Armfield Engineering Education

HEAT TRANSFER AND THERMODYNAMICS

HT16 Radiation Errors in Temperature Measurement - Issue 6

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 past 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. The HT16 provides a means of demonstrating these sources of error and suitable methods by which the errors can be reduced or eliminated.

Order Specification |
Technical Details |
Experimental Capabilities |
Essential Armfield Equipment

Optional Armfield Accessories |
Requirements |
Shipping Specification and Dimensions

Ordering Specification

• A small scale accessory to demonstrate how temperature measurements can be affected by sources of thermal radiation
• Comprises 3 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 thermocouples. A band heater heats the duct wall adjacent to the thermocouple beads.
• Heater rating 216 Watts at 24V DC.
• K-type thermocoules measure the air temperature upstream and the surface temperature of the heated duct section.
• The air flow is manually adjustable overthe range 0-7m/s.
• The air flow is measured by a vane type anemometer in the outlet duct.
• 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 is supplied.

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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 which are located on the centreline of the duct adjacent to the heated section. The heater is insulated on the outside and operates at low voltage for increased operator safety.

The power output from the heater is 216 Watts nominally at 24V DC. The temperature of the heated wall can be changed by varying the power supplied to the heater using the power control on the HT10X. The actual temperature of the heated surface is measured using a K-type thermocouple which is attached to it. The effect of the duct wall temperature on the measurement thermocouples can be demonstrated. The effect of air velocity past the test thermocouples can be demonstrated by rotating a throttle plate at the inlet to the fan.

A vane type anemometer within the fan outlet duct allows the air velocity through the heated section to be measured over the operating range 0 - 7m/s. Three thermocouples with different styles of bead are installed in the duct to demonstrate the differences in readings obtained. A radiation shield, which remains close to the gas temperature, can be raised or lowered over the thermocouples to demonstrate the change in readings when a radiation shield is used.

A 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.

All temperatures are measured using K-type thermocouples, each fitted with a plug for direct connection to the service unit HT10X.

The accessory is designed to be used in conjunction with the HT10X Heat Transfer Service Unit and is assembled on a mounting plate to stand on the bench top alongside the HT10X.

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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

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