Teledyne 4250 Installation And Operation Manual page 126

4250 Flow Meter
Section 4 Options and Accessories
Figure 4-7 pH Probe (with protective cap)
4-16
The pH probe is a combination of two electrochemical half-cells.
Together they provide a low-voltage signal that corresponds to
the hydrogen-ion concentration of a solution. If you look at the
pH probe, you will see a glass bulb on one end. This is called the
glass mono electrode.
The glass is of special composition, sensitive only to hydrogen
ions, and is exposed to the solution to be measured. The specific
sensitivity to hydrogen ions prevents interference from other
ions that may be present in the solution. It is essential to prevent
grease fouling of this membrane. The glass membrane produces
an electrical potential proportional to hydrogen ion activity. The
other electrode, called the reference electrode, completes the
circuit between the glass electrode and the solution.
The Isco pH probe combines both electrodes in a single housing
and also contains an amplifier to reduce the extremely high
impedance of the circuit. This improves the reaction of the probe
to stray capacitance and reduces interference caused by electrical
noise in the vicinity.
pH measurement is affected significantly by temperature, like
any other chemical reaction. Consequently, temperature compen-
sation is provided. in the flow meter. The Isco pH probe has a
built-in temperature sensor that is exposed for faster response.
The pH probe connects to a Parameter Module that plugs into
the Parameter Port on the flow meter. The probe has a 25 ft. (7.6
m) cable. For greater distances, contact the factory. The
maximum distance between the module and the flow meter is
1,000 ft. (304.8 m).
When the ion-selective electrode and the reference electrode are
connected to a high-impedance voltmeter and submerged in a
solution, ions move to the surface of the membrane. The electrical
charge on the ions creates a potential difference across the barrier
between the solution and the membrane. This potential, or voltage
difference, is proportional to the activity of the ions in the solution.
The potential, when read by a sensitive voltmeter, translates into
a reading of pH. With an Isco flow meter, the voltage is sent first
to a preamplifier inside the probe to reduce the impedance of the
circuit and improve the signal-to-noise ratio, and then on to the