Aquametrix 2200P pH Analyzer/Controller User Manual - Manual (Page 11)

Model 2200P Page 9
performs an extensive system of checking values and parameters. All of the
operating parameters are stored in non-volatile memory, without the need of a
battery.
4.1.4 The Model 2200P operates like a normal analog converter with a number of
additional functions made possible by the microprocessor in the instrument.
Some of these are:
• Recall and easy adjustment of relay and output parameters
• Push button calibration
• Automatic Buffer recognition
• A HOLD function, which freezes, outputs when in menu mode
• Continuous checking of the probe during measurement
• Continuous self-check and watch-dog timer to ensure correct system operation
• Simulated input for testing
• Password protection of stored values
• Temperature Output
• Cycling feature for Control Relays A and B
4.2 Calibration
4.2.1 All pH systems need to be calibrated when first placed in service and thereafter
from time to time. The frequency of calibration can only be found by the operator’s
experience with each process. Calibration must always be performed when a new
sensor is placed in service.
4.2.2 Calibration is accomplished by using buffer solutions, of known pH, and adjusting
the instrument to show the known pH value. Buffers are available in 500 mL bottles
and in 20 L packs from AquaMetrix. The procedure for performing calibration is
very simple and is given in Sections 5.2 and 5.3.
4.3 Temperature Compensation
4.3.1 Temperature compensation is an important component of pH measurement. The
potential of a pH electrode is approximately 59 mV per pH unit in a solution at 25°C.
However, at 100°C the potential is approximately 74 mV per pH unit. Provided you
are using a probe with temperature compensation, the 2200P will automatically
compensate for varying temperature, assuming there has been sufficient time for
the probe to attain the process temperature.
4.4 Relay Outputs
4.4.1 Three SPDT (Single Pole Double Throw) relays are provided. The normally open
contacts, (NO), are open when no alarm or control is active.
4.4.2 The two control relays can be programmed to close on either rising or falling pH.
They are normally used to control a valve, pump or auxiliary device. They may be
set to close at any point on the scale. The Relay Off setpoint, sometimes termed
hysteresis, defines the point at which the relays open. See Sections 4.7, 5.5 – 5.10.
4.4.3 The third relay functions as a high-low alarm. It has two programmable setpoints
(high and low). The deadband is fixed at 2% of full span. The alarm relay can be
programmed to signal memory loss in the controller. See Sections 4.7, 5.11, and
5.12.
4.4.4 All relays can be programmed for "fail-safe" operation, which reverses the normal
operation of the relay. In fail safe mode the Normally Closed contact functions as