Measuring Vacuum

Evangelista Torricelli

How did Torricelli come up with the “torr” unit? Knowing that science is a discipline driven by facts with no room for emotion or ego, we can conclude the fact that both “torr” and “Torricelli” have the first four letters is a complete coincidence.

When the force due to the mass of mercury equals the atmospheric pressure, the level of column stabilizes.

Residential & Industrial Cleaning Co.

Carpet Cleaning Specialists Since 1988

The length of the mercury in the tube may vary, but the height above the surface of the mercury in the bowl will stay the same.

Phone: 877-209-6117 (toll free in the US)

704-821-1233

704-261-1233

E-mail: CustomerService@ricinc.net

Measuring Vacuum

The units for measuring vacuum are, oddly enough, inches of mercury. Of course, in countries using the metric system, the measurement is millimeters of mercury.

“Why Mercury?”, you ask—as did I. This results from measurement made by an Italian mathematician named Evangelista Torricelli (1608-1647), who Galileo asked to work on the problem of water pump capabilities.

Torricelli devised an experiment in which he filled a tube sealed on one end with mercury. Holding his finger over the open end of the tube, he inverted it, lowered the open end into a bowl full of mercury, removed his finger, and observed how far the mercury inside the tube fell. The portion of the tube that was vacated held a vacuum, and the distance from the top of the column of mercury to the top of the mercury in the bowl was the differential pressure between the vacuum in the tube and the ambient air (what we call barometric pressure today). This was recorded in millimeters of mercury, resulting in “standard” atmospheric pressure at sea level of approximately 760 millimeters of mercury, Using Torricelli’s units, this is 760 torr.

Converting to English Imperial units, millimeters of mercury becomes in inches of mercury. Today that is generally shown using the symbols for both inches and mercury as “Hg.

Pump manufacturers added another twist. They measure the differential between the ambient air pressure and the intact of the pump—the better a pump was, the more negative the pump’s “Hg rating. . Since this was not intuitive, pump makers decided to use the absolute value of the differential. So a pump producing no differential would be rated at 0 “Hg, while a pump capable of producing a differential equal to one atmosphere would be rated at 29.92 “Hg. In practical terms, a pump rated at 0 “Hg would not be able to raise water above its ambient level. Mercury is about 13.6 times as dense as water, so a pump rated at 15 “Hg should be able to raise water a little over 200 inches, or about 17 feet.

The density of mercury (13.6 g/mL) compared with the density of water (1.000 g/mL). 1 mL mercury balances 13.6 mL water. Mercury is one of the heaviest liquids known.

What Are the Units and Why?

From mercury to vacuum