Gas mixtures containing nitrous oxide

United States Patent Office _Patented ,

3,192,106

GAS MIXTURES CONTAINING NITROUS OXIDE Arthur Bracken and Colin C. Wilton-Davies, London,

England, assignors to The British Oxygen Company ₅Limited, a company of Great Britain

No Drawing. Filed Aug. 13,1962, Ser. No. 216,301 Claims priority, application Great Britain, Aug. 15,1961, 29,417/61

Claims. (Cl. 167—52)

This invention relates to gas mixtures containing nitrous oxide.

Nitrous oxide is used on a considerable scale as an anaesthetic gas and is normally administered in admixture with oxygen. For this purpose the two gases are 15 stored in separate pressure containers, usually termed cylinders, the oxygen being contained as a high pressure gas and the nitrous oxide as a liquefied gas at a lower pressure. The required mixture is commonly prepared at the time of use by controlled discharge of the respective ²⁰gases. Conventionally, the control means include a pressure regulator associated with each cylinder and some kind of proportioning device which meters the flow of the gases to a mixing chamber.

Oxygen is one of the so-called “permanent” gases, that ²⁵is to say; it remains in the gas phase at ambient temperatures whatever the pressure. The pressure in an oxygen cylinder progressively falls as the contents are used and does not vary much with the ambient temperature. A pressure gauge can therefore, serve as a simple contents indicator.

On the other hand, nitrous oxide can be and usually is stored under pressure in the' liquid phase. Under static conditions the same pressure will be indicated whatever the depth of the liquid in the cylinder and a pressure gauge does not therefore serve to indicate the cylinder content, so long as any nitrous oxide remains in the liquid phase. Under conditions of use, if the rate of withdrawal exceeds that of vaporization of the liquid, the ₄₀pressure will fall but will return to its original value when withdrawal ceases if there is still liquid in the cylinder. Variations in ambient temperature can also have a marked effect on pressure. No simple means for determining the contents of a nitrous oxide cylinder under normal condi- 45 tions of use is available and there is therefore a danger that the supply of nitrous oxide will run out unexpectedly.

The control of the two gases to give a mixture of constant predetermined composition is also a difficult matter and requires somewhat elaborate proportioning mechan- 50 ism constructed to close tolerances. Departure from the pre-set tolerances are liable to occur in use resulting in variations from the indicated composition. Regular testing and servicing are therefore essential where such mechanisms are concerned.

It is known that gaseous mixtures containing nitrous oxide can exist in a homogeneous state at pressures up to a partial pressure of nitrous oxide of 50 atmospheres. At higher partial pressures, liquefaction of part of the nitrous oxide would be expected so that the mixture would no longer be homogeneous. Thus for a mixture containing 70% of nitrous oxide and 30% of oxygen one would expect the upper limit for a homogeneous mixture to be about 72 atmospheres. It is, however, standard practice for the “permanent” gases to be distributed in cylinders at a pressure of 132 atmospheres or higher. Atmospheres above 300 are not practical, and are not contemplated by the present invention. The cylinders are designed for such duty and it is uneconomic to use them for the storage and transport of gases at considerably lower pressures.

Contrary to expectations, it has now been found possible to store a gaseous mixture containing up to about 75% nitrous oxide at a partial pressure of nitrous oxide which can be substantially in excess of 50 ats. without any liquefaction taking place.

It is thus possible to conserve a mixture of nitrous oxide with a “permanent” gas such as oxygen in a cylinder at filling pressures normally used for the “permanent” gases and to withdraw from such cylinder a mixture which remains homogeneous at all demand rates. Storage and dispensing apparatus is thereby simplified and the need for a proportioning device avoided.

According to this invention, a method of forming a homogeneous mixture of a “permanent” gas and .up to 75% of nitrous oxide comprises charging the gases in the predetermined required proportions into a pressure container until a pressure is reached at which the partial pressure of the nitrous oxide is greater than 50 atmospheres.

