Campbell Scientific 255-100 Novalynx Analog Output Evaporation Gauge - Manual (Page 12)

255-100 Novalynx Analog Output Evaporation Gauge
(3) Add more water to the evaporation pan and measure the water’s depth in
the pan.
(4) Measure the voltage at that depth using the gauge and datalogger
Instruction 4, ExDelSE or BrHalf with a multiplier of 1.0 and an offset of
0.0.
(5) Calculate the multiplier and offset using the method described in Section 5
of the NovaLynx instruction manual.
Example
This example calculates the multiplier (m) and offset (b) for a specific
evaporation gauge. Because the multiplier and offset vary with each gauge,
your numbers will be different than this example. In this example the pan is
filled to a depth of 2” and the datalogger reads 100 mV on the gauge. The pan
is then filled to a depth of 8” and the datalogger reads 2100 mV. This provides
data for a two point calibration.
FIGURE 7. Determining the Multiplier and Offset
for an Evaporation Gauge
Measurement instructions use the following formula when applying multiplier
and offset to readings:
Y = mX + b Where 'm' is the multiplier, 'b' is the offset, 'X' is the raw reading,
and 'Y' is the output
(X1, Y1) = (100 mV, 2”)
(X2 , Y2) = (2100 mV, 8”)
m = (8” - 2”) / (2100 mV - 100 mV) = 0.003”/mV Multiplier
b = Y1 - mX1 = 2” - (0.003” * 100 mV) = 1.7” Offset
7. Automatically Refilling the Evaporation Pan
A simple solution for automatic refilling of the evaporation pan is provided by
the 255-620 Evaporation Pan Automatic Refill System. The 255-620 consists
of an Electronic Water Timer powered by 2 'AAA' batteries and an Automatic
Float Valve. The 255-620 must be connected to a user-provided water supply.
The water supply can be a pressurized system or a storage tank that sits higher
than the evaporation pan. The Automatic Float Valve will stop tank filling
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