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Deciphering Vacuum Gauge Readings

Other than assembly instructions for Ikea furniture, there is little in life more confusing than high vacuum measurement. Some people speak of 'inches of vacuum', others use words like 'Torr' or 'Microns'. Here is how it works:

Mercury in a “perfect” manometer is pushed up by atmospheric air to 760mm. This is Atmospheric Pressure (1 ATM), or 760 mm Hg, or 760 torrs. 1 torr is therefore 759/760 vacuum, i.e. 1/760 residual pressure. Oil-Sealed Vacuum Pumps achieve vacuum levels a thousand times deeper: 1 x 10-3 torr, or 1 millitorr.--Rough vacuum starts with a relative approach where the ambient atmosphere is considered a 0 inches vacuum.

--Perfect vacuum, then, would be 760mm vacuum or 29.92 inches. Usually, we know this in its rounded-off form of “30 inches Hg vacuum”

--Evacuating to 1⁄2 ATM, or 380 torrs residual pressure, gives a reading of 1⁄2 vacuum or 15 inches vacuum.

--Evacuating to 2/3 ATM or 253.3 torrs residual pressure gives a reading of 20 inches vacuum or 20” Hg.

Measuring pressure is an absolute measurement because it reads the same regardless of ambient atmospheric pressure levels. Conversely, vacuum readings rely on ambient pressure. In Denver, CO, for example, ambient pressure is only 25/30ths of Sea Level’s pressure. Thus a millitorr-range pump will evacuate to a reading of 25” Hg on a rough gauge in Denver while the same pump and rough vacuum gauge will deliver a reading of 30” Hg when at sea level. Users must account for elevation when using a relative vacuum gauge.

It gets even more confusing because when using the 0-30" scale, the bigger the number, the better the vacuum is. But when we take that last, 30th inch and begin to split it into small pieces, the smaller the number the better. Confusing!

Lots of people prefer the 'Torr' scale. It starts at 760 torr, which is the same as the atmosphere and goes down from there. When we approach 1 Torr and lower, we take the last Torr and split into 1,000 pieces, called 'Millitorr' so 1 Torr = 1,000 millitorr. With this scale, the smaller the number the better the vacuum, as we try to get as close to 1 millitorr as possible.

In most practical applications, 1 millitorr would be considered a 'perfect' vacuum, but actually much, much better vacuum levels can be achieved. In a future post, I will discuss how we can achieve and measure Vacuum Pump Pressures below 1 millitorr.