Metex 2200P pH Analyzer/Controller - Manual (Page 12)

Model 2200P Page 10
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
the Normally Open contact. In the event of power loss to the unit, the relay will de-
energize and the device connected to the Normally closed contact will be turned on.
This option is selected with the DIP switches on the swing out board. See Section
4.7.
4.4.5 An option provides cycling for Relay A and Relay B to obtain closer process control.
When the process is between the relay on and relay off setpoints the relay will cycle
on and off, based on the cycle times selected by the user. This enables the process
to mix with the chemical being added and reduces overshooting. See Section 5.10.