Gas Sample Collecting Device

Abstract

Sample gas is diluted with inert gas at or near atmospheric pressure and the mixture is collected in a flexible sample bag hermetically sealed in a container by evacuating air from the container. The reduced pressure outside the sample bag and resulting expansion of the bag causes the gas mixture to be drawn into the bag.

Description

BACKGROUND OF THE INVENTION

This invention is concerned with the problem of collecting an unadulterated sample of air within a bag where it can be stored for periods of as much as several hours until it is laboratory tested for pollutants.

In the past, bags have been attached to the end of a blower. The air passing through the blower is subject to being contaminated by oil, dust and other foreign matter in the blower. To overcome this problem, some investigators have used a peristaltic pump wherein the air sample is forced along a resilient tube by squeezing the tube along a portion of its length.

However, the peristaltic pump does not overcome another problem that arises from storage of the air sample from the time it is collected until the time it is tested in the laboratory. This problem arises where the air sample contains reactive components such as hydrocarbons, which are present in automobile exhaust emissions. The reactive hydrocarbons cause the air sample to undergo chemical change during the storage period, so that the chemical composition of the air sample when tested in the laboratory is not the same as it was when it was collected in the field.

SUMMARY OF THE INVENTION

In accordance with the invention, gas sample collecting apparatus is provided that includes a sealed, rigid-walled container that encloses a flexible gas sample bag with a space being provided between the bag and container. Conduit means are provided for mixing an inert gas, such as nitrogen, with a sample gas and for introducing the mixture into the sample bag. The gas sample bag is filled by providing exhaust means for evacuating gas from the space between the exterior of the sample bag and the interior walls of the container.

The collected gas sample is relatively free of any foreign contaminants because the exhaust means is isolated from the path of the inlet gas. The inert gas dilutes the concentration of the reactive hydrocarbons and thereby slows down their reaction rate. The change in chemical composition of the collected gas sample during the storage period is thereby reduced and the composition of the sample gas at the time it is tested in the laboratory is more nearly representative of the composition at the time it was collected.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic view of the gas sampling apparatus according to the invention; and

FIG. 2 is an electrical wiring diagram therefor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a rigid-walled, cylindrical container 10 is hermetically sealed at the top by a lid 12 and suitable gasket, not shown, the lid 12 being fastened to the container 10 by a clamp 14. The container 10 may be 2 feet in diameter and 2 1/2 feet deep and may be constructed of suitable metal of sufficient thickness and strength to withstand the force of atmospheric pressure when the interior thereof is evacuated of air.

A carton 16 is disposed within the container 10. The carton 16 holds a sample bag 18 that is to be filled with a quantity of sample gas, such as air, for analysis in the laboratory as to the chemical composition of its pollutants. The sample bag 18 is made of strong flexible material that is chemically inert to the constituents of the gas being sampled. For testing hydrocarbons in air it has been found satisfactory to use a bag made of Teflon brand polytetrafluoroethylene.

The carton 16, which may be made of cardboard or similar material of inexpensive, light weight, stiff construction, is closed at the bottom and also at the top except for the outlet 19 of the sample bag 18. The carton 16 protects the sample bag 18 from damage and facilitates handling and storage of the sample bag 18 after the sample gas is collected. The carton 16 is preferably rectangular in shape. The carton 16 is perforated with holes 20 to facilitate removal of air from the space between the exterior of the sample bag 18 and the interior walls of the container 10. Another function of the carton 16 is to facilitate pressure testing of the sample bag 18 to detect pinholes or other leaks prior to the actual collection of the sample gas.

The neck of the sample bag 18 is closed around the end of an inlet conduit 21 and fastened thereto by a fastening strap 22. The inlet conduit 21 conducts a mixture of sample gas and inert gas into the sample bag 18. For this purpose, the inlet conduit 21 extends out of the container 10 through the lid 12 and through a support plate 24 resting on the lid 12 where it is joined to a tee 26.

The sample gas, such as ambient air, is fed to one side of the tee 26 through an open tube 28, a solenoid valve 30, a calibration orifice 32, and a pipe section 34 connected to the tee 26. Similarly, the inert gas, such as nitrogen, is fed under pressure from a gas supply 36, through a pressure regulator 37 through a solenoid valve 38, a calibration orifice 40, and a pipe section 42 connected to the tee 26. The pressure regulator 37 is coupled to the air supply tube 28 to sense the ambient pressure, and serves to reduce the nitrogen supply pressure to a known value slightly above that of the ambient air pressure, or about 1 psi gauge and to regulate it at that pressure.

The calibration orifice 40 for the nitrogen inert gas is an assembly of a central orifice plate 44 disposed between two gaskets 46, all sandwiched together and fastened between two outer flanges 48. The orifice plate 44 has a single central opening of a diameter selected to give a predetermined flow rate of nitrogen.

The calibration orifice 32 for the sample gas or air is similar to the calibration orifice 40 except that its orifice size is somewhat smaller in diameter. The flow rate through the orifices is proportional to the pressure drop across the orifice and the area of the orifice. Since the respective pressure drops across each orifice plate are regulated, the respective flow rates of the inert gas and the sample gas are a function only of the diameter of the orifices. Thus the dilution ratio may be varied by simply inserting orifice plates of different diameter.

