I am building a DIY hoover and want to test different technical configurations of the device (hose diameter, hose length, motor RPM, motor voltage, cavity size, etc).

My objective for testing these different configurations is to find the utility sweet spot of the device. For example I want a large diameter hose (lets say 3 in) that is somewhat long, lets say (10 ft). In order to achieve this I will have to have a smaller vacuum cavity or higher RPM motor. I am hoping to use the hoover's sucking ability as a prime metric in figuring out the perfect sweet spot.

Based on this answer:


As of this writing, a ”6.5 peak horsepower” Shop-Vac® vacuum rated at 9.3A@120V produces 185 CFM (87 l/s) of airflow and 64 inches (160 mbar) of pressure.

A "normal" shop-vac has a 64 inches of pressure. If I were to build a manometer to measure the pressure it would have to be over 2 yards high. That does not seem like a very helpful size of an instrument.

If I increase the diameter pipe of the manometer can I make a smaller one? Is there a formula I can use to figure out what diameter pipe I should use in my manometer to measure a pressure difference of 0 to 75 inches without building a extremely tall manometer?

I am new to manometers but they seem like a great tool to test my configurations against. I just feel like there is a better way of building a manometer that can measure 75 inches of pressure without being crazy tall.

Thank you!


closed as off-topic by Sebastian Riese, Bill N, user36790, Qmechanic Oct 14 '15 at 14:13

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  • $\begingroup$ The pressure in a column of fluid is independent of the diameter. You could however use a denser liquid (that is why old-school manometers used mercury ... but due to its toxicity this is not done anymore). Otherwise very thin tubes may cause measurement offsets due to surface tension effects. $\endgroup$ – Sebastian Riese Oct 13 '15 at 21:33

The height of the liquid in the manometer is independent of the diameter for the sizes that you will be using. The height IS dependent on the pressure drop across the manometer and the density of the fluid in the manometer. Since the differential pressure across the manometer "legs" is equal to the density of the fluid multiplied by the acceleration due to gravity, multiplied by the difference in height between the manometer legs, you can achieve a lower difference in height between the manometer legs by using a denser liquid. Choices include mercury (which may be difficult to get), concentrated salt water (corrosive and many not be dense enough), carbon tetrachloride (very dense, maybe difficult to acquire), etc. A google search quickly provided specific gravities of liquids (the ratio of liquid density to the density of water) that may be useful for your application. See http://www.galloup.com/downloads/ref/specific_gravity_chart.pdf


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