How Pressure/Level Sensors Work - Seametrics INW LevelSCOUT Instructions Manual

TABLE OF CONTENTS INW LEVELSCOUT INSTRUCTIONS

How Pressure/Level Sensors Work

Liquids and gasses do not retain a fixed shape. Both have the ability to flow and are
often referred to as fluids. One fundamental law for a fluid is that the fluid exerts
an equal pressure in all directions at a given level. Further, this pressure increases
with an increasing depth of "submergence". If the density of a fluid remains constant
(noncompressible...a generally good assumption for water at "normal" pressures and
temperatures), this pressure increases linearly with the depth of "submergence".
We are all "submerged" in the atmosphere. As we increase our elevation, the pressure
exerted on our bodies decreases as there is less of this fluid above us. It should be
noted that atmospheric pressure at a given level does vary with changes in the weather.
One standard atmosphere (pressure at sea level at 20º C) is defined to be 14.7 PSI
(pounds per square inch).
There are several methods to reference a pressure measurement. Absolute pressure is
measured with respect to an ideal vacuum (no pressure). Gauge pressure is the most
common way we express pressure in every day life and is the pressure exerted over and
above atmospheric pressure. With this in mind, gauge pressure (Pg) can be expressed
as the difference between the absolute pressure (Pa) and atmospheric pressure (Patm):
Pg = Pa - Patm.
The LevelSCOUT is an absolute sensor and will require an outside barometric reference
to obtain "gauge" type readings.
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