WatchDog Temperature Scanning System |
PD138 and PD765 to Scan Thermocouples
In the following application example, four oven thermocouples are scanned and channeled to a PD765 Temperature Meter where the PV is displayed and checked for low or high alarm conditions. All four ovens share the same low and high alarms, so the alarm inputs are joined together on the PD138
The PD138 MINIMUX® II is programmed in the following manner to satisfy this application:
- Select alarm Sequence A
- Stop-on-alarm is off
- Program dwell times:
- Channel 1 for ten seconds
- Channel 2 for five seconds
- Channel 3 for fifteen seconds
- Channel 4 for five seconds
- Channels 5-8 are disabled
The PD765-6R2-00 Trident is programmed in the following manner to satisfy this level application:
- Select T/C
- Select °F
- Program alarm set and reset points:
The MINIMUX® II scans each input and reflects the alarm state. The PD138 shown above is scanning input 1 (Green LED) allowing the Trident to display the current temperature of 250°. No alarm is present since the temperature is within limits. The MINIMUX® II only responds to new alarm conditions detected in the currently active channel. The PD138 also avoids false alarms by detecting alarm conditions only after dwelling on a channel for 2 seconds, or 0.05 seconds before it has been programmed to go onto the next channel if the dwell time had been programmed for less than 2 seconds. This gives the PD765 time to stabilize and accurately display the temperature and to check for alarm conditions
The MINIMUX® II switches to (scans) input 2 channeling it to the PD765-6R2-00 where a temperature of 345° is found to cause a high alarm condition. The PD138 is programmed for Sequence A, so the channel 2 LED alternately flashes yellow and green, the internal horn sounds, and the PD138’s alarm relay transfers causing an external horn to sound. After acknowledging the alarm the channel LED will glow a steady yellow (while selected), or steady red (when not selected) until reset
The MINIMUX® II switches to (scans) input 3 channeling it to the PD765-6R2-00 where a temperature of 220° is found to cause a low alarm condition 2 seconds into the dwell time. Channel 3 LED alternately flashes yellow and green (shown yellow), the internal horn sounds and the PD138’s alarm relay transfers causing an external horn to sound |
