system can be found here (a plumbing website).

• Nobody is going to run a steam boiler at a gauge pressure of 10-20 psi, and in fact, industrial boilers often operate in the range of 50-150 atmospheres. Any boiler pressure higher than 10-20 psi would require a VERY long open tube. Dec 12, 2021 at 5:44

## 1 Answer

This design would indeed measure the tank pressure, which does not depend on cross sectional area but only on height. But this would be very inadvisable because you'd be creating a leak in a tank that should be sealed. Heat/vapor would leak out, and the tank could not build to any appreciable pressure because the water would shoot out everywhere (1 psi equals about 2.3 feet of water). A simple bourdon pressure gauge is very cheap and effective

• Thank you for confirming such operation can be valid, and the 27in/psi measured-distance-to-pressure conversion. Simplicity and availability is normally implied: "all you need is clear tubing and a barbed garden hose fitting." Flaws you noted are acknowledged. Not proposed for continuous use (and imagine keeping ready to shut off the flow at the drain-pipe valve during "measurement"). Also not mentioned was that most descriptions note "tube up to the ceiling" - which might suffice in height, since most home systems ideally operate at ounces of pressure and are set to cut-off > ~2 psi. Dec 12, 2021 at 23:56
• And appreciate your familiarizing me with the name Bourdon re: gauges. Can't see why a Bourdon gauge, similarly adapted, would not work. (Although as mentioned above, such a low pressure gauge connected through a protective loop (aka "pigtail") to the tank somewhere above the water line would be much preferred, but is rarely installed by default (whereas a gauge to measure dangerously high pressures is present, but is useless to distinguish low operating pressures). Dec 12, 2021 at 23:56