GB2207759A

GB2207759A - Air temperature measurement

Info

Publication number: GB2207759A
Application number: GB08717536A
Authority: GB
Inventors: Christopher John Faisel Terry
Original assignee: ALCOM Ltd
Current assignee: ALCOM Ltd
Priority date: 1987-07-24
Filing date: 1987-07-24
Publication date: 1989-02-08
Also published as: GB8717536D0

Abstract

A device for measuring ambient air temperature is constructed with a surface part substantially thermally insulated from the interior of the device. This is achieved by providing a layer 3 of thermally insulating foam sandwiched between thin skins 4, 5 of a rigid material forming a housing of the device. Temperature sensors S1 and S2 are disposed respectively just inside the outer thin skin 5 and just inside the inner thin skin 4. By summing suitably weighted outputs from the temperature sensors, the ambient air temperature can be derived independently of the temperature within the housing. Equilibration of the housing and the ambient air temperatures, to avoid the housing acting as a heat sink or source for sensor S1, is therefore not necessary. The derived temperature may be used to correct the output of ultrasonic distance measurements. <IMAGE>

Description

AIR TEMPERATURE MEASUREMENT This invention relates to air temperature measurement.

In equipment, particularly of a portable nature, which requires accurate measurement of ambient air temperature, as in, for example, ultrasonic distance measuring apparatus where the variation of the speed of sound with air temperature must be compensated for, a problem exists which can seriously affect the measurement accuracy.

This problem arises as a result of the appreciable thermal capacities and thermal resistances associated with the components and housing of the equipment. If the equipment is moved from one ambient temperature to another, the internal components will initially act as heat sources or heat sinks to create thermal gradients with respect to the ambient atmosphere which will gradually decay as thermal equilibrium is established.

The thermal gradients extend beyond the boundary of the housing and include a layer of air surrounding the equipment, the so-called boundary layer. In an attempt to maintain accuracy, it is common practice to mount the temperature sensing device in a probe which extends from the housing to beyond the boundary layer but this has disadvantages since the appreciable width of the boundary layer and the requirement for very low thermal conductivity between the equipment and the sensor necessitates a long probe which may not be practical or aesthetically desirable.

According to the present invention there is provided a device having temperature measuring means for measuring the temperature of a medium surrounding the device, the device comprising a housing having a surface part substantially thermally insulated from the interior of the housing and in which the temperature measuring means includes a first temperature sensor arranged to detect the temperature at said surface part and a second temperature sensor arranged to detect the temperature at the interior of the housing whereby the temperature of the surrounding medium can be derived from the temperatures detected by the first and second temperature sensors and from the thermal resistances between the interior of the housing and the surrounding medium and between the surface part and the surrounding medium.

The housing can include equipment for transmitting a signal and receiving it after reflection from an article thereby to determine the distance between the article and the device, in which the temperature derived for the surrounding medium can be ' used to control calculating means arranged to calculate said distance using the speed of the signal through the medium, which speed varies with temperature.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic section through a device in accordance with an embodiment of the invention; Figure 2 is a diagrammatic sectional view from the front of the device of Figure 1; and Figure 3 is a simplified circuit diagram of the relevant part of the circuitry used in the device.

Figure 1 shows an air temperature measurement device having a housing 1 containing a power source, for example a battery 2 and circuitry in -the form of a printed circuit board 13. The housing may contain distance measuring equipment including components (not shown) for transmitting a signal to an article (such as the wall of a room) and for receiving the signal after reflection from the article. There is then provided circuitry for ascertaining the time difference between transmission and reception thereby to determine the distance between the device and the article by utilising information pertaining to the speed of the transmitted signal in the medium surrounding the device, e.g. air.

Such equipment is described for example in British Patent Specification No. 2153998 to the present applicant. As is known, the speed of a transmitted signal varies with the temperature of the medium through which it is transmitted. It is hence desirable in such distance measuring equipment to provide a suitable form of temperature compensation.

To this end, an appropriate surface la of the housing, that is one which would normally be unobstructed in use, is thermally insulated to as great a degree as is practicable from the rest of the equipment. This- is achieved in the described embodiment by constructing the corresponding wall from a layer 3 of thermally insulating foam, such as expanded polystyrene, sandwiched between thin skins 4, 5 of a rigid material, such as metal or plastic, forming the housing. In the described embodiment, the housing has the following dimensions: 10.16cm x 7cm x 2.54cm, the layer 3 has a thickness of 0.63cm; and each of the thin skins has a thickness of 0. 5mum.

The area of this thermally isolated surface la should be as large as possible, preferably comparable in size to the overall housing dimensions. For example 50% of the area of the front surface could be thermally isolated in this manner. The front surface of the housing, which bears the signal transmission and reception equipment, is advantageously chosen since this is the one held remote from the hand of a user. Also, it will not be obstructed by walls etc. of a room whose dimensions are to be ascertained.

A first temperature sensor, S1, is attached to the inside surface of the outer thin skin 5 by means of adhesive or a built in clip 6 as shown, and is connected to the circuitry on the printed circuit board 13 via appropriately long runs of fine gauge wire 7 so as not to reduce unnecessarily the thermal resistance between the first temperature sensor S1 and the internal components.

