GB2160971A

GB2160971A - Temperature monitoring

Info

Publication number: GB2160971A
Application number: GB08516159A
Authority: GB
Inventors: Thomas Land
Original assignee: Land Infrared Ltd
Current assignee: Land Infrared Ltd
Priority date: 1984-06-26
Filing date: 1985-06-26
Publication date: 1986-01-02
Also published as: GB8416201D0; GB2160971B; GB8516159D0

Abstract

A method of monitoring the temperature of a surface (4) is described. The method comprises sensing the apparent temperatures (T1, T2,. . .) of the same portion of the surface (4) within at least two different wavelength bands ( lambda 1, lambda 2, . . .). The reciprocal (1/T1, 1/T2, . . .) of each apparent temperature is determined, and fractions of the reciprocal temperatures are algebraically summed to yield the reciprocal of the surface temperature T:- 1/T = alpha /T1 + beta /T2 + . . ., where alpha + beta + . . . . = 1. <IMAGE>

Description

SPECIFICATION Temperature monitoring The invention relates to methods and apparatus for monitoring the temperature of a surface.

The temperature of a surface that is much hotter than its surroundings can be determined by measuring its radiance (J) that is the amount of radiation emitted by unit area of the surface per second, within a suitable band of wavelengths. The radiance of a perfectly black surface at a temperature T is given by Wien's formula: - C2 J=C1A-5exp (1) AT where C1, C2 are constants and A is the effective wavelength at which the radiance of the surface is measured.

In practice, the radiance of a surface is always less than the radiance of a perfectly black surface at the same temperature by factor e which is the emissivity of the surface. If the temperature is to be measured accurately, the emissivity of the surface must be known to within a reasonable degree of accuracy. In a majority of industrial temperature measurements, sufficient accuracy can be obtained by measuring the radiance within a single wave band if the band is judiciously chosen. However, there remain a number of surfaces such as unoxidised solid and liquid metals where such methods fail to yield sufficient accuracy.Large errors arise because the metal surface has a low emissitivity whose value is critically affected by traces of oxide or other surface contamination that readily arise at high temperatures and by surface irregularities in metal streams that lead to cavities and multiple reflections between different parts of the surface of the stream.

One proposal for overcoming this problem has been to measure the radiance of the surface within two different wavebands and to deduce the true temperature from the ratio of the radiances. This is only satisfactory if the surface has the same emissivity at both wavelengths since the emissivity terms in the two radiance equations will then cancel out when the ratio is determined. Unfortunately, it is just the surfaces that are difficult to measure with single waveband thermometers whose emissivities vary substantially with wavelength. Furthermore, the variation of emissivity with wavelength is not easy to predict.

In accordance with one aspect of the present invention, a method of monitoring the temperature of a surface comprises sensing the apparent temperature of the same portion of the surface within n different wavelength bands where n is greater than one; determining the reciprocal of each sensed temperature; and algebraically summing fractions of the reciprocal temperatures, the algebraic sum of the fractions being unity.

By apparent temperature, we mean the temperature of a perfectly black surface which would have the same radiance as the surface that is being measured.

The fractions may have positive and negative values.

The invention may best be understood by considering the method mathematically. The apparent temperature T1 at a wavelength Ai will be given by

where e1 is the emissivity of the surface at the wavelength Ai, and T is the true temperature of the surface. From equation (2), it can be shown that

Similarly at a given wavelength A2, the apparent temperature T2 is given by

Combining equations (3) and (4) we obtain

which is of the functional form described above, ie

If three or more wavelengths Ar, A2, .3... are employed with radiance temperatures T1, T2, ..... a similar analysis shows that the functional form is preserved, ie

where α, ss, γ ... are functions of #1 A2 A3... and the corresponding emissivities #1, #2, #3....

It should be noted that the functional form (7) is preserved if an obscuration, variable with wavelength, for example due to fume or smoke in the instrument sight path, affects the measured radiance temperatures.

Further the functional form (7) is preserved if the emissivities #1, #2, #3... are "effective" values accounting for non-uniformity of surface condition within the measured area.

It should also be noted that, from (3), (4); the difference of the reciprocal temperatures

is independent of temperature and is in fact characteristic of the surface whose temperature is measured. For several wavelengths this remark applies to all the reciprocal temperature difference combinations 1 1 Tj Tj The fractions may be determined manually and be preset based on a priori knowledge of the measurement situation. For example, where the apparent temperature is sensed within two different wavelength bands centred on A1, A3 then by setting the fractions a, ss to be: o = A2/(A2A1) p -A1/(A2-A1) then the true temperature is measured, as can be seen from equation (5) above, provided the emissivity does not vary with wavelength.

