Despatch 926E Equipment Manual (C-155)Rev 2/11 - Manual (Page 32)

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An over-temperature circuit protects the test chamber and test objects by removing
power to heaters when the test chamber exceeds the maximum rating of the chamber
(274

C).
An adjustable high limit allows the maximum operating temperature to be selected or
varied for each test. This dial type unit will interrupt the power to the heater if the
chamber temperature exceeds the high limit setting.
The detailed description of the temperature control circuits is similar for all test chamber
models; therefore, only the temperature control circuit for Model 924 is discussed in
detail.
Basically the same as in 936 and 926 which also have horizontal air flow.
Models 935, 934 and 936 use two additional heaters: HTR2 located under the opening
gasket of the chamber and HTR3 located on the shaft bushing of motor B1. The 935
has an additional heater HTR4 located on the shaft bushing of the second motor B2.
Heater HTR2 is controlled by thermoswitch S5, which is set at +10

F ±5

F.
The thermoswitch is located on the rear of the Transite liner and senses the door
opening temperature of the liner and door opening frame assembly. The heaters
prevent the test tray door from frosting excessively, the door seal from freezing at
temperatures of -300 F. When the temperature test chamber is not being operated at
extremely low temperatures, thermal switch S5 is open, disabling HTR2. A factory set
thermoswitch limits the chamber to the maximum chamber operating temperature.
The line power to the heaters is interrupted. The power will restart automatically when
the temperature is dropped below the over-temperature set point.