Differential gas sensing in-line monitoring system

A method for determining the calibrated concentration of an impurity species in a flowing gas stream, in a low concentration range below a predetermined concentration value, and in a high concentration range above said predetermined concentration value, said method comprising:

• providing an in-line monitoring system comprising:
• (a) means defining a flow path for a flowing gas stream;
• (b) a gas purifier through which gas deriving from the flowing gas stream is flowable to remove impurity species therefrom;
• (c) means for sensing the impurity species concentration of gas deriving from the flowing gas stream;
• (d) means for selectively flowing gas from the flow path through the gas purifier to yield purified gas depleted in said impurity species;
• (e) means for flowing purified gas from said gas purifier to said sensing means;
• (f) means for selectively flowing gas from the flow path to the sensing means without passage of the gas through the gas purifier;
• (g) means operatively coupled to said sensing means for determining a calibrated equilibrium value of impurity species concentration in the flowing gas stream from impurity species concentration in unpurified gas sensed by said sensing means in calibrative relationship to impurity species concentration of purified gas sensed by said sensing means;
• (h) flow control means operatively coupled with gas flowing means (f) and (g), and arranged to selectively, alternatively and repetitively flow gas deriving from the flowing gas stream through gas flowing means (f) to the gas purifier for a first selected period of time t.sub.1, and subsequently through gas flowing means (g) for a second selected period of time t.sub.2, in a repeating cycle, wherein in the low concentration range below said predetermined concentration value, t.sub.2 >t.sub.1, and wherein in the high concentration range above said predetermined concentration value, t.sub.1 >t.sub.2 ;
• flowing gas from the flowing gas stream selectively from the flow path through the gas purifier to yield purified gas depleted in said impurity species, alternatingly with flowing gas from the flow path through the gas flowing means (f), and passing the gas to said sensing means;
• and
• selectively operating said flow control means (h) to flow gas through means (f) to the gas purifier for a first selected period of time t.sub.1, and subsequently through gas flowing means (g) for a second period of time t.sub.2, in a repeating cycle, wherein in the low concentration range below said predetermined concentration value, t.sub.2 >t.sub.1, and wherein in the high concentration range above said predetermined concentration value, t.sub.1 >t.sub.2.

An in-line monitoring system for determining a calibrated concentration of an impurity species in a flowing gas stream, in a low concentration range below a predetermined concentration value, and in a high concentration range above said predetermined concentration value, said system comprising:

• a purifier for removing impurity species from gas derived from the flowing gas stream;
• a means for sensing the impurity species concentration of gas derived from the gas flow stream in a sequential and repetitive sensing operation wherein in a first sensing mode the gas is purified of impurity species by passage thereof through said purifier prior to sensing thereof and wherein in a second sensing mode the gas in unpurified, and wherein in the first sensing mode and second sensing mode, the sensing mode cycle times are varied in accordance with the impurity species concentration, so that in said low concentration range below said predetermined concentration value, the second sensing mode cycle time is greater than the first sensing mode cycle time, and wherein in said high concentration range above said predetermined concentration value, said first sensing mode cycle time is greater than the second sensing mode cycle time, and
• a means for determining a calibrated equilibrium value of impurity species concentration in the flowing gas stream from sensing concentrations in the first and second sensing modes.

An in-line monitoring system for determining a calibrated concentration of impurity species in a flowing gas stream, comprising:

• a sensor including a binding medium having irreversible binding affinity for the impurity species, for sensing impurity species concentration in a gas;
• means for purifying gas derived from said flowing gas stream to yield purified gas;
• means for flowing unpurified gas derived from said flowing gas stream to said sensor and for flowing purified gas from said gas purifying means to said sensor, such that said unpurified gas is sensed by said sensor in a differential mode wherein sensing time of the sensor in exposure to said unpurified gas is less than 50% of the operating time of the sensor, whereby the sensor service life is extended relative to continuous sensing of unpurified gas;
• means for determining a calibrated concentration of impurity species in said flowing gas stream from impurity species concentrations of said purified and unpurified gas sensed by said sensor;
• and
• means for controlling (1) sensing time of the sensor in exposure to said unpurified gas, relative to (2) sensing time of the sensor in exposure to purified gas, in response to impurity species concentration of unpurified gas sensed by said sensor.