In a more complex example where it is known that the variation of emissivity of the surface with wavelength is of the form 1n.=a+bA (9) but with a and b unknown then by using three wavelengths and setting

the true surface temperature is measured, as can be verified from the three wavelength equivalent of equation (5).

The three wavelengths may be centred on 550nm, 700nm and 900nm respectively with a bandwidth of 40nm.

Examples 1, 2 may be generalised to the case of assuming 1 ne = a + bA + xA2 + ... choosing an appropriate number of wavelengths and deriving a, ss, y. . from the appropriate order equivalents of equations (3)-(5).

Thus in the general case where 1 ne = a1 + a2A + ... aA-1 then the nth factor is given by:

Instead of the manual method described above, the method may further comprise determining the differences between the reciprocals of all the sensed temperatures; determining n-1 of the fractions by reference to the determined reciprocal temperature differences; and determining the nth fraction by subtracting the other fractions from unity.

For example suppose we wish to measure the temperature of a pool of liquid metal with patches of oxide floating on the surface. Suppose further that the emissivity properties of both oxide and metal are known, but the fraction x of the instrument field of view filled by oxide is unknown and varies with time.

Let the emissivity of the oxide be eO, independent of wavelength. Let the emissivity of the metal be e1, 82 at two wavelengths A1, A2 respectively.

Effective emissivity values using wavelengths A1 and A2 are now

e1(eff) = x.0 + (1-x) 81 1 # (11) .2(eft) = x.0 + (1-x) 82 J Equation (8) now becomes

This allows the unknown x to be derived from the measured 1 /T1 - 1 /T2.

Given x, appropriate values of .1(eft) 82(eft) are given by equation (11) and by reference to equation (5) appropriate values for a, p can be determined.

In practice, the apparently complex calculation above may be implemented as a simple lookup table of a versus (1 /T1 - 1 /T2) with p then taken as 1-a.

The method may further include obtaining the reciprocal of the algebraic sum of the fractions of the reciprocal temperatures in order to determine the true temperature.

It should be noted that throughout this analysis all temperatures are assumed to be in degrees Kelvin. Suitable adjustments can be made to determine the equivalent temperature in degrees Fahrenheit or Celsius.

In accordance with a second aspect of the present invention, a radiation thermometer comprises means for collecting radiation from a surface and transmitting it to detection means sensitive to radiation in n different wavelength bands, where n is greater than one; means for determining the apparent temperature of the surface within each wavelength band; means for determining the reciprocal of each apparent temperature; and means for generating an output signal representative of the algebraic sum of fractions of the reciprocal temperatures, the algebraic sum of the fractions being unity.

Preferably, the apparatus further comprises means for storing sets of values corresponding to n-1 of the fractions. Conveniently, this means is a look-up table.

The sets of values may correspond to different measurement situations and/or differences between reciprocal sensed apparent temperatures.

In another example, the apparatus may further comprise means to calculate the fractions from the wavelengths about which each band is centred. This is appropriate where either the emissivities are known to be the same or to be of the form 1 ne = a + bA + Conveniently, at least some of the means are provided by a suitably programmed computer.

An example of a radiation thermometer in accordance with the invention in which radiation is sensed in two wavebands will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of the apparatus; Figures 2 and 3 are circuit diagrams of parts of the apparatus shown in Fig. 1; and, Figures 4 and 5 are flow diagrams illustrating the operation of the computer in manual and automatic examples.

The thermometer comprises a conventional radiation pyrometer 1 including a lens 2 which focuses radiation onto a field stop 3. The pyrometer 1 is arranged to receive radiation from a surface 4 such as a flowing metal stream. Radiation passes through the field stop 3 into a conventional beam splitter 5 such as a semi-silvered mirror which passes the radiation to a pair of detectors 6, 7. Each detector has a filter 6A, 7A for passing radiation of a particular waveband centred on wavelengths A1, A2.

The detector 6 is shown in more detail in Fig. 2. The detector comprises a silicon cell 6B connected to an operational amplifier 6C, the silicon cell 6B generating a current related to the intensity of the incident radiation.

The output signals from the operational amplifiers of the detectors 6, 7 are fed to amplifiers 8, 9 whose output signals S1, S2 are fed to conventional analogue-to-digital converters 10, 11.

