U-Tube Manometer Madness, Part 2

Let’s dive into some more manometer madness today, shall we?  Last week I tried to clear up some weird stuff I saw online about U-tube manometers.  And then I found out that I might have made things worse, at least for some people.  But it’s not completely my fault, as you’ll see below.

Scaling up

Almost everything I said in last week’s article was correct.  The reason I felt compelled to write the article is that the instructions for the RadonAway U-tube manometer say to ignore the height of the column of liquid below the zero on the scale.  As a physicist, I know that the weight of the whole difference in height matters.

But there were two things I didn’t know at the time.  First, the scale on the manometer isn’t in inches.  Look at the photo above.  The full manometer scale goes to 4.5, but when I put my tape measure next to it, I found that the actual height of those 4.5 units is only ~2.75 inches.

I thought the instructions resulted in a pressure reading that was only half what it should be.  But the scale on the manometer accounts for that by having twice as many numbers.

Aha!  They’re doing some scale magic to change how you read the pressure.  That’s some manometer madness right there.

Gauge oil

The second thing they did also resulted in an adjustment to the scale.  We want the answer in inches of water column (i.w.c.).  But the blue liquid isn’t water.  It’s what they call gauge oil.  Its density is close to that of water, but not equal.  And that’s why the scale isn’t 1 per 0.5 inch.  It’s closer to 1 per 5/8 inch.

More scale magic.

Scale magic video

Here’s a little summary of those first two points.

Connecting to the radon pipe

There’s one other thing to explain about my manometer setup.  Some of you who install radon mitigation systems may have noticed it.  Look at the photo below.  The hole in the radon pipe is an inch or so above the top of the U-tube.  The instructions, however, say that the top of the manometer should be 2 inches above the hole in the pipe.

Now, the reason for that positioning guidance has nothing to do with pressure.  Well, not directly anyway.  The problem with the way it’s installed above is that the soil gases inside the pipe usually include water vapor.  If it gets in the vinyl tube and condenses, that can mess up the pressure measurements.  Worse, water can drain down into the U-tube, further messing things up.  So, I’ll get this fixed right away.

I’m sorry I made a bit of a mess about this.  But I understand it a lot better now and hope you do, too.  I spoke with Rick Saulen, RadonAway’s top technical guy, for over half an hour yesterday, and he cleared it all up.  I also asked him to let people know about their scale tricks to prevent other over-educated folks like me from overthinking things.

So there’s another hit of manometer madness for you.  Next time I’ll try not to make such a mess and instead keep things on the level.  (Oh, sorry about that.)

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.

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