GB2175693A

GB2175693A - Air temperature instrument

Info

Publication number: GB2175693A
Application number: GB08513634A
Authority: GB
Inventors: David Martin Moore
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1985-05-30
Filing date: 1985-05-30
Publication date: 1986-12-03
Also published as: GB8513634D0; GB2175693B

Abstract

In an instrument for measuring ambient air temperature a thermometer (1) and a radiation shield (4) are mounted within a vertical duct (7). Air is drawn upwardly over the thermometer (1) by a fan (12). The instrument is protected from solar and ground radiation by shields (18) and (10). An extended annular plate (9) reduces the ingestion of air from around the duct (7). Instruments in accordance with the invention will function over a wide range of wind speeds such as 0 to 50 ms <1> and air temperature ranges of -15 DEG C to 35 DEG C. <IMAGE>

Description

SPECIFICATION Air temperature instrument The present invention relates to air temperature instruments and particularly relates to aspirated thermometers for meterological field work.

Temperature and vertical temperature gradient are meteorological parameters of fundamental interest in the determination of atmospheric stability and heat flux. Meteorological temperature measurements may be required in geographically remote locations so that the instruments used for making such measurements must be portable, robust, maintenance free, and have low electrical power requirements. A typical power supply for a remote aspirated thermometer is 12V, 12 AH capacity. The vertical temperature difference between 1 and 10 metres height from ground level may be only 0.15C deg under neutral conditions. Hence accuracies of the order of 0.1"C over a wide range of meteorological conditions are required.

A known way to measure air temperature is to enclose a temperature sensor such as a mercury-in-glass thermometer, platinum resistance thermometer or thermistor inside a sky/ground radiation shield and to aspirate the sensor by using a fan.

Temperature errors and problems arise in known air thermometers because some or all of the following apply: a. all the surfaces of the instrument facing the sensor are not at the same temperature as that of aspirated air flowing over the sensor, b. the thermal time constant of the sensor and/or its radiation shield is such that unacceptably long response times are introduced, c. the sensing element is not dry, d. air which has been warmed or cooled by external surfaces of the instrument or by exhaust air from the fan enters the instrument and is drawn over the thermometer e. there is self heating from an electrical input to a resistance thermometer or thermistor, f. there is heat conduction along the material which supports the sensor, g. there is reverse air flow through the instrument at high wind speeds.

The present invention provides an air temperature instrument which is accurate over a wide range of meteorological conditions such as wind speeds from 0 to 50 m/s, and an air temperature range of - 150C to +35"C.

According to the present invention an air temperature instrument comprises a vertical duct housing a centrally mounted air thermometer a radiation shield for the thermometer comprising a thin-walled tube coaxial with the duct and extending around and spaced from the thermometer, a motor driven axial flow fan axially aligned with, and above, the tube, and a diffuser section between the duct and the fan wherein, in use, the fan draws ambient air upwardly through annular passages defined between the tube end, the thermometer and between the tube and the duct and thence through the diffuser section.

The air flow rate past the thermometer may be between 10 and 3 m/s, preferably 4 m/s.

The diffuser area ratio may be between 1:1.1 to 1:3 and is preferably 1:1.7.

The duct wall serves to protect the thermometer radiation shield from damage and must be a good thermal insulator. Preferably the outer surface of the duct is highly reflective at solar wavelengths. The duct may be of double skin construction and may have an intermediate insulative layer of, for example plastics foam, to reduce radiative and convective heat transfer from the outer skin to the inner skin.

The thermometer radiation shield should have a low thermal capacity and have high thermal conductivity so that it rapidly reaches the temperature of the aspirated air flowing over its surfaces. Preferably the outer surface of the shield is highly reflective.

The thermometer is mounted coaxially within the tube which comprises the radiation shield by suspension means which cause minimum air flow interference and is a poor conductor.

One suitable suspension means comprises fine, Kelvar (RTM) filaments.

