Container for Calibration Gas

The present invention relates to containers for holding gases which are suitable for the calibration of gas sensing devices. In particular it relates to low pressure containers.

Gas sensing instruments are used throughout many industries utilising different types of sensor. The sensors often have characteristics such that the sensitivity can change, or 'drift', over time. In order to ensure accurate readings, the instruments are calibrated or checked for correct operation using a known gas concentration.

The current state-of-the-art for calibration is using either refillable or non- refillable gas cylinders with appropriate regulators at the required flow rate. These are high pressure containers which contain the required gas concentration for calibration e.g. 20ppm carbon monoxide in air, filled to a significantly higher pressure than atmosphere.

The advantage of these cylinders is that they can be used for a number of calibrations, however the cylinders are high pressure and therefore shipping the cylinders to customers is expensive as they are classed as dangerous goods because they are compressed gas.

Up until now, there has been no alternative to these cylinders for calibration so there is a high expense and low convenience for the products, leading some customers to neglect the calibration of their instruments.

According to the present invention there is provided a container for calibration gas comprising a flexible reservoir to hold a calibration gas, and

a hermetically sealable connector to allow fluid connection of the reservoir to a device to be calibrated.

Preferably the reservoir is at least partially enclosed within a cover.

The reservoir is suitably formed from any material which is capable of holding the desired gas within the reservoir. The material should be flexible to allow it to expand when filled with the gas, and to be compressed when the gas to be expelled from the reservoir. The material may, for example, be rubber, latex, polychloroprene or a nylon fabric. A problem associated with some materials is the tendency for gas to escape from the reservoir, especially where gases with small molecules are stored, e.g. hydrogen and helium. It may therefore be desirable to use a material which has a very small pore size (e.g. a metallised plastic film), or which has been treated to minimise diffusion across the wall of the reservoir (e.g. with a polymer gel such as Hi Float). Various materials for the storage of gases are well known in the art.

The connector may be any connector suitable for connection to a device in need of calibration. Suitable connectors, such as gas tight valves, are well known in the art, and it is well within the person skilled in the art to select an appropriate connector. The connector must be sealable such that, prior to use, the reservoir is hermetically sealed and the calibration gas does not escape.

The connector may be attached to the reservoir in a number of ways. It may suitably for formed integrally with the reservoir. Alternatively, the connector may be joined to the reservoir by gluing or it may be fused (e.g. by heat or ultrasonic welding).

The connector may suitably contain a flow control device which is able to regulate the amount of gas which is permitted to escape from the container during use. The flow control device may simply be a restriction in diameter of the lumen of the connector, or it may be a more elaborate system.

Suitably the container is filled with a gas appropriate for a calibration purpose. It will be clear that essentially any gas which is useful for calibration may be provided within the reservoir. Exemplary gases, may be any noble gas (e.g. He, Ne, Ar, Kr and Xe), other elemental gases (e.g. N ₂ , O ₂ , H ₂ , Cl, and F), and gaseous compounds (e.g. CO, CO ₂ , H ₂ S, NO, NO ₂ , hydrocarbon gases, and SO ₂ ). Clearly there are too many gases to list individually. In some cases the gas may be a particular mixture of different isotopes, e.g. C ¹² O ₂ and C ¹³ O ₂ , in a desired ratio.

The gas is suitably at a relatively low pressure, typically at or slightly above atmospheric pressure. Preferably the gas is at from about 1 to about 1.5 atmospheres absolute. Because the calibration bag is at near atmospheric pressure there is not a problem with transport safety regulations compared with highly pressurised products.

The cover may suitably enclose substantially all of the reservoir. Desirably the cover is formed primarily of a relatively rigid material. The rigid material may be, for example, plastic or cardboard. The cover may provides some protection for the reservoir to avoid the risk of puncture.

