Hanna Instruments HI 23 Series Instruction Manual page 27

Note
ALARM RELAY
4. The deviation, Ti and Td can be calculated from the following:
• Deviation = Tx * max. slope (EC/TDS)
• Ti = Tx / 0.4 (minutes)
• Td = Tx * 0.4 (minutes).
5. Set the above parameters and restart the system with the
controller in the loop. If the response has too much over-
shoot or is oscillating, then the system can be fine-tuned
slightly increasing or decreasing the PID parameters one
at a time.
Connecting an external device (e.g. chart recorder) to the
controller, the procedure is easier and doesn't need to hand
plot the process variable (EC or TDS).
The alarm relay functions in the following manner:
During alarm condition, the relay is de-energized. When
not in alarm condition, the relay is energized.
High alarm set at 1400 µS
Example:
Low alarm set at 600 µS
An hysteresis will eliminate the possibility of continuous se-
quences 'energizing/de-energizing' the alarm relay when the
measured value is close to the alarm setpoint. The alarm
hysteresis amplitude is 1.5% of full scale.
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