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T-901, T-903 and T-905
Submaster Thermostats
Operation
This series of submaster thermostats is so designed
that the set point may be remotely readjusted by
changing pressure to the readjusting bellows.
(See Fig. 2)
A master-submaster system consists of a primary
controller (master thermostat) and submaster
thermostat so connected that output (master
pressure) from the master thermostat readjusts the
set point of the submaster thermostat according to a
predetermined schedule. A dial on the submaster
thermostat also provides manual selection of set
point at the instrument. These thermostats are
available with liquid filled remote-bulb-and-capillary,
insertion and immersion measuring elements.
Pneumatic Control Manual 717.1
Repair Parts for Discontinued Products Section
Instruction Bulletin T-901-A
Issue Date 16B4
Control Action
These thermostats are available in two-position and
proportional action models. Both models are
furnished either direct or reverse acting. A direct
acting thermostat increases control pressure when
temperature increases. A reverse acting thermostat
decreases control pressure when temperature
increases. A simple change of pivots converts the
thermostats from one action to the other. See Fig. 1
and Table I.
Readjusting Action
Depending on the pivots used, an increasing master
pressure either raises or lowers the set point. When
increasing master pressure raises set point, the
thermostat has direct readjustment. When increasing
master pressure lowers set point, the thermostat has
reverse readjustment.
Sensitivity
Sensitivity of a pneumatic thermostat is defined as
the change in the control air pressure per unit change
in temperature. It is expressed as psi per degree.
Sensitivity of this series is adjustable from 1/8
through 5 psi per degree by moving a slider located
on the control mechanism channel.
Fig. 1: T-901 Series Submaster
Thermostat Showing Pivot Locations
Differential
Differential of a two-position controller is defined as
the degrees temperature change required to actuate
the relay. The sensitivity adjustment described above
becomes a differential adjustment in two-position
action. Differential is adjustable 2 through
30 F degrees.
Operation of Control Mechanism
The liquid in the measuring element expands or
contracts with temperature changes. This movement
is transmitted to the control port lid through a system
of levers as shown in Fig. 2. Output pressure to the
controlled device varies with control port lid
© Johnson Controls, Inc.
Code No. LIT-7171715X
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movement. The amount of movement depends on
the location of the sensitivity slider which causes the
control port lid to move. Thus, the lid moves with
respect to the control port, located directly beneath
the lid. This varies the amount of air leaving the
control port and results in varying control port
pressure. This varying pressure is used to operate a
relay of large capacity located in the thermostat.
Operation of Readjusting Mechanism
Set point depends on the position of the control
mechanism assembly with respect to the control port.
The assembly is so pivoted that turning the set point
dial positions the assembly to establish set point.
The readjusting bellows contacts the free end of the
readjusting mechanism assembly. As master
pressure changes, the bellows moves, repositioning
the control mechanism. Movement of the control
2 T-901/T-903/T-905 Instruction Bulletin
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mechanism changes location of the set point screw
with respect to the control rod, thus changing the set
point.
Range of Remote Readjustment
The range of remote readjustment is the number of
degrees the set point can be changed as master
pressure changes from zero to maximum. The
readjusting bellows moves a fixed amount for each
psi change in master pressure. Since the movement
is transmitted through the readjustment spring, Fig. 3,
to the readjusting mechanism, the amount of channel
movement depends upon where the pin rests on the
spring. The pin, also shown in Fig. 3, is rigidly
attached to the post. Turning the dial and post
results in a varying channel movement from 0 to
100 percent of bellows movement. The readjusting
dial is calibrated in degrees F of remote readjustment
for a full change in master pressure. The possible
minimum and maximum ranges vary with the
maximum master pressure available. The “Range of
Remote Readjustment” for various master pressure
ranges is shown in Table II.
Stops for Readjustment Mechanism
Sometimes it is necessary to readjust the submaster
thermostat over only a portion of the master pressure
range. Special stops are available to make this
possible. These stops, Fig. 4, may be arranged to
limit readjustment at the lower end only, or in both
directions, as required.
