INPUT CONDITIONER BOARD ADJUSTMENT (Refer to Fig. 6, 3, 12)
The input conditioner board has four adjustments: chlorine zero chlorine span, pH zero, and pH span. Since
the pH sensor output is used to correct the chlorine sensor reading, it must be adjusted first. Two solutions of
known pH are required, one of which has a pH of seven. With the sensor in this solution the zero adjustment
(R851) should be set to give a reading of 7.00. With the sensor in the second solution the pH span (R855)
should be adjusted to read to the value of this solution.
Chlorine zero is adjusted by R828. With the front panel FUNCTION, DISPLAY, and RANGE switches at
TFC/AUTO, CL, and 1ppm; and the chlorine and pH sensors in a chlorine free solution, R828 should be
adjusted to read zero on the display.
Chlorine span is adjusted by R820. With the front panel FUNCTION and DISPLAY switches at TFC/AUTO and
CL; and the chlorine and pH sensors in a solution of known TFC, R820 should be adjusted to display the proper
The sensor may be simulated by inserting a current into input terminal 7. Input terminal 8 should be used for
the return path of this current to bypass the sensor bias voltage. The input impedance between terminals 7,
and 8 is 14.7 kilohms.
R851, R855, R828, and R820 can be adjusted independent of the processor display board by observing the
outputs of the input conditioner. It should be noted, however, that no temperature compensation is made of the
input conditioner board and that the following adjustment criteria apply at 25 degrees C. R851 should be
adjusted while observing the voltage at pin M which is related to pH as follows:
526.5 x (pH-7.00) mV
R851 (offset) and R820 (span) are adjusted while observing voltage at pin K which has the relationship, for EM
0.220 x (ppm concentration)-4.167
The Model 924 input conditioner provides a battery to maintain bias voltage on the chlorine sensor when power
is off. This battery is charged during normal operation. A fully charged battery will supply sensor bias for at
least forty hours. When the instrument is turned on after a long off-period, the charge rate can be safely
increased by installing a five thousand ohm resistor across R858 for ten operating hours.