A second temperature sensor, S2, is situated inside the housing and ' is sited in such a way as to assume the temperature; T2, of the interior of the housing and in particular of components near the inner skin 4 of the thermally insulating wall. The second temperature sensor S2 can be attached to the inner thin skin 4 in the same manner as the first temperature sensor S1. As an alternative, the thin skin 4 could be dispensed with and the temperature sensor S2 secured directly to the printed circuit board.

The object of the above construction is to raise the thermal resistance between the temperature sensors 5,2 and S1 to a value comparable to, or greater than, the thermal resistance between the first temperature sensor S1 and the air beyond the boundary layer. The effect of this is that the temperature sensor S1 will quickly assume a temperature T1 somewhere between T2 and the ambient air temperature, T3, but, in accordance with the design of the device, probably nearer to the latter than the former.

Assuming the thermal resistances involved are independent of temperature, the following relationship will hold: T2 - T3 = K ................. (1) T1 - T3 where K is a constant dependent on the thermal resistances involved. As is well known, thermal resistance is the resistance to heat flow across a material layer. Where two layers of material having different thermal resistWnces are in contact with one another the above equation is derived by equating heat flows across the layers. In the simple case, the constant K is then the ratio of the two thermal resistances. In the described embodiment of the present invention, there are several thermal resistances involved and it would be difficult to calculate a precise value from all the actual thermal resistancas involved.The constant K is thus determined empirically as the ratio of the effective thermal resistance between the temperature sensor S2 and the air and the effective thermal resistance between the temperature sensor S1 and the air.

It is evident that the value of T3 can be calculated from the values of T2 and T1 as K is constant for a given housing construction. Reworking (1), the following equation is obtained: KT1 - T2 T3 = ............................... (2) K-l Hence, by choosing appropriate weighting coefficients for the outputs of the temperature sensors S1 and S2, the effect of a sensor at the ambient temperature, T3, can be simulated. Effectively, the output of the first temperature sensor S1 must be multiplied by K/(K-1) and the output of the second temperature sensor S2 must be multiplied by -1/(K-l) before summing the two outputs.

This operation can be simply carried out by the circuit as shown in Figure 3. Figure 3 shows an operational amplifier 10 connected to sum the inputs applied to its inverting input. The feedback resistor is designated R3. Its non-inverting input is connected to ground. The inputs to this circuitry are respectively the signal sl from the first temperature sensor fed through unity gain inverting amplifier 11 via a resistor R1 and the signal s2 from the second temperature sensor fed via a resistor R2.Hence the output of the operational amplifier 10 is

Hence, the resistor values are selected to satisfy the following equations: R3 K = ........................................... (3) Ri K-1 and R3 1 - = ................... (4) R2 K-1 The resulting output is thus related solely to the ambient temperature T3 and is independent of the temperature within the housing. Thus the effect on measurement accuracy of moving the equipment from one ambient temperature to another is eliminated as are the thermal effects of hand contact and internal power dissipation.

In practice, there may be shorter secondary thermal time constants in the equipment which need to settle before full accuracy is obtained, and also the value of K is dependent on the speed of air movement over the temperature sensing surface. These are factors which will affect the degree of improvement obtainable but still a considerable reduction in the time to reach full accuracy is to be expected.

Claims

CLAIMS:

1. A device having temperature measuring means for measuring the temperature of a medium surrounding the device, the device comprising a housing having a surface part substantially thermally insulated from the interior of the housing and in which the temperature measuring means includes a first temperature sensor arranged to detect the temperature at said surface part and a second temperature sensor arranged to detect the temperature of the interior of the housing, whereby the temperature of the surrounding medium can be derived from the temperatures detected by the first and second temperature sensors and from the thermal resistances between the interior of the housing and the surrounding medium and between the surface part and the surrounding medium.

2. A device as claimed in claim 1 in which the housing includes equipment for transmitting a signal and for receiving said signal after reflection from an article thereby to determine the distance between the article and the device, in which the temperature derived from the surrounding medium can be used to control calculating means arranged to calculate said distance using the speed of the signal through the medium, which speed varies with temperature.

3. A device as claimed in claim 1 or 2 which includes means for deriving the temperature of the surrounding medium using an empirically derived constant representing the ratio of the thermal resistance between the interior of the housing and the surrounding medium and the thermal resistance between the surface part and the surrounding medium.

4. A device as claimed in any preceding claim in which means for deriving the temperature of the surrounding medium includes means for summing a signal derived from the first temperature sensor and multiplied by a first weighting co-efficient with a signal derived from a second temperature sensor and multiplied by a second weighting co-efficient, in which the first and second weighting co-efficients are respectively: K , and 1 K-i K-i, K being a constant dependent on the ratio of said thermal resistances.

5. A device as claimed in any preceding claim which comprises a layer of thermally insulating foam between the interior of the housing and said surface part to effect said thermal insulation.

6. A device having temperature measuring means substantially as hereinbefore described with reference to, or as shown in, the accompanying drawings.