MIL-F-80056C
thermocouples as the temperature sensing element. The controllers shall provide
thermocouple upscale fail-safe protection that will shut off the heat source at
approximately
above maximum temperature.
3.4.12.1 Type I and Type 11. The Type I and Type 11 furnaces shall be provided
with a single indication-recording, controlling type instrument. Unless otherwise
specified (see 6.2), the instrument shall be of the time proportioning type control
and shall operate a contractor through an intermediate relay system. When
specified (see 6.2), the Type II, Class C furnace shall be provided with saturable
core reactor or a continuous proportional SCR control system. Either system
furnished shall have an accuracy of 0.25 percent or better, of full scale reading
and shall be capable of maintaining the set control temperature within plus or
minus
within operating range of the furnace.
3.4.12.1.1 Temperature recorder. The temperature recorder shall be of the strip
chart, self-balancing type. Unless otherwise specified
(see 6.2), the calibrated
chart width shall be not less than six inches, and the chart length shall be not
less than 80 feet to the roll. The minimum chart speed shall be one inch per hour.
The chart mechanism shall be furnished in a dust-proof housing with a glass door
that pemits full view of chart width, and not less than seven inches of chart
1ength.
3.4.12.1.2 Control indicator scale. The temperature control indicator scale
shall be approximately the same length as the temperature recorder chart width.
The calibrated scale range shall be not less than 100° F above the maximum
temperature of the furnace. The increments between successive temperature marks
shall be not greater than 20° F, with the spacing of numbered marks not greater
than 400° F. The control indicator shall be provided with two pointers. One
pointer shall be adjustable to the desired control temperature, and one pointer
shall indicate the actual furnace temperature.
each
3.4.12.1.3 Excess temperature controller. When specified (see 6.2),
The excess
furnace shall be provided with an excess temperature controller.
temperature control circuit shall be independent of the control circuitry. The
controller shall provide for selecting the over-temperature control point and shall
function to shut-off the heat source to the furnace when the over-temperature
control point is reached. The excess temperature controller shall provide for
furnace shut-down in the event of thermocouple failure,
or failure of other
components in the circuit. In case the furnace is shut-down by the excess
temperature control system, it shall sound an audible alarm and the furnace shall
remain off until started manually. Calibrated accuracy shall be within one percent
of full scale reading.
3.4.12.1.4 High-Low temperature controller. When specified (see 6.2), in lieu
of the excess temperature controller specified in 3.4.12.1.3, the furnace shall be
provided with a High-Low temperature controller. The High-Low control circuit
shall be independent of the control circuitry and the controller shall provide for
selecting the high temperature control point and the low temperature control point.
The High-Low controller shall interrupt the heat source to the furnace when the
high temperature control point is reached and sounds an alarm. The temperature
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