Preferably the duct has a lip comprising a horizontal annular plate extending from its base. The lip reduces the possibility of ingesting air flow from around the outer surface of the duct and acts as a radiation shield for a ground radiation shield beneath the lip.

An embodiment of the invention will now be described by way of example only with reference to the drawing which is a sectional view of air temperature instrument.

The instrument comprises a circular duct having an inner wall 5 and an outer wall 7 of white plastics material and an intermediate layer of polystyrene foam 6. A thermometer 1, comprising a platinum resistance thermometer encased in an elongated ceramic body is suspended by upper and lower sets of four filaments 2a to 2d composed of Kevlar (RTM) which extend radially though mounting holes on the inner wall of a tubular radiation shield 4 composed of stainless steel foil. The radiation shield is similarly mounted by means of the Kevlar filaments 2 symmetrically within the duct. A diffuser section 8 extends from the upper end of the duct to an axial flow fan 12 having blades 13. Air from the fan passes through a circular air outlet 19 in a plate 15 above the fan 12.The air flow is directed outwardly radially between the plate 15 and a cap 16 supported on bolts 17.The upper part of the instrument is protected by a radiation shield 18 comprising a large diameter plastics disc having a downwardly extending a nular lip.

At the base of the duct an insulative plastics annular plate 9 extends horizontally from the duct entrance and reduces ingestion into the duct of air from around the outer surface of the duct. A lower disc 10 spaced from the duct entrance and supported by nylon bolts 11 serves as ground radiation shield. The plate 9 also acts as a solar radiation shield for the disc 10.

In operation, the fan 12, which is driven by a small electric motor (not shown), runs at constant speed and induces air flow, in the direction of the arrows shown in the drawing, over the surface of the thermometer 1. By correct choice of the fan and the diffuser size a suitably high, eg 4 m/s, air flow veiocity can be maintained in the duct. It is important to maintain a high aspiration rate in order to prevent reverse flow in high wind speeds, and also to reduce the possibility of ingesting air which has been heated or cooled by contract with the extension of the instrument, and also to maintain the thermometer as close as possible to the air flow temperature.

Claims

1. An air temperature instrument comprising a vertical duct, housing a centrally mounted air thermometer, a radiation shield for the thermometer comprising a thin-walled tube coaxial with the duct and extending around and spaced from the thermometer, a motor driven axial flow fan aligned with and above the tube, and a diffuser section between the duct and the fan wherein, in use, the fan draws ambient air upwardly through annular passages defined between the tube and the thermometer and between the tube and the duct and thence through the diffuser section.

2. An air temperature instrument as claimed in claim 1 wherein the air flow rate past the thermometer is between 3 and 10 ms

3. An air temprature instrument as claimed in claim 1 or claim 2 wherein the diffuser area ratio is between 1:1.3 and 1:3.

4. An air temperature instrument as claimed in claim 1 wherein the duct wall is of double skin construction having an intermediate insulative layer.

5. An air temperature instrument as claimed in claim 4 wherein the insulative layer is polystyrene foam.

6. An air temperature instrument as claimed in claim 4 wherein the outer surface of the duct wall is reflecting.

7. An air temperature instrument as claimed in claim 1 wherein the thermometer is a platinum resistance thermometer.

8. An air temperature instrument as claimed in claim 7 wherein the thermometer is encased in a ceramic body.

9. An air temperature instrument as claimed in claim 1 and further comprising means for suspending the thermometer from the radiation shield.

10. An air temperature instrument as claimed in claim 9 wherein the means for suspension comprises filaments of Kevlar (RTM).

11. An air temperature instrument as claimed in claim 1 and further comprising means for suspending the radiation shield from the duct housing.

12. An air temperature instrument as claimed in claim 11 wherein the means for suspension comprises filaments of Kevlar (RTM).

13. An air temperature instrument as claimed in claim 1 and further comprising means for restricting the injection of air at the lower end of the duct.

14. An air temperature instrument substantially as described with reference to the drawings.