Apparatus and method for generating calibration gas
An apparatus and method for generating a low concentration of gas within a carrier gas flow employing one or more miniature one-piece cylinders filled under pressure with a pure gas, or a concentrated gas balanced with an inert gas or gas mixture. Released from the cylinder through a pierced or other controlled opening, the flow of the gas is regulated by a pressure regulator and a micro orifice to be blended into a steady stream of diluent gas, typically ambient air, to form a desired gas concentration. No gas is generated if the pressure of the gas in the cylinder, which is monitored constantly by a pressure transducer, is below a predetermined level. The apparatus can be built into a portable device, or an automated docking station (or calibration station) for testing and calibrating gas detection and monitoring instruments, or into a fixed gas detection system for performing such functions.
1. An apparatus for providing calibration gas to a gas monitoring instrument, comprising:
a calibration gas source comprising a miniature gas cylinder containing a calibration gas, and including a pierceable seal or open/close valve;
an outlet for providing test gas to the monitoring instrument; and
a gas conduit for providing diluted cylinder gas to the outlet, the gas conduit comprising:
a receptacle means for receiving the miniature gas cylinder in gas-tight manner, and including means for rupturing the seal or opening the valve;
a manifold;
pressure transducer means for determining pressure of the gas in the conduit;
a valve for releasing the gas into the gas conduit;
a micro orifice for providing a controlled flow of calibration gas in the conduit;
means for diluting gas in the conduit with air; and
control means for releasing test gas to the conduit upon determination of sufficient pressure in the manifold.
2. The apparatus of claim 1 , wherein the gas dilution means comprises a tee fitting with a leg of the tee fitting connected to a source of air.
3. The apparatus of claim 2 , wherein the source of air is atmospheric air, and a filter is disposed between the atmospheric air and the tee fitting.
4. The apparatus of claim 3 , additionally comprising a pump in combination with the tee fitting to provide a controlled flow of diluted calibration gas to the outlet.
5. The apparatus of claim 1 , wherein the means for diluting comprises a source of compressed air connected by a valve to the conduit.
6. The apparatus of claim 1 , wherein the gas source comprises a plurality of miniature gas cylinders, each cylinder being received in a said receptacle means.
7. The apparatus of claim 6 , wherein each gas cylinder is connected to a common manifold, with a valve disposed between each receptacle means and the common manifold.
8. The apparatus of claim 7 , wherein a single pressure transducer is connected between the valves and the common manifold.
9. The apparatus of claim 8 , wherein the transducer controls a single pressure switch controlling flow of gas to the manifold.
10. The apparatus of claim 7 , wherein a pressure transducer is connected between each valve and the common manifold.
11. The apparatus of claim 1 , wherein the miniature gas cylinder has a volume of about 10–100 ml.
12. The apparatus of claim 1 , wherein the receptacle includes a regulator.
13. The apparatus of claim 1 , wherein the receptacle includes venting means for excess gas pressure.
14. The apparatus of claim 1 , wherein the gas source is a single cylinder containing a plurality of gases.
15. The apparatus of claim 1 , wherein the micro orifice has a diameter between 0.5 and 50 μm.
16. The apparatus of claim 1 , wherein the calibration gas in the cylinder has a concentration of at least 0.1% by volume, and is diluted with a gas or gas mixture with which the calibration gas does not react.
17. The apparatus of claim 1 , wherein the calibration gas in the cylinder has a concentration of 1–100% by volume.
18. The apparatus of claim 17 , wherein the concentration is at least 98% by volume.
19. A method for providing a calibration gas to a gas detection instrument, comprising the steps of:
disposing in a receptacle a miniature gas cylinder containing a calibration gas, and opening a seal in the cylinder to permit gas to flow into a gas conduit;
determining pressure of the gas in the conduit, and verifying that said pressure is at least a predetermined level;
passing the gas which is at least at said predetermined pressure through a micro orifice to control the flow of the gas to a predetermined rate;
diluting the gas flowing at said predetermined rate with a diluting gas flowing at a known rate; and
passing the diluted gas to an instrument to be calibrated.
20. The method of claim 19 , wherein the calibration gas in the miniature cylinder has a concentration of at least 0.1% by volume, and is diluted with a gas or gas mixture with which the calibration gas does not react.
21. The method of claim 19 , wherein the calibration gas in the cylinder has a concentration of 1–100% by volume.
22. The method of claim 21 , wherein the concentration is at least 98% by volume.
23. The method of claim 19 , wherein the orifice has a diameter between 0.5 and 50 μm.
24. The method of claim 19 , wherein the diluting gas is filtered ambient air.