Many years ago, when I was working in the maintenance dept of our local chicken processing factory, one of our gas fired boilers developed a problem. It would start ok, but when it tried to switch from start to run it just cut out. The electrician couldn't find any electrical problems. I suspected the gas supply. So I made up a crude manometer using about ten foot of 10mm bore clear plastic pipe. I fixed it to a length of plank so that it formed an elongated 'U', added water until the water level was about half way up the pipe (naturally on both sides of the 'U'), and connected one end of the pipe to a spigot on the side of the gas supply pipe.
A mark was made on the board coinciding with the level of the water.
And the boiler was switched on.
At first the gas pressure showed, as expected, a positive pressure, not much, about 2" of water if I remember correctly. But when the system tried to go to 'run' the pressure dropped back past the original mark. In other words it was, briefly, pulling a slight vacuum. And there was the problem.
A gas boiler system must never have a negative pressure in it as there is the potential to draw in air. This can lead to an explosive gas/air mix. So the electronics detected the negative pressure and shut the system down.
For very low pressure measurement, manometers are simple, rugged and virtually fool proof. There is nothing to go wrong.