Prior to using the bag 18 for sampling, it must first be pressure tested to detect pinholes and other leaks. This is done by filling the bag with inert gas to a low pressure, such as 5 psi. The cardboard carton 16 will prevent the flexible sample bag 18 from overexpanding and bursting. The sample bag 18 is determined to be satisfactory for use if it can maintain nominal pressure for an hour or so without the addition of more pressurizing gas. After pressure testing, the sample bag 18 is deflated and ready for use.

In order to fill the sample bag 18 with a mixture of the inert gas and sample gas, means are provided for reducing the pressure inside the container 10. An exhaust tube 50 extends from inside the container 10 through the support plate 24 to a flow meter 52. The flow meter 52 is connected to a conduit 54 and to one branch of a tee 55. A second branch of the tee 55 is connected to a metering valve 56 and to an open tube 57. A third branch of the tee 55 is connected to a vacuum pump 58 through a conduit 60. The metering valve 56 controls the amount of outside air that is allowed to bleed into the exhaust system from the open tube 57. The vacuum pump 58 exhausts the air within the container 10, thereby reducing the pressure on the exterior of the sample bag 18. With reduced pressure on its exterior, the sample bag 18 expands and sucks in the gas mixture.

The flow rate of the gas mixture, which is read on the flow meter 52, is adjusted simply by bleeding outside air through the metering valve 56 to vary the exhaust rate. Flow rate adjustment by means of the metering valve permits the use of an inexpensive constant running vacuum pump motor.

Typically, a flow rate of about 1/2 liter per minute has been used to fill, in about 3 hours, a sample bag of 100 liters in size with a gas mixture containing 10 parts nitrogen and one part air, giving a dilution ratio of 10 to 1. The diameter of the orifices for the nitrogen and air were 0.032 inch and 0.010 inch respectively. The vacuum pump 58 and solenoid valves 30 and 38 may be controlled by a timer to shut off at the end of a predetermined period.

An electrical vacuum switch 62, mounted in the wall of the container 10 is coupled to the vacuum pump 58 and the solenoid valves 30 and 38 to turn them off automatically if the sample bag 18 is filled before the allotted time. When the sample bag 18 has completely filled the protective carton 16, an abrupt rise in vacuum will occur that is sensed by the vacuum switch 62, causing the latter to open the electrical supply circuit shown in FIG. 2, whereupon the vacuum pump 58 shuts down and the solenoid valves 30 and 38 close.

The material used for the sample bag 18 must be inert to the gas being sampled. For sampling air containing automotive exhaust emissions, a material such as Teflon brand polytetrafluoroethylene has been found satisfactory. All of the parts in the inlet line that come in contact with the sample gas, such as piping, orifice plate, solenoid valve, should also be made of like inert material.

When the sample bag 18 is filled with the gas mixture, the lid 12 is lifted to provide access to the bag 18. The neck of the bag 18 may then be tied off below the inlet conduit 21, the fastening strap 22 released, and the bag 18 slipped off the inlet conduit 21. Thereafter, the carton 16 containing the bag 18 may be removed from the container 10 and transported to the laboratory for analysis.

Claims

1. Gas sample collecting apparatus, comprising:

A. a closed, rigid-walled container provided with an opening and a cover for hermetically sealing and unsealing said opening;

B. a carton nested within said container and smaller in size relative thereto to permit insertion of said carton within said container and withdrawal therefrom through said opening;

C. a flexible gas sample bag within said carton and having a neck through which gas may be introduced;

D. said carton being stiff in construction and substantially closed except for the provision of an outlet for accommodating the neck of said sample bag and of a multiplicity of holes for permitting ready exhaust of the interior thereof and of the space between said carton and said container and to limit expansion of said sample bag to the volume enclosed thereby;

E. a source of pressurized inert gas;

F. a first gas flow path for conducting said inert gas to a common junction;

G. a second gas flow path for conducting sample gas to said common junction where it is mixed with said inert gas;

H. a third gas flow path extending from said common junction to said sample bag for conducting the mixture of inert gas and sample gas to said sample bag;

I. means in said first and second gas flow paths including means coupled therebetween for adjusting the relative flow rates of said inert gas and said sample gas to attain a desired dilution ratio; and

J. an exhaust system for evacuating the space between the exterior of said sample bag and the interior walls of said container so as to cause said sample bag to fill with said gas mixture to a volume limited by the interior of said carton;

K. means for sealing the neck of said gas sample bag, said bag being separable from said third gas flow path upon filling with said gas mixture, thereby to permit removal of said carton containing said sealed

2. The invention according to claim 1, wherein said exhaust system of clause (J) includes means for adjusting the evacuation rate by bleeding

3. The invention according to claim 1 wherein said gas flow adjusting means of clause (I) includes

1. a calibration orifice in each of said first and second gas flow paths respectively, for restricting the flow of gas therethrough, the ratio of the diameter of the calibration orifice in said first path to that of the calibration orifice in said second path being greater than one to give a desired dilution ratio; and

2. pressure regulator means disposed in said first gas flow path and coupled to said second glas flow path to sense the gas pressure therein and to reduce the pressure of said inert gas to a predetermined value

4. The invention according to claim 1, and further including

L. a vacuum switch within said container to sense the pressure therewithin; and

M. means coupled to said vacuum switch to deactuate said exhaust system and interrupt the flow of inert gas and sample gas in said first and second gas flow paths, respectively, when the pressure sensed by said vacuum

5. The invention according to claim 4, and further including

N. a solenoid valve in each said first and second gas flow paths; and

O. means electrically coupling said vacuum switch, said solenoid valves, and said exhaust system.