Preferably the mixture is compressed to at least 132 atmospheres.

I Acording to another aspect of this invention there is provided a homogeneous gaseous medium comprising up to 75% nitrous oxide in-admixture with a “permanent” gas confined in a container at a pressure such that the partial pressure of nitrous oxide is greater than 50 atmospheres.

The “permanent” gas most commonly used with nitrous oxide is oxygen. For inducing a state of analgesia, a nitrous oxide/oxygen mixture containing nitrous oxide in the range of 50 to 60% is frequently administered. The provision of the gases in pre-mixed form at high pressure, for example, 132 ats., eliminates the need to use gas mixing equipment at the time of administration. This has especial advantage in reducing the impedimenta of the travelling midwife attending domiciliary confinements.

For inducing a state of anaesthesia, a more potent drug such as halothane or cyclopropane may be added as a constituent of the nitrous oxide/oxygen mixture. Up to about 1% halothane, volume by volume, or up to 5% cyclopropane may be included in the nitrous oxide/oxygen mixture as the potent adjuvant, while maintaining the contents of the pressure container entirely in the gaseous phase. When cyclopropane is the potent adjuvant, the mixture may be inflammable and the usual precautions should be taken to guard against risk of static discharge.

In formulating the mixture in accordance with the in55 vention, the required proportion of nitrous oxide is preferably charged into a valved pressure container first, the quantity being determined either by weight or by volume. The required proportion, of “permanent” gas then added, either continuously or intermittently so as to allow equili60 brium to take place. The attainment of equilibrium, namely the complete disappearance of the liquid phase, may be accelerated by agitating the contents. For example, the container may be shaken during charging with oxygen, or the container may be rolled about after charging.

3,192,106

Alternatively the container may be inverted so that the oxygen bubbles through the liquid nitrous oxide.

In an alternative method of formulating the mixture, the nitrous oxide and oxygen are charged simultaneously into the container. The two gases are preferably premixed, either in a gas holder from which they are pumped into the container, or continuously by the method known as “streaming in” in which the flow rates of the separate gas streams are adjusted to produce the correct mixture.

The invention may be illustrated by the following example.

A standard compressed gas cylinder was used having a nominal compressed gas capacity of 220 cu. ft. at 132 atmospheres pressure. A weighed amount of nitrous oxide was charged into the cylinder, following which oxygen was introduced until the pressure reached 135 atmospheres. The composition of the mixture was found on analysis to be 74.9 of nitrous oxide and 25.1% of oxygen. The cylinder was allowed to discharge, and at intervals samples of the issuing gas were taken from the cylinder in its normal upright position and in the inverted position. The nitrous oxide content of the samples was determined by measurement of the difference between the refractive index of the gas mixture and that of oxygen. The results obtained which are subject to an error of ±0.2% were as follows:

<td> Cylinder press, p.s.i.</td><td> Cylinder position</td><td> Temperature, °C.</td><td> Nitrous oxide percent by vol.</td>

<td> 1,700......-..........-</td><td> Upright__________ Inverted__________</td><td> 21.1 21.1</td><td> 74.7 74.7</td>

<td> 1,500-.-......-_______</td><td> Upright___________ Inverted-_________</td><td> 23.0 23.0</td><td> 74.9 74.7</td>

<td> 1,300................-</td><td> Upright-__________ Inverted__________</td><td> 23.8 23.8</td><td> 75.0 75.0</td>

<td> 1,100-__......-.......</td><td> Upright___________ Inverted-__________</td><td> 20.2 20.2</td><td> 74.6 74.6</td>

<td> 900—.................</td><td> Upright_______ Inverted__________</td><td> 22.2 22.2</td><td> 74.7 74.7</td>

<td> 500-------------------</td><td> Upright__________ Inverted______...-</td><td> 23.0 23.0</td><td> 74.7 74.7</td>

<td> 300.........-........</td><td> Upright—_____— Inverted__________</td><td> 24.8 24.8</td><td> 75.2 75.2</td>

<td> 100.....-............-</td><td> Upright...-------- Inverted._____——</td><td> 27.2 27.2</td><td> 75.2 75.2</td>

From these results it is apparent that (1) the composition of the mixture is unchanged if the cylinder is inverted; there is therefore no partial liquefaction of the nitrous oxide, and (a) the composition of the mixture remains constant as the cylinder is emptied.

