Landcal Blackbody Source
Type P1600B2
User Guide
Page 16
6 Using the Furnace
6.1 Introduction
The furnace has been designed to create an enclosure of uniform temperature,
ideal for the calibration of radiation thermometers or thermocouples. The cone
point of the refractory cavity target block installed in the furnace is placed in
the area of minimum gradients within the furnace. When calibrating radiation
thermometers, the target size requirements of the thermometer should,
whenever possible, be fulfilled by the cone. If the thermometer views the
walls of the target block, results of greater uncertainty will be achieved. When
calibrating thermocouples, the hot junctions must be inserted into the cone area
and the furnace aperture plugged with ceramic fibre to reduce conduction and
convection losses.
6.2 Measuring Thermocouple
Provision has been made to measure the temperature of the target block using
a Platinum thermocouple, which can be inserted from the rear of the furnace
into the target. A recrystallised Alumina sheath must always be used to protect
the thermocouple from mechanical damage and contamination. The optimum
position for the measuring thermocouple to be placed is 470mm from the rear
cover. When placed in this measuring position in the furnace, the temperature
(as measured by the thermocouple) should match the cone point radiance
temperature to within the uncertainty of the thermocouple.
The output from the thermocouple must be measured via a cold junction
enclosure of known temperature to an indicator or digital voltmeter having a
resolution of 0.1°C.
This is the recommended way to obtain the true temperature of the target
cavity. The temperature indication on the controller must not be used as an
accurate measurement of target cavity temperature.
6.3 Survey Thermocouple
The temperature gradients along the length of the target can be assessed. This
should only be necessary if doubt exists about the condition of the furnace. The
temperature gradients within the furnace are dependant on control temperature,
so if the furnace is used over a large temperature span, several assessments at
different temperatures may be required.
To carry out an assessment of the temperature gradients along the length of
the target, it is recommended that a thermocouple be inserted down the open
end of the furnace until it is almost touching the cone point. Refer to Fig. 10 for
assistance with the position of the thermocouple.
After the thermocouple has achieved thermal stability, measure the temperature
of the furnace with both the measuring and survey thermocouples. Withdraw
the survey thermocouple (leave the sheath in position) by 20mm / 0.75in, wait
until the output stabilises and again measure the two temperatures. Repeat
for a series of 5 to 10 survey thermocouple immersion depths, i.e. from fully
inserted to 100 to 200mm / 4 to 8in back from full.
To determine the magnitude of the gradients, subtract the survey thermocouple
temperature from the measuring thermocouple temperature. Typically, all
values found will be within ±5°C of the measuring thermocouple.
