User Guide

CALIBRATION: FOUR STAGE THERMOSTATS — TC-288

Adjustments

Dial

The dial (a) setting (see Figure-17) determines the operating point of the first, or low, stage

switch. It may be locked in place by tightening the recessed Allen set screw in the top of the

dial shaft (see Figure-16).

Differential between Switches

The differential between the “R” and “B” contacts of the SPDT switches is easily adjusted by

moving the differential contact springs (b) on the thermostat arm (c) (see Figure-17). The

differential may be varied by loosening the differential nut (d). Use a 5/16" open-end wrench,

and slide the contact springs to the left to increase the differential of all four switches (or to

the right to decrease the differential). This varies the tension brought to bear on the switch

pins and adjusts the differential between 3° and 7°. (Differential factory set at 3° minimum.)

Each division on the thermostat arm represents approximately 1°. This adjustment may

have some effect on the differential between stages and therefore should be the first adjust-

ment made in resetting the differential of the control.

Differential between Stages

The differential between the first and second stage switches is easily adjusted by turning the

second stage adjustment screw (AA) (see Figure-17). Use a 3/16" open-end wrench, turning

to the left to decrease the differential and to the right to increase the differential. (Differential

is factory set at 3° minimum.)

The differential between the second and third stage switches is adjusted by turning the third

stage differential adjustment screw (BB) to the right to decrease the differential and to the

left to increase the differential. (Differential is factory set at 3° minimum.)

The differential adjustment between the third and fourth (or high) stage switches is accom-

plished by loosening the lock screw (CC) and rotating the eccentric stud, which is just to the

left of the lock screw, thereby physically changing the distance between the trip pins of the

first and fourth stage switches. Turning this eccentric stud CW decreases the differential and

turning it CCW increases the differential.

The total differential between the first and fourth stages is adjustable between 0° and 20°.

The differential between the first, second, third, and fourth stages may be varied but the total

may never exceed 20°. For example, if the differential between the first and second stages

is set at 4° and the differential between the second and third stages is set at 6°, then 10° is

the maximum differential that can be obtained between the third and fourth stages.

Calibration Procedure

The use of test lights is strongly suggested. However, as they may not always be available,

the control should be calibrated as follows:

1. Immerse the sensing bulb for about 10 minutes in any medium of known temperature

before proceeding. (A pail of water may be used and temperature determined with an

accurate thermometer).

2. Turn regulating knob to setting corresponding to bulb temperature.

3. Turn calibrating screw (DD) (see Figure-17), using a 3/16" open-end wrench, either to

the right or to the left, until the first stage switch actuating points straddle the bulb tem-

perature (indicated by the dial). For example, if the bulb temperature is 70°, turn the cal-

ibrating screw until the first stage switch actuating points (clicks) are an equal number

of degrees from 70°, namely 68° and 72°.

Figure-16 Dial Adjustment of Four Stage Thermostats.