Hach 900 MAX - Manual (Page 94)

Page 92
Alarm Relays
8990cm.fm
Section 6
1. Enable one of the alarm conditions.
2. Select an action to occur when the alarm is activated.
3. Set either a High trip point or a Low trip point.
4. After entering the trip point enter the deadband value. The deadband is
the area between the alarm “turn on” and “turn off.”
Note: Log rainfall to use an alarm on a rainfall condition; likewise, log flow in order to
implement an alarm on a flow rate of change.
Set Point Alarm Conditions:
Note: Rainfall and Flow Rate of Change alarms are HIGH set point conditions; they
take no deadband and they are time dependant.
Deadband
After entering the trip point, enter a deadband value. The deadband is the
area between alarm “turn-on” and “turn-off”.
The purpose of setting the deadband is to eliminate alarm chatter which may
occur if the “turn-on” and “turn-off” values are too close together. Small
fluctuations occurring when the reading is at or near the trip point can rapidly
toggle an alarm relay on and off.
In the pH example ( Figure 28 ) the deadband is set to 0.10 pH. When the pH
reached 6.9 (lower dashed line), the alarm tripped, but the alarm did not turn
off until the pH came back up to 7.00. This difference is the deadband setting
which should be set according to the characteristics of the item
being measured.
• Level • Cabinet Temperature (refrigerated samplers)
• Flow • Analog Channels 1–3
• Flow Rate of Change • Analog Ch. 4 or D.O.
• pH • Analog Ch. 5 or D.O. Temp.
• ORP • Analog Ch. 6 or Conductivity
• Process Temperature • Analog Ch. 7 or Conductivity Temp.
• Rainfall