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INW - PT2X Instruction Manual
Do’s and Don’ts
How Pressure Sensors Work
Liquids and gasses do not retain a fi xed shape. Both have the ability to fl ow and are
often referred to as fl uids. One fundamental law for a fl uid is that the fl uid 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 fl uid 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 fl uid 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 defi ned 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.
To measure gauge pressure, atmospheric pressure is subjected to one side of the
system and the pressure to be measured is subjected to the other. The result is that the
differential (gauge pressure) is measured. A tire pressure gauge is a common example
of this type of device.
Recall that as the level of submergence increases (in a noncompressible fl uid), the
pressure increases linearly. Also, recall that changes in weather cause the absolute
atmospheric pressure to change. In water, the absolute pressure (Pa) at some level of
depth (d) is given as follows:
Pa = Patm + kd
where k is simply a constant (i.e.: 2.307 feet of water = 1 PSI)
Don’t
support the device with the connector.
Use a strain relief device to take the
tension off the connectors.
Don’t
allow the device to free-fall down a
well as impact damage can occur.
Don’t
bang or drop the device on hard
objects.
Do
handle the device with care.
Do
store the device in a dry, inside area
when not in use.
Do
install a desiccant tube if using a gauge
sensor.
Do
install the device so that the connector
end is kept dry.