The following table compares the volumes of mixed gas available from identical cylinders holding different N₂O/oxygen mixtures at different pressures at 19°. C. Unit volume was taken as the volume of oxygen obtained from the cylinder charged with this gas to a pressure of 2000 p.s.i.g.

<td rowspan="2"> Pressure, p.s.i.g.</td><td colspan="4"> Percent volume of nitrous oxide in oxygen</td>

<td> 0</td><td> 47</td><td> 56</td><td> 100</td>

<td> 2,850_______________</td><td> 1.485</td><td> 2.504</td><td> 2.778</td><td> Unsafe (cyl. full of liquid).</td>

<td> 2,500_______________</td><td> 1.300</td><td> 2.225</td><td> 2.476</td><td> Do.</td>

<td> 2,000_______________</td><td> 1.000</td><td> 1. 665</td><td> 1.925</td><td> Do.</td>

<td> lisoo_____________</td><td> 0.731</td><td> 1.145</td><td> 1.247</td><td> Do.</td>

<td> 1,000______________</td><td> 0.469</td><td> 0.678</td><td> ' 0.696</td><td> Do.</td>

<td> 750_______________-</td><td> 0.357</td><td> 0. 485</td><td> 0.485</td><td> 2.667.</td>

<td> 500_________________</td><td> 0.244</td><td> 0.308</td><td> 0.308</td><td></td>

The following is illustrative of the case where a more potent drug, halothane, is included as an adjuvant in a nitrous oxide/oxygen mixture.

A cylinder of 538 gms. water capacity was evacuated, and 3.75 ml. halothane (7.01 g.) was admitted. The cylinder was then charged to a pressure of 1,620 p.s.i.g. at 20° C. with a mixture of 68.51% volume nitrous oxide and 31.49% volume oxygen. Samples withdrawn from the cylinder as it was progressively emptied, gave the halothane content indicated below:

<td> Cylinder contents in litres N.T.P.</td><td> Cylinder pressure, p.s.i.g., 20° C.</td><td> Halothane volume, percent v./v.</td>

<td> 151.6</td><td> 1,620</td><td> 0.518</td>

<td> 148.8</td><td> 1,600</td><td> 0.528</td>

<td> 139.1</td><td> 1,540</td><td> 0.535</td>

<td> 111.2</td><td> ' 1,390</td><td> 0. 538</td>

<td> 69.3</td><td> 1,045</td><td> 0.524</td>

<td> 49.8</td><td> 870</td><td> 0.512</td>

<td> 13.6</td><td> 345</td><td> 0.539</td>

<td> 4.31</td><td> 160</td><td> 0.538</td>

<td> 1.06</td><td> 50</td><td> 0. 523</td>

<td> 0.6</td><td> 20</td><td> 0. 516</td>

The small differences in halothane content were caused by sampling difficulties. They are not significant and show that the halothane content remained substantially constant.

Mixtures of nitrous oxide with “permanent” gases other than oxygen can be prepared in a manner similar to that described above to yield a mixture of constant composition, even if the partial pressure of nitrous oxide is in excess of 50 ats. A specific example of such a .mixture is a nitrous oxide/nitrogen mixture containing up to 75% nitrous oxide in a pressure container to 132 ats. Such mixtures are of use in leak detection, using infra red apparatus where it is necessary for the test procedure to be conducted at relatively high pressure. Such high pressure testing is sometimes necessary for detecting leaks in water mains and other hydraulic equipment such as tubular condensers.