Pneumatic detector for chromatographic analyzer

• US 4,033,171 A
• Filed: 02/14/1975
• Issued: 07/05/1977
• Est. Priority Date: 02/14/1975
• Status: Expired due to Term

• Current Assignee: The Foxboro Company (Schneider Electric SE)
• Original Assignee: The Foxboro Company (Schneider Electric SE)
• Inventors: Kalinoski, Richard W., Karas, Edwin L., Annino, Raymond
• Primary Examiner(s): Queisser, Richard C.
• Assistant Examiner(s): Kreitman, Stephen A.
• Application Number:
  US05/549,929
• Time in Patent Office: 872 Days
• Field of Search: 73/23, 73/23.1, 73/55, 73/56, 73/54, 73/205 D, 73/211, 137/624.14, 23/232 C, 23/232 R, 23/254 R
• US Class Current: 73/23.24
• CPC Class Codes: F15B 5/003 characterised by variation of the pressure in a nozzle or the like, e.g. nozzle-flapper system
• CPC Class Codes: G01N 30/62 Detectors specially adapted therefor

Abstract

A gas chromatograph having a pneumatic detector adapted to produce a component concentration measurement signal that is relatively unaffected by changes in carrier flow rate. The detector includes an orifice connected in the flow stream at the output end of the chromatographic column to produce a pressure signal responsive to density. This orifice signal also responds to flow rate of the fluid passing through the column. A capillary is connected in series with the orifice to produce a second flow-responsive pressure signal which is subtracted from the orifice signal in a way to effectively eliminate changes in the final measurement output signal due to changes in flow rate while leaving the signal variations due to changes in density, i.e. concentration. The orifice and capillary pressure signals are combined subtractively by means of a balanceable force-bar arrangement to which the pressure signals are applied by bellows of predetermined relative sizes, and which is automatically maintained in balanced condition by a rebalance bellows operated by a pneumatic nozzle-flapper feedback device producing the detector output signal.

Claims

1. For use in a gas chromatograph including a separation column into which a sample mixture can be injected to be conducted through the column by a carrier gas so that the mixture components are physically separated to emerge from the column at different times;

a pneumatic detector for measuring the concentration of the separated components without significant error due to variations in fluid flow rate;

said detector comprising;

an orifice connected to receive the gas from the column and to produce a first pressure signal responsive to gas density and to gas flow velocity;

a capillary connected in series with said orifice to produce a second pressure signal responsive to gas viscosity and to gas flow velocity; and

means to subtractively compare said first and second pressure signals and to develop a concentration measurement signal in accordance with such comparison;

said comparison means includng a balanceable force bar;

first and second pressure-responsive means adapted to apply opposed forces to said force-bar;

means connecting said first and second pressure signals to said first and second pressure-responsive means respectively;

pneumatic means responsive to small movements of said force-bar and adapted to maintain said force-bar in balance;

said first and second pressure-responsive means being arranged to provide a force-effectiveness ratio different from unity so as to obtain effective cancellation of the flow-related elements of such signals, whereby said measurement signal is not significantly affected by variations in fluid flow rate.

2. A detector as claimed in claim 1, wherein said orifice is upstream of said capillary;

said orifice pressure signal being developed at a point upstream of said orifice and directed to said first pressure-responsive means;

said capillary pressure signal being developed upstream of said capillary and directed to said second pressure-responsive means; and

the force effectiveness of said second pressure-responsive means being substantially greater than the force effectiveness of said first pressure-responsive means.

3. In chromatographic apparatus of the type including a separation column through which a carrier fluid conducts a sample fluid mixture to be separated into its components as it passes through the column, the apparatus including an orifice in a line connected to the column output to serve as a detector means to produce an output signal responsive to a characteristic of the fluid sample components which characteristic is in turn proportional to concentration of such components, and wherein the output signal of such detector means also is undesirably responsive to fluid flow rate;

that improvement for minimizing the response of said output signal to changes in flow rate of the fluid passing through the column, comprising;

compensating means comprising a capillary connected in series with said orifice downstream thereof and arranged to produce a compensating signal responsive to the fluid flowing therethrough;

means for combining said output signal and said compensating signal in opposed sense to develop a sample concentration measurement signal;

said combining means including a force-balanceable member;

first pressure-responsive means coupled to the upstream side of said orifice and arranged to apply a corresponding force in one direction to said force-balanceable member;

second pressure-responsive means coupled to the juncture between the downstream side of said orifice and the upstream side of said capillary and arranged to apply a corresponding force in an opposite direction to said force-balanceable member;

means responsive to small movements of said force-balanceable member for automatically adjusting the net force applied thereto so as to maintain said member in force balance, whereby the adjustments of net force can serve to provide the concentration measurement signal; and

means for setting the relative rates-of-change of said output and compensating signals with respect to changes in fluid flow rate therethrough to produce at least substantial cancellation of the effects of such changes in flow rate on said concentration measurement signal, while effecting changes in said concentration measurement signal in response to changes in said fluid sample characteristic.

4. Apparatus as claimed in claim 3, wherein said second pressure-responsive means has an effective area significantly greater than said first pressure-responsive means.

5. In gas chromatographic apparatus of the type including a separation column through which a carrier gas conducts a sample gas mixture to be separated into its components as it passes through the column, the apparatus including component concentration detector means in a conduit connected to the column output for producing an output signal responsive to a characteristic of the gas sample components which characteristic is in turn proportional to the concentration of such components;

that improvement in said concentration detector means comprising;

an orifice for producing a first pressure signal responsive to characteristics of the gas flowing therethrough;

a capillary connected in series with said orifice for producing a second pressure signal responsive to the characteristics of the gas flowing therethrough;

means for combining said first and second pressure signals in opposed sense to develop the component concentration measurement signal;

said combining means including a movable member;

first pressure-responsive means coupled to said first pressure signal and arranged to apply a corresponding force in one direction to said movable member;

second pressure-responsive means coupled to said second pressure signal to apply a corresponding force in an opposite direction to said member;

means responsive to movements of said member for automatically adjusting the net force applied thereto so as to balance the forces on said member and to produce an output signal responsive to the amount of force required to establish said balance of forces to serve as the concentration measurement signal; and

means for predeterminedly setting the relative rates-of-change of said first and second signals with respect to changes in gas flow rate therethrough to produce at least substantial cancellation of the effects of such changes in flow rate on said concentration measurement signal, while effecting changes in said concentration measurement signal in response to changes in said gas sample characteristic.

6. Apparatus as claimed in claim 5, wherein the relative rates-of-change of said signals is predeterminedly set by fixing the pressure-responsive areas of said first and second pressure-responsive means to be substantially different whereby to produce forces of different magnitude from said pressure-responsive means for equal pressures thereof, and simultaneously providing the desired relative rates-of-change of said signals for substantial cancellation of the effects of changes in flow rate.