Digital signals from the converters 10, 11 which are related to the intensities of the incident radiation on the detectors 6, 7 are fed to a computer 1 2. The computer may comprise a suitably programmed microcomputer or based on a single chip eg. INTEL 8031, an EPROM for storing the program, and a small RAM.

The action of the computer 1 2 is illustrated by the flow diagram in Fig. 4. Firstly, the computer 12 reads the signal S, and from a conventional look-up table determines the apparent temperature T,. The look-up table (LUT) will previously have been set up in a manner to be described below. The value T1 is then inverted to obtain a value 1 /T,. Next, the signal S2 is processed in a similar way to obtain a value for 1 /T2. A value for the fraction a is then read which has been preset. Fig. 3 illustrates a simple circuit for enabling a to be preset comprising a potentiometer 1 3 across which is applied a reference voltage.The output voltage from the potentiometer 1 3 is fed to an analogue-to-digital converter 14 whose digital output, representing a, is fed to the computer 12. ss is then calculated from the equation a + p = 1 and the computer then computes the equation: 1 /T = a1 /T, +p1/T2 The result of this equation 1 /T is then inverted to obtain T which will be in degrees Kelvin and this is converted to degrees Centigrade by adding 273. The computer then outputs a digital signal repreesnting the temperature T in degrees C either to a digital-to-analogue converter for a display (not shown) or to a printer (also not shown).

The processing just described represents a manual system in which a is preset manually depending upon the surface 4 whose temperature is being measured. The system may be readily extended to use three or more wavebands and to deal with situations where emissivity varies with wavelength.

In an alternative, automatic arrangement, the potentiometer 1 3 and analogue-to-digital converter 14 can be omitted. In this example, shown in Fig. 5, the steps carried out by the computer are slightly different. Initially, the reciprocal temperatures 1 /T1 and 1 /T2 are determined as before. At this stage, the difference between the reciprocal temperatures 1 /T1-1 /T2 is calculated and this difference is used to address a LUT containing values of a against reciprocal temperature difference. The remaining processing is the same as in the Fig. 4 example.

It is a simple matter to calibrate the thermometer in order to obtain values for T1 and T2 corresponding to wavelength. The pyrometer is first sighted on a perfectly black surface and is adjusted so that it measures correctly the radiant temperature at each waveband. To achieve this, the thermometer is first set so that 100% of the output is derived from the measurement 1 /T1 and none from 1 /T2 and the value S, corresponding to this temperature is noted by the computer 1 2. This is then repeated for the other waveband and the computer 1 2 then can determine the different temperatures corresponding to signals S1 and S2.

In more complex arrangements, not shown, where more than two wavelength bands are used with corresponding additional detectors, the fractions a, ss, y etc may be determined in a similar way to that described in Figs. 4 and 5 or they may be calculated using equations similar to equation 10 above.

Claims

1. A method of monitoring the temperature of a surface, the method comprising sensing the apparent temperature of the same portion of the surface within n different wavelength bands where n is greater than one; determining the reciprocal of each sensed temperature; and algebraically summing fractions of the reciprocal temperatures, the algebraic sum of the fractions being unity.

2. A method according to claim 1, for monitoring the temperature of a surface whose emissivity varies as follows: 1 n8 = a1 + ..... .

the method comprising determining at least n-1 of the factors by using the formula:

3. A method according to claim 1, further comprising determining the differences between the reciprocals of all the sensed temperatures; determining n-1 of the fractions by reference to the determined reciprocal temperature differences; and determining the nth fraction by subtracting the other fractions from unity.

A. A method according to any of the preceding claims, further including obtaining the reciprocal of the algebraic sum of the fractions of the reciprocal temperatures in order to determine the true temperature.

5. A method of monitoring the temperature of a surface substantially as hereinbefore described with reference to the accompanying drawings.

6. A radiation thermometer comprising means for collecting radiation from a surface and transmitting it to detection means sensitive to radiation in n different wavelength bands, where n is greater than one; means for determining the apparent temperature of the surface within each wavelength band; means for determining the reciprocal of each apparent temperature; and means for generating an output signal representative of the algebraic sum of fractions of the reciprocal temperatures, the algebraic sum of the fractions being unity.

7. A thermometer according to claim 6, further comprising means for storing sets of values corresponding to n-1 of the fractions.

8. A radiation thermometer according to claim 6 or claim 7, wherein at least some of the means are defined by a suitably programmed computer.

9. A radiation thermometer substantially as hereinbefore described with reference to the accompanying drawings