Something that comes up a lot with manometers is confusion about what you're actually seeing displayed on the screen, because there's two different types of measurements: there's a measurement that is from one hose to atmosphere or whatever the zeroed out level is, and then there is a measurement Between the two probes, which is what we would call a delta p, the pressure differential between the two i'm going to show you on this really popular field, piece, manometer, the sdm and six. How to think about this, and hopefully will prevent you from making a pretty significant mistake. So first thing: let's turn it on once we get it turned on we're gon na zero it in this case you just hold zero for one second gon na zero out and right now you can see that we have the arrow here pointing at p1 and what That means is, is that this big number up here is representing p1 to atmosphere, and this smaller number here is our delta p. So this is going to show our difference in pressure between the two probes, and this is going to show just our one p1 probe to atmosphere right now.

You can see they are the same and that's because uh right now, those two things are exactly the same, because they're referenced to atmosphere. But let's go ahead and take a i'm just going to use a turkey baster, which i actually use quite a bit for cleaning out float switches on drain lines. It's a nice little tool for that. So i'm going to take this and go to our p1 and i'm just going to apply a little bit of pressure.

So you can see that the two are exactly the same so long as this p2 stays referenced to atmosphere. But let's watch what happens if we change this p2 you'll see that it changes on the second one, because the pressure differential in between these two probes is changing, but you'll notice that our p1 measurement, the large one, isn't changing at all. Now, let's switch it to our p2 measurement. So when i do that you'll notice, our little indicator goes over to p2 and now, what's shown here on top is going to be p2 and now this is going to be delta p.

If i change the pressure here, it's going to display the same on both for now. But if i change the pressure on p1 now, because i'm only looking at p2 - it's not going to change my p2, because this is only reference to atmosphere and, of course, that zero. Now, if we wanted to do something a little bit more complicated, so i'm gon na go ahead and trap a little bit of positive pressure in that p2. Now, if i affect p1, we're gon na see the delta the difference between the two on the bottom and the absolute pressure on p2 represented on top all this matters, because you have to know what you're looking at, if you're, measuring something like total external static pressure.

You would want to be looking at the bottom number because that's going to show you the differential in between both of them. So if i take my you know, i take one of them and i put it in the return. I take the other and put it in in the supply that bottom number is going to represent the differential between both, whereas i'm going to see the individual ones as i toggle back and forth between p1 and p2. If you're measuring pressure on a gas valve, you would want to be toggling, you know if you, if you take one and you hook it on your inlet to your gas valve and the other to your outlet or manifold pressure, you're going to want to toggle back And forth p1 and p2 to see what those are, because the delta p doesn't mean much so you're not going to care very much about what this number is on.

A gas valve you're really going to care about this top number. Here there are some gas furnaces. Like we mentioned some high efficiency furnaces in cases in which the gas valve is outside of the combustion chamber, where you actually have to reference one of these tubes to the combustion chamber in order to set the gas valve accurately jim pointed out that those are actually Pretty rare nowadays just depends on the design of your furnace, but generally speaking, you're going to just be toggling in between the two measuring your inlet as p1 generally and then the outlet of your gas valve going to your manifold as p2. But just again, don't get these too confused because they represent very different things.

This represents whichever one you're at in reference to atmosphere, and then this other one is your delta p or your difference in pressure between these two probes. All right, that's it. Thanks for watching.