Campbell Scientific CS440 Liquid Level Sensor - Manual (Page 10)

CS440 Liquid Level Sensor
6
After this command is executed, the input location within the logger called
“Level” holds the measured pressure, reported in PSI. This result may be
further processed within the logger or stored to final storage memory.
Multiplier and Offset
The Multiplier and Offset can be changed to scale the result or perform unit
conversions. The P2 instruction measures mVolts.
V=IR
At 4 mA, V = 4 mA * 100 ohms = 400 mV
At 20 mA, V = 20 mA * 100 = 2000 mV
Pressure = M * mV output + b
Where; M = multiplier, and b = offset
M= Pressure range / (mV2 – mV1)
If the sensor outputs 0 psi at 4 mA (or 400 mV), and 5 psi at 20 mA
(or 2000 mV) then the multipliers is calculated as follows for a 50 psi
sensor:
M = 50 psi / (2000 mV – 400 mV) = 0.03125 psi/mV
To convert to depth in feet., (Depth = 2.31 * psi).
M = 0.03125 psi/mV * 2.31 ft/psi = 0.7218 ft mV
The offset is typically applied in the field to adjust the reading to
whatever datum is used.
6. Maintenance
Periodic evaluation of the desiccant is vital for keeping the vent tube dry. To
assess the effectiveness of the desiccant, use one of the following:
• An indicating desiccant that changes color when it’s losing its drying
power
• An enclosure humidity indicator such as our #6571 humidity indicator card
6.1 Every Visit, At Least Monthly
• Collect data
• Visually inspect wiring and physical conditions
• Check indicating desiccant or enclosure humidity indicator; service
desiccant if necessary