Instruction manual

7. SALINITY (DISSOLVED OXYGEN ONLY). The solubility of oxygen in water depends on the concentration of dis-

solved salts in water. Increasing the concentration decreases the solubility. If the salt concentration is greater than

about 1000 ppm, the accuracy of the measurement can be improved by applying a salinity correction. Enter the salin-

ity as parts per thousand. One percent is ten parts per thousand.

8. pH CORRECTION (FREE CHLORINE ONLY). Free chlorine is the sum of hypochlorous acid (HOCl) and hypochlorite

ion (OCl

). The relative amount of each depends on pH. As pH increases, the fraction of free chlorine present as HOCl

decreases and the fraction present as OCl

increases. Because the sensor responds only to HOCl, a correction is nec-

essary to convert the sensor current into a free chlorine reading. The Solu Comp Xmt uses both automatic and man-

ual pH correction. In automatic pH correction the transmitter continuously monitors the pH of the sample and corrects

the free chlorine reading for changes in pH. In manual pH correction, the user enters the pH of the sample. Generally,

if the pH varies more than about 0.2 units over short periods of time, automatic pH correction is best. If the pH is rel-

atively steady or subject only to seasonal changes, manual pH correction is adequate.

9. pH SETTINGS (FREE CHLORINE ONLY). If you are measuring free chlorine with continuous (automatic) pH correc-

tion, there are additional pH settings to make.

a. PREAMPLIFIER. The raw pH signal is a high impedance voltage. A voltage follower or preamplifier, located either

in the sensor or transmitter, converts the high impedance signal into a low impedance one. Normally, high imped-

ance signals should be sent no further than about 15 feet.

b. REFERENCE OFFSET. Ideally, a pH sensor in pH 7 buffer should have a voltage of 0 mV. The difference between

the measured voltage in pH 7 buffer and the ideal value is the reference offset. Typically, the reference offset is

less than 60 mV.

c. DIAGNOSTICS. The Solu Comp Xmt continuously monitors the pH sensor for faults. If it detects a fault, the trans-

mitter displays a fault message.

d. GLASS IMPEDANCE. The transmitter monitors the condition of the pH-sensitive glass membrane in the sensor by

continuously measuring the impedance across the membrane. Typical impedance is between 100 and 500 MΩ.

Low impedance (<10 MΩ) implies the glass bulb has cracked and the sensor must be replaced. An extremely high

impedance (>1000 MΩ) implirs the sensor is aging and may soon need replacement. High impedance might also

mean that the glass membrane is no longer immersed in the process liquid.

10. DUAL SLOPE CALIBRATION (FREE AND TOTAL CHLORINE ONLY). The Model 499ACL-01 (free chlorine) and

499ACL-02 (total chlorine) sensors lose sensitivity at high concentrations of chlorine. The Solu Comp Xmt has a dual

slope feature that allows the user to compensate for the non-linearity of the sensor. For the vast majority of applica-

tions, dual slope calibration is unnecessary.

11. INPUT FILTER. The raw sensor current can be filtered to reduce noise. Filtering also increases the response time. The

filter is the time required for the input to reach 63% of its final reading following a step change.

MODEL XMT-A-FF/FI SECTION 7.0

PROGRAMMING THE TRANSMITTER