GB2188190A - Temperature standard - Google Patents

Abstract

A temperature standard apparatus includes a light bulb (12) with a cylindrically-shaped helically-wound filament (14). The bulb has a glass envelope (20) with two optical flats (22) at opposite ends, each perpendicular to the helix axis (24). Gas such as nitrogen fills the envelope. The gas within the filament is at substantially uniform temperature equal to that of the filament, and may be used as a temperature standard for laser based thermometric techniques such as coherent anti-Stokes Raman spectroscopy (CARS). <IMAGE>

Description

SPECIFICATION Temperature standard This invention relates to a temperature standard apparatus, and to a method, for providing a region of a gas at a well-defined temperature.

According to the present invention there is provided a temperature standard apparatus comprising a light bulb with electrically conducting means defin ingacylindricaltubewhoselength is greaterthan its diameter, and an envelope surrounding the conducting means and defining two optical flats arranged such that a light beam may be passed through one optical flat to propagate through the centre of the cylindrical tube and to emergethroughtheotherop- tical flat, the envelope being filled with a gas comprising molecules with two or more atoms.

The conducting means preferably comprises a helically wound filament with close wound turns, for example at 1/2 mm spacing, and might be about4 mm in diameter and about 20 mm long. it is desirably of tungsten, so as to be able to reach high temperatures. The gas may be nitrogen or hydrogen; clearly it must not react with the filament material atthe temperature of operation.

The apparatus may also comprise electrical means for raising the filament to a desired, substantially constant, temperature, such as a stabilized d.c. electrical supply. It may also include an optical pyrometerfor monitoring or measuring the temperature of the filament.

In operation of the apparatus a region of gas of a well-defined temperature of up to 3500 K can be created within the filament. A lasertechnique such as coherent anti-Stokes Rahman spectroscopy (CARS) may be used to observe this region of gas, laser beams passing through the optical flats and being focussed within this region of gas.

The gas within the filament is substantially stagnant, particularly at high temperatures where the gas viscosity increases so that gas flow out of the ends of the tube or between the adjacent turns of the helix is inhibited. Over a ra nge of tem peratures the gas tem- perature is equal to the filament temperature; the exact relationship between these temperatures can be determined by fluid dynamic modelling ofthe gas.

Current high temperature standards use pyrometric measurements oftungsten filaments as a reference standard and thus the method ofthis invention permits the gas temperature to be related to internationally accepted standards. The pocket of gas can readily be made much largerthan a typical CARS probe volume, consequently CARS thermometry of a known temperature gas can be easily performed and a given CARS instrument accurately calibrated. The apparatuscan operate on a range of gases, mostim- portantly it can operate on nitrogen. Previous devices have utilised flames which cannot easily be calibrated fortemperature since thermocouples or optical measurements are complicated by the reac tire flame environment.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings in which: Figures la, band c illustrate the essentials of the CARS process; Figure 2 shows the theoretically predicted CARS spectrum for nitrogen at different temperatures; F!gure3showsa diagrammaticviewofatem- perature standard apparatus; and Figure4shows graphically a CARS spectrum observed with the apparatus of Figure 3.

Referring to Figure 1a, which illustratesthees- sentials of the CARS process, three laser beams are focussed by a lens 1 to cross within a probed volume of a gas, and emerge through a lens 2. Two beams are of frequency (ss1 and are generated by a common laser, while the other beam is of frequency S.

To generate the signal efficiently the beams are combined in the medium in a suitable geometry to achieve phase matching. (Ifthe pump and Stokes lasers are properly vectored then the generated CARS signal will be in-phase from all points within the intersection volume of the lasers; thus the CARS signal will grow coherently in a specific direction.) The geometry illustrated in Figure 1 (a) is commonly known as BOXCARS. The result is a coherent generation of a 'laser-like' signal at a frequency ( as = 2 wl . Commonly the laser at owl is termed the pump laser, and the laser at w, istermed the Stokes laser because it is frequency shifted to the 'red' ofthe pump laser.The signal beam at (x)as is termed anti Stokes because of its 'blue' shift from the pump laser. The properties ofthe anti-Stokes signal are de- termined both by the properties ofthe input lasers and the probed medium. Most practical applications of CARS utilise the 'broadband' technique where the Stokes laser, normally a dye laser, is designed to operate with a 1 50cm -1 or 3-4 nanometer bandwidth.

Suitable choice ofthe difference frequency between and (oS allows a particular molecular Raman reson- ance to be selected. CARS signals are efficiently gen eratedwhen a Raman resonance is driven by the difference frequency of the input lasers. Because a broadband spectra source is used forthe Stokes laser all features of a Raman spectrum of a single species may be simultaneously generated. The energs level diagram is shown in Figure 1 (b) andthe spectrum is shown in Figure 1(c). The pump laser source is a Q-switched Nd: YAG laser with a 10-20 Hz repetition rate and frequency doubled to 532 nm.

One third oftypically 300 millijoules at 532 nm would be used to optically pump a broadband dye laserto provide the Stokes source. The spectral properties of the generated anti-Stokes beam are analysed and recorded using a multi-channel detector.

