Teledyne 4220 - Manual (Page 121)

4220 Flow Meter
Section 5 Options and Accessories
5-13
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 sa
olution, 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
parameter module to allow greater operating distance from the
flow meter. The flow meter determines the pH value and displays
it.
5.9.1 pH Probe Calibration The 4220 provides a two- or three-point calibration for the pH
probes with commercially-prepared calibrated buffer solutions.
Calibrations of 4 and 7, 7 and 10, and 4, 7, and 10 are all pos-
sible. For accurate readings you must clean and re-calibrate the
probe on a regular basis. How often you need to do this depends
on the condition of your flow stream. Flow streams with a high
grease content will coat the sensing surfaces of the probe quickly,
clogging them and slowing the response time or stopping it alto-
gether. Installation in very greasy flow streams is not recom-
mended.
Note
For pH probe calibration, Teledyne Isco recommends that you
use a glass container for the buffer solutions to ensure that the
following conditions are met:
The probe must be properly submerged in solution, and there
must be no air trapped under the probe membrane, or the
reading may become incorrect and/or erratic.
Do not touch the probe until after you have performed step 6.
1. To calibrate the pH probe with the flow meter, first go to
program Step 1: PROGRAM, SETUP. Select PROGRAM.
2. Step through the units of measure with the Enter key
until you reach the menu that says pH UNITS - NOT
MEASURED, pH. Select pH. You must do this in step 1 or
the pH menu will not appear later.