What's your Boost gauge telling you...
12-07-2004, 10:19 AM
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What's your Boost gauge telling you...
There seems to be some misunderstanding on what the numbers on your boost gauge is telling you. I'd like to explain what you're seeing and the correct verbiage used to describe it.
Lets start with the definition of Atmospheric Pressure. Pressure can be described in numerous units of measure. I will use the English unit of measure for pressure (PSI) for this example
Picture an imaginary column of air that rises from the surface you're standing on to space. The mass or weight (in pounds) of the column of air acting over the area of the column (in square inches) at the surface is defined as pressure. Units of measure are pounds per square inch or PSI. So a one square inch column of air rising to space will weigh about 14.7 lbs.
Positive pressure (above atmospheric) is referred to as Boost in the automotive aftermarket circle. The reference to the atmosphere is referred to as gauge pressure typically.
Negative pressure (below atmospheric) is referred to as vacuum. Negative boost is the incorrect verbiage for describing vacuum.
The charge air pressure sensor in our car uses an absolute scale. That means it measures pressure relative to an absolute zero reference (or vacuum chamber) inside the sensor. So it senses the atmospheric pressure (about 14.3 to 14.7 psi typical) and then adds the boost pressure to it and reports it to the ECU.
Boost gauges typically have two scales, Inches of Mercury to measure Vacuum and Psi (pounds per square inch) for measuring positive pressure (boost) relative to the atmospheric pressure. Some boost gauges use the Bar scale (another common unit of measure for pressure).
The relationship between Inches of mercury and Psi and psi to Bar is as follows.
1 inch of Hg (mercury) is equal to 0.4911541 PSI
So 20 in of Hg of manifold vacuum would equate to 20 x 0.4911541 = 9.82 PSIa of vacuum or about 4.9 psi less than the atmospheric reference.
1 psi is equals to 0.06894757 Bar
So 9.82 psi equates to 9.82 x 0.06894757 = 0.677 Bar of vacuum or 0.33 Bar less than the atmospheric reference.
Lets start with the definition of Atmospheric Pressure. Pressure can be described in numerous units of measure. I will use the English unit of measure for pressure (PSI) for this example
Picture an imaginary column of air that rises from the surface you're standing on to space. The mass or weight (in pounds) of the column of air acting over the area of the column (in square inches) at the surface is defined as pressure. Units of measure are pounds per square inch or PSI. So a one square inch column of air rising to space will weigh about 14.7 lbs.
Positive pressure (above atmospheric) is referred to as Boost in the automotive aftermarket circle. The reference to the atmosphere is referred to as gauge pressure typically.
Negative pressure (below atmospheric) is referred to as vacuum. Negative boost is the incorrect verbiage for describing vacuum.
The charge air pressure sensor in our car uses an absolute scale. That means it measures pressure relative to an absolute zero reference (or vacuum chamber) inside the sensor. So it senses the atmospheric pressure (about 14.3 to 14.7 psi typical) and then adds the boost pressure to it and reports it to the ECU.
Boost gauges typically have two scales, Inches of Mercury to measure Vacuum and Psi (pounds per square inch) for measuring positive pressure (boost) relative to the atmospheric pressure. Some boost gauges use the Bar scale (another common unit of measure for pressure).
The relationship between Inches of mercury and Psi and psi to Bar is as follows.
1 inch of Hg (mercury) is equal to 0.4911541 PSI
So 20 in of Hg of manifold vacuum would equate to 20 x 0.4911541 = 9.82 PSIa of vacuum or about 4.9 psi less than the atmospheric reference.
1 psi is equals to 0.06894757 Bar
So 9.82 psi equates to 9.82 x 0.06894757 = 0.677 Bar of vacuum or 0.33 Bar less than the atmospheric reference.
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