The overall shape of a spectrum is dependent on the population ofthe individual vibrational and rotational levels of the molecule. For investigating air-fed combustion, the normal probed molecule is nitrogen, which is diatomic. It can vibrate, rotate and translate, though only in specific quantum states (V and J; translational motion always appears continuous because of uncertainties in the molecular and measurement reference frames). Typically, strong CARS signals are generated from O-branch spectra where transitions involving a vibrational quantum change of one (AV = +1 ) and a rotational quantum change of zero (AJ = 0) are observed.

The O-branch spectrum of an ideal harmonic oscillatorwith no vibrational rotational coupling would appearassinglespectral line. In practicethis is not observed. Transitions between individual vibrational states (AV = +1) exhibit a number of side bands which are increasingly shifted to lowerfrequency with increasing J. The actual splitting of adjacent lines is nearly quadratic in J and varies from 0.02 cm-1 to 1 cm-1. More significant shifts are seen for transitions arising from differentvibrational levels.

These are shifted duetothe anharmonic character of the molecular oscillator and are spaced by app roximately30 cm-1. Theoretical predictionasafunc- tion oftemperature for typical CARS spectra of nitrogen are shown in Figure 2. The individual rotational side bands are not full resolved, by most broadband CARS instruments, and show only as a slight modulation on the low frequency side of the principal vibrational features. As temperature increases, higher rotational levels are populated. Thus between 500-1000 Kthevibrational band arising from V = 0 broadens; above 1000 K highervibrational states become important and a new band at 2300 cm appears. The gas temperature can beat least approximately determined from the shape of the observed CARS spectrum.

Referring now to Figure 3, a temperature standard apparatus 10 consists of a light bulb 12 whosefilament 14 is electrically connected to a d.c. variable power supply 16. A calibrated two-colour radiation pyrometer 18 is arranged to viewthe surface ofthe filament 14. The filament 14 is of tungsten wire of diameter 0.5 mm wound helically and with twenty two turns each of diameter 4 mm, the gaps between adjacent turns being 0.5 mm. The filament 14 is surrounded by a heat resistant glass envelope 20 with two high quality optical flats 22 at opposite ends of the light bulb 12, each set perpendicular to the longitudinal axis 24 of the filament 14 (shown by a broken line), the axis 24 passing through the centre of each optical flat 22.The envelope 20 is filled with nitrogen gas at a desired pressure.

In operation of the apparatus 10thed.c. power supply 16 is energised and adjusted to raisethefilament 14to a desired temperature; the temperature of the filament 1 4can, if desired, be measured with the optical pyrometer 18. The apparatus 10 can be used in conjunction with a CARS spectrometer (not shown) as described with reference to Figure 1, the laser beams entering through one optical flat 22, crossing in the centre of the filament 14, and emerging through the other optical flat 22.

As mentioned above the gas temperature can be calculated from the shape of the CARS spectrum. Referring to Figure 4there is shown the CARS spectrum of nitrogen as determined experimentally (shown by the broken line), when the filamenttemperature as measured by the pyrometer 18 is 3063K. The spectrum shape gives an apparent, calculated temperature of 3071 K. The close agreement between these temperatures indicates that the gas within the filament 14 is at the same temperature as the filament 14; thatthis is so is also supported bytheoretical modelling ofthetemperature distribution ofthe gas within and close to the ends ofthe cylindrically- shaped enclosed gas volume.

Experiments have established that over a wide range oftemperatures the gas within the filament 14 is of uniform temperature equal to that of the surface of the filament itself. The temperature can be varied over a wide range by varying the power dissipation.

The enclosed gas can be used as a secondarytemperature standard, being calibrated with a CARS in strumentwhich has itself been calibrated with a thermostatically controlled gas cell.

It will be appreciated that the diameter and length of the filament might differ from that described, and indeed the helical filament might be replaced byan electrically heated tungsten tube. Each such filament will have a range of operating temperatures atwhich the gas temperature within the coil is sufficiently uniform to permit CARS, or another laser-based thermometric determination. This range will depend on the filament configuration and the gas pressure, and can be determined by theoretical modelling.

Claims (7)

1. Atemperature standard apparatus comprising a light bulb with electrically conducting means defin- ing a cylindrical tube whose length is greater than its diameter, and an envelope surrounding the conducting means and defining two optical flats arranged such that a light beam may be passed through one optical flat to Propagatethrough the centre of the cylindrical tube and to emergethroughthe other optical flat, the envelope being filled with a gas comprising molecules with two or more atoms.

2. An apparatus as claimed in Claim 1 wherein the conducting means comprises a helically-wound filamentwith close-wound turns.

3. An apparatus as claimed in Claim 2wherein the turns are spaced about 0.5 mm apart, and of diameter about 4 mm, and the tube so defined is of length about 20 mm.

4. An apparatus as claimed in anyoneofthepre- ceding Claims wherein the gas comprises nitrogen or hydrogen.

5. An apparatus as claimed in any one of the preceding Claims also comprising electrical means for raising thefilamentto a desired, substantially constant, temperature.

6. An apparatus as claimed in anyone of the preceding Claims also comprising an optical pyrometer for monitoring or measuring the temperature ofthe filament.

7. Atemperaturestandard apparatussubstanti- ally as hereinbefore described with reference to, and as shown in, Figure 3 of the accompanying drawings.