Armfield Engineering Education
HEAT TRANSFER AND THERMODYNAMICS
HT16 Radiation Errors in Temperature Measurement
HT16C Computer Controlled Radiation Errors in Temperature Measurement -
Issue 1
Radiative heat transfer between a thermometer and its surroundings may significantly affect the temperature reading obtained from the thermometer, especially when the temperature of a gas is to be measured while the thermometer ‘sees’ surrounding surfaces at a higher or lower temperature than the gas. The error in the reading from the thermometer is also affected by other factors such as the gas velocity over the thermometer, the physical size of the thermometer and the emissivity of the thermometer body.
In this equipment a group of thermocouples are used to measure the temperature of a stream of air, at ambient temperature, passing through the centre of a duct while the wall of the duct is elevated in temperature to subject the thermocouples to a source of thermal radiation. Each thermocouple gains heat by radiation from the heated wall and loses heat by convection to the air stream and conduction along the wire. The net result is an increase in the temperature of the thermocouple above the temperature of the air stream which it is supposed to measure. The result is an error in the reading from the thermocouple. A radiation shield can be positioned in the duct to show the effect of screening the thermocouples from thermal radiation from the duct wall.
On the HT16C the heater power, the air flow rate and the position of the
radiation shield can all be controlled via the HT10XC, either from the front
panel controls or from the software. On HT16, these parameters are adjusted
manually.
Right: HT16C Computer Controlled Radial Heat Conduction Accessory
shown with the HT10XC - Service Unit
Ordering Specification
• A small scale accessory to demonstrate how temperature measurements can be in.uenced by sources of thermal radiation.
• Comprises three K-type thermocouples with different styles of bead mounted in a vertical air duct. A fan at the base of the duct provides a variable air flow over the cylinder. A band heater heats the duct wall adjacent to the thermocouple beads.
• Heater rating 216 Watt at 24V DC.
• K-type thermocouples measure the air temperature upstream and the surface temperature of the heated duct section.
• On HT16C the air flow is electronically adjustable over the range 0 - 9m/s by a variable speed fan.
• On HT16 the air flow is manually adjustable
• The air flow rate is measured by a vane type anemometer in the outlet duct.
• A radiation shield can be lowered over the thermocouples to demonstrate the improvement in reading accuracy when the thermocouples are shielded from the source of radiation (On HT16C this is electronically activated, on HT16 it is manually positioned.)
• The accessory is mounted on a PVC baseplate which is designed to stand on the bench top and connect to the Heat Transfer Service Unit without the need for tools.
• A comprehensive instruction manual describing how to carry out the laboratory teaching exercises in combined radiation and convection (free and forced) and their analysis as well as assembly, installation and commissioning is included.
Experimental Capabilities
> Errors associated with radiative heat transfer:
- Effect of wall temperature on measurement error
- Effect of air velocity on measurement error
- Effect of thermocouple style on measurement error
> Methods for reducing errors due to radiation:
- Design of a radiation resistant thermometer
- Use of a radiation shield to surround the thermometer
Technical Details
The equipment comprises a tubular metal duct through which air, at ambient temperature, is blown vertically upwards by an electric fan.
A section of the duct wall is heated from the outside by an electric band heater and provides the source of radiation to the test thermocouples. Three thermocouples with different styles or sizes of bead are installed in the duct to demonstrate the differences in readings obtained. The temperature of the heated wall can be changed by varying the power supplied to the heater. The actual temperature of the heated surface is measured using another thermocouple which is attached to it. The effect of the duct wall temperature on the measurement thermocouples can be demonstrated. A further thermocouple is installed upstream of the heated section to measure the temperature of the ambient air passing over the thermocouples at the core of the duct.
The effect of air velocity past the test thermocouples can be demonstrated by adjusting the air flow. On the HT16C this is achieved by a variable speed fan with electronic control. On HT16 the fan is fixed speed with a manually adjustable throttle plate.
A vane type anemometer within the fan outlet duct allows the air velocity through the heated section to be measured.
A radiation shield, which remains close to the air temperature, can be raised or lowered over the thermocouples to demonstrate the change in readings when a radiation shield is used.
On HT16C this radiation shield is controlled by an electro-mechanical servo actuator under software control. On HT16 the radiation shield is positioned manually.
Essential Equipment
Essential Armfield Equipment
HT16C requires the HT10XC Computer Controlled Heat Transfer Service Unit.
HT16 can use either HT10XC or HT10X Heat Transfer Service Units.
Requirements
All electrical requirements are obtained from the service unit.
NOTE: the supply rating of the HT16/HT16C must
be the same as
that of the HT10X/HT10XC that it is used with, i.e.
HT16-A, HT16C-A: 230V/1ph/50Hz
HT16-B, HT16C-B: 115V/1ph/60Hz
HT16-G, HT16C-G: 230V/1ph/60Hz
Shipping Specification
HT16
Volume: 0.1 m³
Gross Weight: 9 kg
HT16C
Volume: 0.2 m³
Gross Weight: 15 kg
Overall dimensions
HT16
Height: 1.22 m
Width: 0.30 m
Depth: 0.35 m
HT16C
Height: 1.19 m
Width: 0.49 m
Depth: 0.44 m
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