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GR1 Gas Laser Refractometer

For the precision measurement of the refractive index of air and gases.....

The GR1 Gas Laser Refractometer is a high accuracy optical interferometer which counts the number of fringes that are created by the change in optical path length through the sample cell as the refractive index of the gas within it changes.

The GR1 has its own special counting unit (ICF 1/1000) to carry out this exacting task. The unit displays and outputs the results in terms of fringe count, refractive index (RI) or, if the appropriate scaling factors are entered, gas concentration.

At the heart of the GR1 lies a special type of self-compensating Jamin interferometer. This ensures that besides being an extremely sensitive instrument, it is also ultra-stable. The GR1 incorporates a patented operating system, developed by the UK National Physical Laboratory (NPL) and manufactured by Index Instruments under licence from the British Technology Group (BTG). The entire system is engineered into a high quality, robust, main frame, designed to withstand the rigours of an industrial environment. It is therefore equally at home in both a research and manufacturing quality control situation.

Schematic Layout


The GR1 Gas Laser Refractometer has a number of important applications: Firstly, it has a major role in the calibration (and recalibration after routine maintenance) of vaporisers used in the administration of anaesthetic gases during human and veterinary surgery. Many hospitals throughout the world could not perform any surgical procedures were it not for their vaporisers, which are calibrated using the GR1. As such, the GR1 can truly be said to be at the forefront of patient care.

Index Instruments were the first commercial company to produce a refractometer of this level of precision and hold the original manufacturing licence granted by BTG. Working in close co-operation with the NPL, Index Instruments took the original concept and turned it into reality as the world's highest accuracy commercial refractometer.

The overall accuracy of anaesthetic gas measurement depends on many factors. These include the accuracy to which the refractive index of the active agent is known and the overall stability of the system. Experience has shown that 1/10th of a fringe, which is easy to measure on the GR1, corresponds to an anaesthetic gas concentration of approximately 0.01%. The ultimate sensitivity of the GR1 is 1/1000th of a fringe (approximately 0.0001%).

Other applications for the GR1 are any situation where it is necessary to measure the absolute refractive index of air and other gases throughout industry and research (ref 3,4,5).

Gas Tube

The standard gas sample tube normally supplied with the GR1 is a trilateral stainless steel type and is an original Index Instruments' design. It consists of a central tube to take the sample of gas to be measured and a pair of outer reference tubes. These are connected internally. The windows are flat and parallel to interferometric standards. All the tube connections are resistant to anaesthetic agents. The tube length is 316.4mm and the volume is approximately 16ml. The reference tube has a volume of approximately 33ml. Other types of sample tube can be supplied to special order (contact us with details of your precise requirements).


To measure the absolute refractive index of the gas, all compartments are evacuated using a pump. Fringes are then counted as sample is slowly re-admitted into the central tube. Changes of the refractive index of the sample can be monitored by sealing the outer reference tubes and recording the movement of interference fringes as sample is pumped through the central tube. Differential measurements between two substances can also be made by flowing a reference gas through the outer tubes and a gas under test through the sample tube. A further option is to seal or evacuate the outer tube, as required, depending on the type of measurement required.


1. Commission to European Communities Report, BCR Information, Applied Metrology EUR 14103 EN 1992 (ISSN 10185593). Evaluation of the Effect of Variations in the Refractive Index of Air Upon Uncertainties of Industrial Length Measurement. K P Birch, R E Ward, G Wilkening and R Reinboth.

2. Precision Engineering, Vol 5, 1983, pages 105-110. Bi-Directional Fringe Counting Interference Refractometer. M J Downs and K P Birch.

3. NPL Report MOM90 1988. The Precise Determination of the Refractive Index of Air. K P Birch and M J Downs.

4. J Phys. E, Vol 211988, pages 694-695. The Results of a Comparison Between Calculated and Measured Values of the Refractive Index of Air. K P Birch and M J Downs.

5. Journal of Optical Society of America A, Vol 8, 1991, pages 647-651. Precise Determination of Refractive Parameters for Atmospheric Gases. K P Birch.

Technical Specifications Download Brochure

The main application for the GR1 gas laser refractometer is for the calibration of vaporisers used in operating theatres in hospitals and veterinary clinics.

GR1 are currently being used worldwide to calibrate vaporises for such anaesthetic agents as

  • Isoflurane
  • Halothane
  • Servoflurane
  • Desflurane

Calibration factors are entered by the user making the GR1 suitable for any similar products.

The GR1 can also be used to measure the absolute RI of air or gases.  The ultimate resolution is 0.001 fringes, equivalent to 1 x 10-9 refractive index.

Index Instruments USA

Tel: +44 (0)1487 814313 Fax: +44 (0)1487 812789
Index Instruments Ltd. Bury Road Industrial Estate, Old Station Road,
RAMSEY, Huntingdon, Cambridgeshire, PE26 1NF, England

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