House of Hacks

Monday, September 21, 2015

How to make a vacuum (or pressure) gauge


Vacuum and pressure are two sides of the same coin. A manometer can measure both. In this episode, Harley shows how to make a DIY gauge to measure relatively low vacuums and pressures for shop use.

Part 1: How to quiet a shop vac
Part 2: Central shop vac efficiency: Coming soon.

Unit conversion web site:
The unit converter

For a written transcript, go to How to make a vacuum (or pressure) gauge

Music and photos under Creative Commons License By Attribution 3.0.
Intro/Exit: "Hot Swing" by Kevin MacLeod (
Incidental music:
“George Street Shuffle” by Kevin MacLeod (
“Mining by Moonlight” by Kevin MacLeod (

SU Carbs photo: by Cgxke at

Photos under Creative Commons Attribution 2.5 Generic:
Organ pipes photo: by Xauxa at


Need a vacuum gauge to sync your carbs, test your pipe organ or shop vac? Today at the House of Hacks I’m going to make an instrument to measure low-pressure vacuums.

Hi Makers, Builders and Do-it-yourselfers. Harley here.

I recently made some changes to my shop vac system and want to quantify the changes to the vacuum’s efficiency. One of the things I need is a vacuum gauge, also known as a manometer. Because I’m thrifty, I decided to make my own.

Since some people find it interesting, I want to start with the theory of operation. If you want to skip directly to the build, you can click here.

There are three types of pressure measurement: absolute pressure, differential pressure and gauge pressure.

Imagine a perfect vacuum. Like absolute zero in temperature, this is an ideal that cannot actually be accomplished, but is useful as a standard to measure from. Now let’s put a container with a gas inside that vacuum. The pressure the gas exerts on the container is the absolute pressure. Or said a different way, absolute pressure is the pressure measured on a scale where zero is a perfect vacuum.

Now let’s put another container inside that vacuum with a different pressure. If we subtract one pressure from the other, we get the differential pressure. Differential pressure is the pressure difference between two systems. If we now connect these two containers together with a U shaped tube and put some heavy liquid in it, like mercury, we can directly measure the difference in pressures. If the pressures are the same, the liquid will be at the same height on both sides of the tube. If the pressure is greater in one side, it will push down on the liquid with more force and cause a difference in the height of the liquid on both sides of the tube. Measuring the difference in heights will tell us the difference in pressures.

Now let’s add a rock that’s big enough to have gravity that will hold onto gas, creating an atmosphere. With our containers next to the rock, instead of being in a vacuum, they are now in this atmosphere. The atmosphere will create a pressure from all the gas above our containers pressing down. Here on earth at sea level, this is about 15 pounds per square inch. If we eliminate one of the containers, the U shaped tube will now measure the pressure inside the remaining container relative to the atmosphere’s pressure.

This is known as gauge pressure. It is the pressure of one system relative to the current atmospheric pressure. For convenience, we set the zero point to atmospheric pressure.

In this context, vacuum and pressure are two sides of the same thing; they both measure the pressure of a gas relative to a known point, in this case atmospheric pressure. Vacuum is simply a pressure lower than atmospheric pressure, or said a different way, a negative pressure.

And this U shaped device is what our manometer will look like.

For this build you’ll need:

A flat mounting surface; I used some left over peg board but it could be a piece of wood or even cardboard if it’s rigid enough. It does need to be moderately long. The longer it is, the higher pressure you can test. I’m making mine about 6 feet long. If it's a lighter color the readings will be easier to make.

Clear tubing. The size isn’t critical; I used some with 3/8” inside diameter and 20’ long.

A means of attaching the tubing to the mounting surface. I used zip ties. If you use wood for the mounting surface, you could also use j-hooks or tubing clamps or anything else to attach the tubing.

Some sort of measuring stick. I used a yard stick but you could use a tape measure or even mark out your own lines with a ruler and marker.

A means of filling the tubing. I used a turkey baster but you could also use something with a small nozzle or funnel.

Some water and dark food coloring. I used blue for high contrast but you can use any color you want, or no color at all.

To make the manometer, start by attaching one end of the tubing close to a corner of the mounting surface. Continue to attach the tubing in a straight line next to the edge until you get close to the other end of the mounting surface. Then create a U in the tube and run it up the opposite edge of the mounting surface.

You should have a long bit of tubing left over. Leave this attached. It is where you will do your testing from.

Attach the apparatus so it’s hanging straight down.

Color the water so it’s fairly vibrant. Again, I used blue to get high contrast, but you can use any color you want.

Pour the water into the tube. The amount of water you use will impact the amount of pressure you can test. If you fill to the half way point of your height, then you have the maximum measurement for the size of your instrument. If everything is setup properly, the water will be at the same height on both sides of the tube.

Attach the ruler so it starts at the top of the water.

Your vacuum gauge is now ready to use.

And it is really easy to use. Simply apply a vacuum to the long end of the tube. The amount of vacuum pressure is indicated by reading how high the water comes up on the ruler and then multiply that number by two. This will give you a measurement in units called “inches of water." In this example I’ve got 7 inches of water.

If you wanted to get real fancy, you could make your own direct reading scale. Just mark off every half inch and label the marks with whole numbers starting with 0. Doing this, you don’t need to remember to multiply by two.

There are websites where you can convert this measurement to other pressure measurements such as inches of mercury or bars or pascals. If you do use other units of measurement, you could mark their values directly on the ruler for convenience.

Depending on your vacuum source, you may need to make an adapter for the tubing. For my shop vac I used a flexible connector and plug found in the ABS area of the plumbing aisle along with a pronged tubing to MPT brass fitting.

I’ll put this to use in a follow-up video for the shop vac system.

In conclusion, I’d love to hear in the comments below what you think about making your own instruments. Is it worth the effort to do it yourself or is it better to just buy something ready made? Am I being too cheap?

Finally, if this is your first time here at House of Hacks: Welcome, glad you’re here. We’d love to have you subscribe. Through these videos I hope to inspire, educate and inform makers in their creative endeavors. Usually this involves various physical media like wood, metal, electronics, photographs and other similar materials. Thanks for letting me encourage your creativity. So subscribe and I’ll see you again in the next video.

Now, go make something. It doesn’t have to be perfect, just have fun!