MODEL 54e pH/ORP | SECTION 6.0 |
| THEORY OF OPERATION |
A new glass electrode has an impedance of approxi- mately 200 megohms As it ages, this value typically increases over time because lithium ions (which carry current) in the glass are slowly depleted by the process. If an electrode cracks, the impedance drops sharply, usually to below 5 MΩ.
The following Diagnostic Checks are possible with the Model 54e pH/ORP:
Cracked Glass Diagnostic. One way to tell that you have a broken or cracked glass electrode is that the controller will read a constant value (usually between
NOTE
A broken electrode may not be detected above 70°C (158°F).
Old Glass Diagnostic. This diagnostic is used for pro- gramming the high impedance limit. For example, if the set point is 1000 megohms, and the impedance rises above this value, the controller will go into a fault mode. The electrode is either worn out, severely coated, or not immersed in the process fluid..
Calibration Warning. Under this diagnostic you can select the percent increase in impedance before a cal- ibration warning fault appears. It is recommended to keep this feature disabled.
High Reference Impedance. The reference is also continuously checked. High values indicate a plugged liquid junction or a coated sensor. The setpoint can be adjusted depending on the sensor used. Always set this value above the value for a new clean sensor.
You may also get this fault if:
1.The sensor becomes excessively coated.
2.The sensor is not immersed in the process
Typical set points:
1.1000 Megohms for old glass diagnostics.
2.10 Megohms for cracked glass.
3.0 Megohms for calibration warning (disabled)
4.40 Kohms for high reference impedance.
NOTE
See Section 5.8 for instructions on chang- ing these setpoints.
6.3 INTERVAL TIMER
The interval timer may be used for periodic sensor cleaning or periodic process adjustment (see Section 5.7 for procedure).
The interval timer settings are:
1. | Timer | - | Enables/disables the interval timer. |
2. | Interval | - | the time period between cycles. |
3.Repeats - the number of relay activations per cycle.
4. | On time | - | the time period of one relay |
|
|
| activation. |
5. | Off time | - | the time period between two or more |
|
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| relay activations. |
6.Recovery - the time period following the final relay activation.
The cycle begins at the Interval time when the Switch is turned on. When the Interval time has expired the analyzer activates hold mode and the relay is activated for the On time period. If the number of Repeats is greater than one, the relay is deactivated for the Off time period and reactivated for the On time period for the number of relay activations selected. When the final relay activation is complete the relay is deactivat- ed for the Recovery time period. Note that no Off time period follows this last relay activation. When the Recovery time period expires the Hold mode deacti- vates, and the cycle repeats, beginning with the Interval time.
Typically, the interval timer is configured with a long Interval, several Repeats of fairly short On times, fairly short Off times and a Recovery time which allows the process to stabilize. Setting Interval to zero results in continuous pulsing and setting Off time to zero will cause a single pulse equal to [On time x Repeats].
High reference impedance can be used to trigger timer activation. In this case, the interval time is used only to allow the analyzer time to determine whether the refer- ence impedance has exceeded the reference imped- ance setpoint. If the reference impedance setpoint is exceeded, the timer cycle will begin. This is an ideal way to start a cleaning cycle in a dirty application.
Note that the hold mode supersedes the Timer State. If the hold mode is on, the present interval time continues to expire and once expired the interval timer is sus- pended until the hold state is removed. For more infor- mation on Controller Mode Priority, see Table
46
