Calibrating Gas Generator

This invention relates, in general, to calibrated gases and their uses and, in particular, to a new and useful calibrating gas generator and to a method of forming a calibrating gas.

Breath-alcohol meters are used increasingly by the police in their patrols. In order to ensure the measuring accuracy, particularly in view of later court proceedings, the breath-alcohol meters are regularly calibrated. To this end is used a calibrator which is of high concentration stability, simple and inexpensive to operate.

A known calibrating gas generator contains two tanks which are filled with an aqueous ethyl alcohol standard. They are thermostated to the same temperature of 34.+-.0.2 deg C. The tanks are series-connected over lines with a pump. The pump conveys ambient air through the filling of the first and subsequently of the second tank. The air absorbs alcohol and water vapor from the liquid, the calibrating gas formed has a defined alcohol concentration which depends on the temperature of the tank. The concentration of the liquid drops, due to the alcohol given off. This effects particularly the first tank. From the second tank is only taken the amount of alcohol which the liquid in the first tank could not absorb, due to the impoverishment of the liquid which had already occurred there. This results in a gradual drop of the liquid concentration in the second tank too, which leads to differences in the concentration of the calibrating gas. Within the framework of the required tolerances, the amount of the calibrating gas that can be produced with a liquid filling must therefore be limited. The frequent replacement of the partly utilized liquid is annoying for the operator, due to the energy expenditure and the waiting periods until it can be operated again, because of the required heating (Operating Instructions 4752.01, Oct. 1981, Draegerwerk AG).

The invention provides a calibrating gas generator with a high concentration stability over a great number of measurements, which is simple to operate and which remain economical.

According to the invention, the temperature of the aqueous ethanol solution in the first solution tank is 37.degree. C., and in the second solution tank 34.degree. C. Curves I and II show the extended concentration stabilitity. With a maximum deviation of 1% from the initial value (nominal value), the number of tests can be increased from 55 to 110. This means essential advantages for a better utilization of the ethanol solution and the resulting savings in waiting time.

Accordingly, it is an object of the invention to provide a method of generating calibrating gas so that its calibrating life may be extended which comprises directing a carrier gas such as air successively through an ethanol solution in at least two tanks while maintaining a first tank at 37.degree. C. and the second tank at 34.degree. C.

A further object of the invention is to provide a calibrating gas generator which includes two series connected tanks with means for directing air so that it bubbles successively through the liquid in each tank and further including means for maintaining the temperatures of the solutions in each tank at approximately 37.degree. C. in the first tank and 34.degree. C. in the second tank.

A further object of the invention is to provide a calibrating gas generator which is simple in design, rugged in construction and economical to manufacture.

For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

In the Drawings:

FIG. 1 is a diagrammatical indication of a calibrating gas generator constructed in accordance with the invention;

FIG. 2 is a diagram showing the tests made with aqueous ethanol solutions in the variation of the concentration over the test life.

In accordance with the method of the invention, the carrier gas such as ambient air is pumped by a pump 7 so that it bubbles through an ethanol solution 4 of a first solution type 1 and is then directed through a connecting line 2 so that it bubbles through an ethanol solution 4 of a second tank 3 and is then delivered out to a test nozzle 8. In accordance with the invention, the ethanol solution in the first tank 1 is mainted at 37.degree. C. and in the second tank 3 is maintained at 34.degree. C. The temperature is controlled by a temperature controller 5 which is connected to a heater 6 and is responsive to a temperature sensor 50 disposed in the respective tanks 1 and 3.

The calibrating gas generator contains a first solution tank 1 and a second solution tank 3 connected over a connecting pipe line 2 both filled with an identical aqueous ethanol solution 4. By means of a temperature control 5 and heater 6 for each, the temperature of the aqueous ethanol solution 4 is brought to 37.degree. C. in the first solution tank and is reheated and maintained at 34.degree. in the second solution tank 3.

Ambient air is pumped into the system by means of a pump 7. It flows in fine bubbles through ethanol solution 4 in the first solution tank 1, then through connecting pipe 2 and in the same manner through the second solution tank 3 and leaves the calibrating gas generator through test nozzle 8 to the calibrating breath-control meter.

In operation, the ambient air in the first solution tank 1 is enriched with alcohol and water corresponding to the solution temperature, which ensures the desired vapor pressure. The same procedure takes place in the second solution tank 3.

The higher temperature of 37.degree. C. in the first solution tank 1 leads to greater evaporation than in the second solution tank 3 with a solution temperature at 34.degree. C. This has the effect that more vapor issues at first from the first solution tank 1 than from the second solution tank 3 and that a gradual enrichment of the calibrating substance even takes place in the latter, which compensates, however, later the decrease of the calibrating substance from the first solution tank 1, due to evaporation.

FIG. 2 shows a curve of the mode of operation, N tests were made with an aqueous ethanol solution, and the concentration C of the calibrating gas was measured in percent:

(a) The temperature of ethanol solution 4 in the solution tanks 1 and 3 was the same with 34.degree. C. The broken curve I shows the decrease of the calibrating gas concentration C. A deviation of 1% from the initial value (nominal value) is already obtained after about 55 tests;

(b) The temperature of ethanol solution 4 in the first solution tank is 37.degree. C. and in the second solution tank 34.degree. C. As it can be seen from curve II, the calibrating gas concentration C increases first by 1% to drop then after about 100 tests to 99%.

Breath-alcohol meters are used increasingly by the police in their patrols. In order to ensure the measuring accuracy, the instruments are regularly calibrated. To this end is used a calibrator which should be of high concentration stability.

The calibrating gas generator contains a first solution tank 1 thermostated to a temperature of 37.degree. C. and a following second solution tank 3 connected over a connecting line 2, which is thermostated to 34.degree. C., both filled with an aqueous ethanol solution 4. A pump 5 conveys air, which is conducted successively through the ethanol solution in the first and second solution tank and bubbles toward a test nozzle 6 while it is enriched with alcohol and water corresponding to the vapor pressure.

The different temperatures of 37.degree. and 34.degree. C. in the solution-tanks permit a greater number of possible calibrations before an impoverishment of the ethanol solution takes place.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.