Johnson Controls T-901 User manual

Pneumatic Control Manual 717.1

Repair Parts for Discontinued Products Section

Instruction Bulletin T-901-A

Issue Date 16B4

Code No. LIT-7171715X

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.

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.

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

Fig. 1: T-901 Series Submaster

Thermostat Showing Pivot Locations

2 T-901/T-903/T-905 Instruction Bulletin

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

T-901/T-903/T-905 Instruction Bulletin 3

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

4 T-901/T-903/T-905 Instruction Bulletin

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

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