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The relay goes through a sequence of activation/deactivation as described below. The intended purpose of this
algorithm is boiler blowdown. A sample cannot be supplied to the sensor continuously in many boilers because
a recirculating loop is not possible, and it would be a waste of hot water to constantly run a sample to a drain. A
valve is opened intermittently to supply a sample to the sensor.
Where a non-ideal installation of the sensor can cause the sample to flash to steam, and give a false low reading,
this can be corrected by taking the reading with the sample held in the pipe with the sampling valve closed, so the
sample is at boiler pressure and therefore back in the liquid state. Enable Trap Sample if this is the case. Because
the conductivity reading cannot be trusted while the valve is open, the blowdown is timed rather than in direct re-
sponse to a sensor reading. Rather than relying upon a fixed time, where the blowdown could be much longer than
necessary if the reading is just barely off the set point value, proportional blowdown adjusts the time appropriately.
If Trap Sample is Disabled, then the blowdown is not timed, and the Hold Time and Maximum Blowdown time are
not used. The blowdown valve will stay open until the conductivity is below set point. In this case the Output Time
Limit menu is available to stop the blowdown if the sensor is unresponsive.
Note that the software will not allow two relays using Intermittent Sampling to be assigned to the same sensor
input; the previous relay set up will change to Off mode.
Output Details
The details for this type of output include the relay on/off state, relay status (HOA mode, Interlock status,
Intermittent Sampling cycle step, etc.), time remaining for the active Intermittent Sampling cycle step, alarms
related to this output, current cycle on time, relay type, the live reading of the conductivity, and the current control
mode setting.
Settings
Touch the Edit icon to view or change the settings related to the relay.
Set point
Enter the conductivity value below which the controller will not start a blowdown cycle.
Proportional Band
(only shown if trap sample is enabled) Enter the conductivity value above the set point
at which the maximum blowdown time will occur. For example, if the Set point is 2000
uS/cm, and the Proportional Band is 200 uS/cm, then if the conductivity is above 2200
uS/cm the blowdown valve will open for the Maximum Blowdown time described be-
low. If the conductivity of the trapped sample is 2100 uS/cm, the blowdown valve will
open for half the Maximum Blowdown time.
Deadband
(only shown if trap sample is disabled) Enter the sensor process value away from the
set point at which the relay will deactivate.
Sample Time
Enter the length of time the blowdown valve will be open in order to capture a fresh
sample of boiler water.
Hold Time
(only shown if trap sample is enabled) Enter the length of time the blowdown valve
will be closed in order to ensure that the captured sample is at boiler pressure.
Maximum Blowdown
(only shown if trap sample is enabled) Enter the maximum length of time that the
blowdown valve will be open, when the conductivity of the captured sample is above
the set point plus the proportional band.
Wait Time
Enter the time to wait to sample the water again once the captured sample is below set point.
Trap Sample
Enable or disable trapping of the sample.
Cond Input
Select the sensor to be used by this relay.
5.3.11
Relay, Manual Mode
Output Details
The details for this type of output include the relay on/off state, HOA mode or Interlock status, accumulated on-
time, alarms related to this output, current cycle on time, relay type and the current control mode setting.