The cover may also provide the function of providing a convenient surface on which pressure may be exerted to expel the gas from the reservoir when required. It is desirable that the cover comprises two essentially flat panels between which the reservoir can be squeezed, thus ensuring essentially complete emptying of the contents. Provision of such a cover

makes the container of the present invention particularly suitable for use in an automated system.

In a further aspect the present invention provides a method of calibrating a gas sensing device comprising: connecting a container as set out above to the device; causing the gas within the reservoir to be expelled into the device; and using said gas to calibrate the device.

According to another aspect of the invention, there is provided a method of producing a container for calibration gas, the method comprising the steps of:

- providing a reservoir to hold a calibration gas; - at least partially filling the reservoir with a calibration gas; and

- sealing a hermetically sealable connector to retain the calibration gas therein.

Preferably the method comprises the step of providing a cover which partially encloses the reservoir.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig 1 shows a container according to the present invention; Fig 2 shows a container according to the present invention packaged within a box for shipping; and Fig 3 shows a net which can be formed into a cover for the reservoir.

As best shown in Fig 1 , a reservoir 12 is formed from a metallised plastic film and provided with a connector 14 at the middle of one end. The

reservoir 12 is essentially a bag which is essentially rectangular in plan view. The connector 14 is located in the centre of one of the edges of the bag 12. The connector 14 is essentially a tube containing a gas-tight valve. When the reservoir is filled with the desired gas, it is prevented from escaping by the valve.

The reservoir 12 is placed within a cover 10 formed from sheet corrugated cardboard. Fig 3 shows a net suitable for use in the formation of a cover. All dimensions shown in the net are in mm. The corrugations run in the directions shown by the arrows in the drawing. The cover 10 encases the bottom and top of the reservoir, with the sides being left uncovered. A hole is provided in the cover 10 through which the connector 14 projects. The grey areas shown in the figure are removed an disposed of to form the hole. The dashed lines show gently perforated fold lines where the net is folded to shape it into the final cover.

For transport it may be preferable that the container is placed in a box 16 or other packaging for additional protection. However, the cover 10 provides a significant degree of protection to the reservoir 12 which can eliminate the need for additional packaging.

The container can be filled by attaching it to a source of calibration gas, opening the valve and allowing the gas to flow into and fill the reservoir, and the closing the valve to retain the calibration gas within the reservoir.

Alternatively the connector 14 may not comprise a valve, but rather may be provided with sealing means to hermetically seal it (e.g. a membrane, cap or bung) following the filling step. Conveniently this sealing means may be removed or punctured when the gas is to be expelled into the

device to be calibrated, e.g. by puncturing a membrane or removing a cap or bung.

In use the container is connected to a device to be calibrated via the connector 14 . The container has a suitable connector to connect to the device in question. The valve (or other sealing means) provided in the connector 14 is opened to allow the gas to flow from the reservoir. The valve may conveniently be opened as a result of the interaction of the connector with the device, e.g. a feature of the device forcing the valve open.

The reservoir 12 is then compressed to force the gas from out and into the device. Conveniently the reservoir 12 may be compressed by applying force to both sides of the cover 10, thus compressing the sandwiched reservoir 12; this is especially conveniently achieved by pressing on one side of the cover 10 when the container is resting on a surface such as a table.

Once calibration is complete, the container is disconnected from the device and may be recycled or disposed of.

This low pressure calibration bag is an inventive way for calibration gas to be packaged and sent to users as currently this can only be done via high pressure cylinders.

The advantages the calibration bag has over current state-of-the-art are: The handling of high pressure gases is potentially a health and safety risk and specific recommendations around the handling of compressed gases exist. Handling gas at atmospheric pressure (or

almost atmospheric pressure) has fewer health and safety risks associated with it.

There are often import restrictions on high pressure cylinders which would not apply to the calibration bag (e.g. transportation and training).

The use of calibration bags is more environmentally friendly as fewer materials are used, they are recyclable and transport restrictions are reduced therefore reducing the carbon footprint of the product.