User Guide

CALIBRATION: THREE STAGE THERMOSTATS — TC-282

Adjustments

Dial

The dial setting determines the operating point of the 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-9).

Differential

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

moving the contact spring on the thermostat arm (see Figure-15). The differential may be

varied by loosening the clamp nut and sliding the contact spring to the left to increase the

differential of all three switches (or to the right to decrease the differential). This varies the

tension brought to bear on the switch trip pins and adjusts the differential between 2°F and

7°F (±0.5°F). Factory set at minimum. Each division represents approximately 1°F. This

adjustment may have some effect on the differential between stages and therefore should

be the first adjustment made in resetting the differential of the controls.

Differential between Stages

The differential between the low stage and middle stages is easily adjusted by turning the

middle stage adjustment screw (see Figure-15) CW to decrease differential and CCW to

increase differential. Factory set at 3°F. The differential between high and middle stages is

easily adjusted by loosening the lock screw and rotating the eccentric stud (see Figure-15),

physically changing the distance between the trip pins of the high and middle stage

switches. Turning this eccentric stud CW decreases differential and turning CCW increases

differential. The total differential available between the low and high stages is adjustable

between 0°F and 20°F. The differential between the low, middle, and high stages can be

varied but the total may never exceed 20°F. For example, if the differential between the low

and middle stage is set at 12, then 8°F is the maximum differential that can be obtained

between the middle and high stages.

Calibration Procedure

After differential adjustments have been made, the unit should be calibrated as follows:

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

2. Turn dial to setting corresponding to bulb temperature.

3. Turn calibration screw (see Figure-15) until the low stage switch actuating points

(indicated by the dial) straddle the bulb temperature. For example, if bulb temperature

is 70°F, turn calibrating screw until low stage switch actuating points (clicks) are an

equal number of degrees from 70°F, namely 68°F and 72°F.

If dial setting is higher than actual bulb temperature, turn calibrating screw slightly in a

CCW direction. If dial setting is lower than bulb temperature, turn screw in a CW

direction.

4. To check differential between stages:

a. Turn dial well above calibration point.

b. Start turning dial slowly toward calibration point until the low stage switch just clicks

(makes to “B”) and note dial reading at this point.

c. Continue turning dial in the same direction until middle stage switch clicks (makes to

“B”) and note dial reading at this point.

Figure-14 Dial Adjustment of Three Stage Thermostats.