Two Negative Samples:
Connect higher negative sample to
L
,lower negative sample to
H
.
Two Positive Samples:
Connect higher positive sample to
H
, lower positive sample to
L
.
One Negative and One Positive Sample:
Connect positive
sample line to
H
, negative sample line to
L
.
For each right angle bend, add four feet to the computed line
length in order to determine correct pipe or tubing size. If
either sample line connector is vented to the atmosphere, at-
tach an elbow to it so that the open end of the connector points
downward. This will help protect the switch from contamina-
tion.
ELECTRICAL CONNECTIONS
Five numbered terminals are provided for field wiring. Ter-
minals are pressure clamp screw type.
SEQUENCE OF OPERATION
Before pressure and power are applied to the
AFS-953
, the
SPDT switch contacts are in the normally closed position as
shown in
Figure 2
(“shelf” condition); relay contacts T-T
(TD-1) and 3-4 (TD-2) are open . When the sample lines are
connected as shown in
Figure 1
, and power is applied to
terminals 1 and 2, the following sequence occurs.
The pre-purge period (20 seconds) commences: the red lamp
is off. The TD1 time delay is energized. The 20-second timing
sequence commences.
At the end of the 20-second timing sequence, T-T contacts
close.
If, at the end of the 20-second timing sequence the switch is
below set point, the SPDT contacts are in NC position, the
red light comes on, the relay contacts 3 and 4 remain open,
and T-T contacts remain closed. This condition is “alarm
mode.”
If, at the end of the 20-second timing sequence the switch is
above set point, the SPDT contacts are in NO position, the
red light does not come on, the TD2 time delay is energized
and the 8-second timing cycle commences. Relay contacts
3 and 4 remain open during the 8-second cycle. After the 8-sec-
ond cycle is completed, contacts 3 and 4 close. T-T contacts
remain closed. This condition is “operating mode.”
Whenever differential pressure drops below the set point, the
SPDT switch contacts change to NC immediately, turning the
red lamp on, and de-energizing the TD2 time delay, which
starts the 8-second timing sequence. The relay contacts 3 and
4 remain closed unless the 8-second timing sequence is com-
pleted: differential pressure must remain below set point for
8 seconds continuously to complete the sequence. After this
8-second period, TD-2 relay contacts 3 and 4 open: the switch
is in “alarm mode.”
When the differential pressure becomes greater than set point,
the SPDT switch contacts change to NO immediately, turning
the red lamp off, and energizing the TD2 time delay. The dif-
ferential pressure must remain greater than set point for 8 sec-
onds continuously before the TD2 time delay closes contacts
3 and 4, returning the switch to normal “operating mode.”
FIELD ADJUSTMENTS
For the standard NEMA-1 enclosure, field adjustments to the
operating set point are made by removing the protective plug
button on the side of the enclosure and inserting a standard
blade-type screwdriver, at least 3 inches long. For the NEMA-4
and NEMA-7/9 enclosures, it is necessary to open the enclo-
AFS-953 Contact Configurations
T - T
(TD-1)
3 - 4
(TD-2)
Red
Light
1. On Shelf Contact Positions
before power is applied
Figure 2
shows
AFS-953
in "on shelf"
condition: before power is applied to the
circuit and before pressure is applied to
diaphragm.
open
open
off
2. Pre-purge Contact Positions
During initial 20-second period after power is applied: contacts TT stay
open. (TD-1)
Power Applied; Differential Pressure
below Set Point (Immediate).
open
open
off
Power Applied; Differential Pressure
above Set Point (Immediate).
open
open
off
During initial 20-second period after power is applied: contacts 3-4 also
stay open., even after 8-seconds have elapsed (TD-2) .
Power Applied; Differential Pressure
below Set Point (After 8 seconds).
open
open
off
Power Applied; Differential Pressure
above Set Point (After 8 seconds).
open
open
off
3. Purged Contact Positions
After 20 seconds with power applied contacts TT (TD-1 ) close, and
remain closed unless power is interrupted.
Differential Pressure below Set Point
(Immediate).
closed
open
on
Differential Pressure above SetPoint
(Immediate).
closed
open
off
After 8 seconds (a total of 28 seconds of uninterrupted operation),
contacts 3-4 (TD-2) can change state.
Differential Pressure below Set Point
(after 8 seconds.)
closed
open
on
Differential Pressure above Set Point
(after 8 seconds.)
closed
closed
off
Power interrupted.
Prepurge cycle repeats.
s u r e t o m a k e a d j u s t m e n t s . T h e a d j u s t m e n t r a n g e i s
0.05±.02" w.c. to 12.0" w.c.
To adjust the set point, turn the
adjusting screw counterclockwise until motion has stopped.
Next, turn the adjusting screw
4 complete turns
in a clock-
wise direction to engage the spring. For the
next 10 turns,
each full turn represents approximately 1.2" w.c.
At higher
operating pressures, there is a slight increase in the switching
differential.
Please note:
To properly calibrate an air pres-
sure sensing switch, a digital manometer or other measuring
device should be used to confirm the actual set point.
