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4220 Flow Meter
Section 5 Options and Accessories
5-11
The Parameter Modules themselves plug into the Parameter
Probe connector on the flow meter.
CAUTION
The pH and D.O. probes require continuous submersion after
installation, or they will lose sensitivity. Prolonged dehydra-
tion of the sensor bulb may damage or even ruin the pH
probe.
5.8 The Temperature
Probe
The temperature probe is the simplest of the three, consisting of
a thermistor inside a metal housing. The thermistor changes
resistance with an increase or decrease in temperature. The flow
meter converts this resistance change to a temperature reading
and displays it, in degrees Celsius or Fahrenheit, as chosen by
menu selection.
Figure 5-2 Temperature Probe
5.9 The pH Probe The pH probe measures the acidity or alkalinity of an aqueous
solution by determining the relative quantity of dissociated
hydrogen ions, H
+
(actually H
3
O
+
) in the solution. A larger
quantity of H
+
ions indicates acidity, while a smaller quantity of
H
+
ions indicates alkalinity. The H in pH stands for Hydrogen
and the p stands for power.
The normal scale for pH runs from 0 to 14, with 0 being most
acidic and 14 being most alkaline. Distilled water at 25° C is
neutral at 7, based on the fact that the dissociation constant
(number of H
+
and OH
–
[hydroxyl] ions present) for pure water
at that temperature is 10
-7
.