Monarch Instrument DC1250 - Manual (Page 16)

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Setting the engineering units is done in the same menu location using the CHAN UNITS
option to set the three alphanumeric characters.
Let us assume we have an input from a pressure
transducer of 1 to 5 volts DC, corresponding to real
world units of zero to 250 PSI (Pounds per Square
Inch of pressure). Assume we choose the 0 to 5 volt
input. The actual output span of the transducer is 4
volts, with 1 volt = 0 PSI and 5 volts = 250 PSI. This
is equivalent to 62.5 PSI per volt. The LO SCALE
which is the "0" volt value is equivalent to -62.5 PSI
and should be set to this value, and the HI SCALE is
set to "250". The engineering units are set to "PSI".
The display will now show zero to 250 PSI for 1 to 5
Volts input. The linear scaling is shown in Figure 2.5.
2.5 ALARMS
The Recorder has four programmable internal alarms with dual relay output. The alarms
can be associated with either analog input channel, can be set as high or low, latching or
non-latching with or without lockout capability. Each relay has a set of potential free
normally open contacts (Form A - SPNO) that are accessible from the rear panel via screw
terminal connections. Alternatively the relays can be set to the failsafe mode, normally
closed (Form B
– SPNC). The relays can be assigned to any, all or none of the alarms.
Refer to section on Alarms and Relays in Advanced Setup Menu.
2.5.1 Alarm Types
A high alarm is active when the input is greater than the setpoint. A low alarm is active
when the input is less than the setpoint. A latching alarm will remain set once activated
and must be reset by the operator. Note that the alarm condition should be removed
before resetting the alarm or the alarm will activate and latch again. A non-latching
alarm will set on alarm condition and automatically reset once the alarm condition is
removed. A differential, known as hysteresis or deadband can be introduced so that
once an alarm is set (at the setpoint), the input must go beyond the deadband before
the alarm resets. This prevents the output relays from chattering in borderline
conditions. The deadband is set in absolute units and adds to the setpoint in a low
alarm, or subtracts from the setpoint in a high alarm. By way of example, if the setpoint
is 100, the hysteresis (deadband) is set to 5, and the alarm is a high type and non
latching, then the alarm will activate when the input exceeds 100 and will remain active
until the input drops below 95 (100 minus 5). Alternatively the user can set a reset
delay from 1 to 255 seconds. This delay must time out once an alarm condition has
been removed before the alarm will reset to normal.
Lockout prevents an alarm from activating until the setpoint has been traversed in the
opposite sense by the input. For example, if we were measuring pressure and had a
low alarm set at 100, we may not wish the alarm to be active on start up when the
pressure may be low or zero. We really wish to monitor a low-pressure situation once
our target pressure has been reached, say 150. The lockout prevents the alarm
activating at startup. The alarm remains deactivated until the input exceeds the
setpoint. Now the alarm arms and any future time that the input drops below the
setpoint, the alarm will activate.
5
4
3
2
1
0
0 125 250
-62.5
(OFFSET)
INPUT
VOLTS
PSI
Figure 2.5
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