IE49682B1

IE49682B1 - Method of doping shielding gas

Info

Publication number: IE49682B1
Application number: IE1156/80A
Authority: IE
Original assignee: Air Prod & Chem
Priority date: 1980-06-05
Filing date: 1980-06-05
Publication date: 1985-11-27

Description

This invention relates to a method of doping shielding gas. During recent years considerable concern has been expressed regarding the adverse physiological effects of ozone which is formed during MIG and TIG welding. In order to reduce the quantity oi ozone which the welder inhales it has been proposed to add certain dopants to the shielding gas employed during such welding. Typically the dopant is present in the shielding gas to the extent of about 1000 ppm by volume and reacts with the ozone formed to render the ozone physiologically inert.

Although such small quantities of dopant can be readily added to cylinders of shielding gas using expensive and sophisticated equipment there is no method readily available for the ordinary customer who stores his shielding gases in liquid form to add such small quantities of dopant to his shielding gases as they are vaporized.

In order to meet this problem we propose a method of doping a shielding gas with a dopant which will react with ozone, which method is characterized in that it comprises the steps of mixing said shielding gas and a carrier gas containing a known quantity of said dopant and having, with said dopant, a thermal conductivity differing substantially from that of said shielding gas, measuring the thermal conductivity of said mixture and adjusting the composition of said mixture until said thermal conductivity reaches a predetermined value. •la 49682 Preferably the thermal conductivity of said shielding gas and said carrier gas, with dopant, differ (at 0°C) by at least 10 cal/cm. sec/°C and more preferably by at least 20 cal/cm. sec/°c.

Typically, in Europe, the shielding gas will be argon or a 5 mixture of argon and carbon dioxide. In such a case the carrier gas is preferably helium. However, when the shielding gas is helium or a mixture of helium and carbon dioxide, such as commonly used in the OSA, the carrier gas is preferably argon.

The dopant may be amongst those already known, for example 10 hydrogen or nitric oxide or may be selected from other dopants which have not heretofore been proposed for this purpose. Such dopants include silane, carbon monoxide and halocarbons with fluorinated halocarbons being particularly preferred.

The concentration of the dopant in the carrier gas will to '5 some extent depend on such factors as the toxicity of the dopant, the accuracy of the mixing equipment and the accuracy of the meter measuring the thermal conductivity of the resulting mixture. However, in general, the carrier gas will contain between 1 and 30% (by volume) dopant and from 1 to 10% being preferred and from 3 to 7% being recommended. 6 8 2 For a better understanding of the invention reference will now be made to the following example: EXAMPLE A bank of 10 cylinders containing 5% (by volume) halocarbon 13 in helium was connected to one inlet of a standard mixing panel. The other inlet was connected to a bulk supply of argon. The valves on the mixing panel were then adjusted until the thermal conductivity of the mixture indicated that the mixture contained approximately 2% by volume helium, i.e. approximately 1000 ppm halocarbon 13.

It should be noted that whilst the present invention is particularly useful to customers who store their shielding gases in liquid form it is also useful to those who keep their shielding IF gases in cylinders arranged in banks of, for example, 10 to 50 cylinders connected to a common manifold.

Claims

1. A method of doping a shielding gas with a dopant which will react with ozone, which method is characterized in that it comprises the steps of mixing said shielding gas and a carrier gas

2. A method according to Claim 1, characterized in that the thermal conductivity of said shielding gas and said carrier gas, with dopant, differ (at 0°C, by at least 10 cal/cm. sec/°C.

3. A method according to Claim 1 or 2, characterized in that the thermal conductivity of said shielding gas and said carrier gas, IS with dopant, differ (at 0°C) by at least 20 cal/cm. sec/°C.

4. A method according to any preceding Claim, characterized in that said carrier gas is argon or helium.

5. A method according to any preceding Claim, characterized in that said dopant is selected from the group consisting of hydrogen, 20 nitric oxide, silane, carbon monoxide, halocarbons and mixtures thereof. 5 containing a known quantity of said dopant and having, with said dopant, a thermal conductivity differing substantially from that of said shielding gas, measuring the thermal conductivity of said mixture and adjusting the composition of said mixture until said thermal conductivity reaches a predetermined value. 10

6. A method according to any preceding Claim, characterized in that said carrier gas contains (by volume) from 1 to 30% dopant.

7. A method according to any preceding Claim, characterized in 25 that said carrier gas contains (by volume) from 1 to 10% dopant.

8. A method according to any preceding Claim, characterized in that said carrier gas contains (by volume) from 3 to 7% dopant.

9. A method of doping a shielding gas substantially as described herein w«K referena t« fta . ζ

10. A shielding gas doped by a method according to any of the preceding claims.