Adjustment
Setting Range of Remote Readjustment
The readjusting dial, Fig. 2, is used to adjust the
range of remote readjustment. It is calibrated in
F degrees for a full change of master pressure.
The range of remote readjustment is obtained from
control system design information, for example:
MASTER PRESSURE TEMPERATURE
0 psi 140°F
15 psi 65°F
The range of remote readjustment is:
140 - 65 = 75 F°.
Set the readjusting dial at 75 degrees.
Checking the Range of Remote Readjustment
1. Set master pressure to either its extreme
maximum or extreme minimum value.
2. Turn the lower adjusting screw until an
intermediate control line pressure is obtained,
e.g. 10 psi. Make a written note of the lower dial
setting at this pressure.
Fig. 4: T-901 Series Submaster Thermostat
Showing Adjustable Stops
T-901/T-903/T-905 Instruction Bulletin 3
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3. Set master pressure at the extreme opposite
value of that used for Step 1.
4. Turn the lower adjusting screw until the control
line value of Step 2 is obtained. Make a note of
the dial setting at this pressure.
5. The number of F degrees rotation of the lower
dial should equal the degree F setting on the
readjusting range dial. If not, follow Steps 6, 7,
and 8.
6. Loosen the readjusting dial and tighten it at the
value obtained in Step 5.
7. Turn the readjusting dial and post to the original
desired value.
8. Repeat Steps 1 through 5 until the readjusting
dial setting equals the actual readjusting range
obtained from the readings.
Adjusting the Set Point
Referring to the example above, the set point of the
submaster thermostat is 140 F when the master
pressure is 0 psi.
1. Set the master pressure to its minimum value,
e.g. 0 psi.
2. Set the sensitivity selector at a position
determined by experience, or about 1/3 of the
distance up from the inner pivots. The sensitivity
should be as high as possible without producing
undue “hunting.”
3. Check the temperature at the measuring
element.
4. Turn the lower adjusting screw until the mid-point
control pressure of the controlled device
operating range is obtained. For example, 6 psi
is the mid-point of a valve having an operating
range of 4.8 psi.
5. Loosen the lower dial and turn it to indicate the
temperature found at the measuring element.
Tighten the dial in this position.
6. Turn the set point adjusting screw until the dial
setting corresponds to the set point determined
by the master pressure.
Referring to the example, this would be 140°F at
0 psi master pressure.
Adjusting the Stops
The procedure for adjusting the stops is
shown by another example.
MASTER PRESSURE TEMPERATURE
5 psi 60°F
10 psi 53°F
This means that 5 psi change in master pressure
must readjust the temperature 60 - 53 = 7 degrees.
In order to use the readjusting dial which is calibrated
for 15 psi maximum master pressure it is necessary
to know how much a 15 psi change in master
pressure will readjust the instrument. Using the
above example, 15/5 x 7 = 21 degrees. The
readjusting dial is then set at 21 degrees. (Refer to
the procedure for Checking the Range of Remote
Adjustment.) Proceed as follows:
1. Adjust the master pressure to 10 psi.
2. Turn the set point adjusting screw until the
control line pressure is at some intermediate
value, for example, 7 psi. Make sure that the
stop screws are not touching the stop plate.
3. Turn the stop screw for the upper pressure
clockwise until it just touches the plate. (This will
be indicated by a change in control air pressure.)
4. Turn the same stop screw counterclockwise until
the former intermediate value is obtained.
5. Repeat this procedure for the other stop screw.
6. Adjust the master pressure to 5 psi.
7. Turn the set point adjusting screw until the
control line pressure is at some intermediate
value as in Step 2.
8. Adjust the stop screws for the lower pressure
limit by repeating the procedure described for the
stop screw for the upper pressure limit, Steps 2
through 5.
Controls Group
507 E. Michigan Street
P.O. Box 423 Printed in U.S.A.
Milwaukee, WI 53202
4 T-901/T-903/T-905 Instruction Bulletin
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