Patent Application
20160123849
Methods and apparatuses for the preparation of known quantities of gases and vapors, including for sensor calibration.
There are many situations in which it is important to know the concentrations of gases and vapors. Examples include, but are not limited to, monitoring of gases and vapors in the environment, in chemical manufacturing processes, in chemical warfare and in medical diagnostic and treatment procedures. Accordingly, a broad spectrum of chemical sensor technologies have been developed to quantitatively detect and measure the gases and vapors of interest. Most of these technologies require that the sensor be calibrated in order to ensure the accuracy of the gas and vapor concentrations being measured.
Many methods are well known for the preparation of gas and vapor “standards” that can be used to calibrate chemical sensors (see, for example, Nelson, G. O.; “Controlled Test Atmospheres: Principles & Techniques”; Ann Arbor Science Publishers; 1982). It is common to use cylinders of compressed gas (e.g., pure air or nitrogen) to which a known amount of the desired calibration gas or vapor has been added (see, e.g., ISO Standard No. ISO 6142:2001(E); “Gas analysis—Preparation of calibration gas mixtures—Gravimetric method”). Such blended compressed gas standards are very reliable and easy to use but suffer from the disadvantage that they are often large, heavy and cannot be transported by aircraft or other means of transport unless elaborate and often expensive safety measures are implemented. Furthermore such gas standards have a limited “shelf-life” after which the accuracy of the standard concentration degrades.
Vapor standards can also be prepared by bubbling a carrier gas (e.g., pure air or nitrogen) through a reservoir (e.g., a vaporizer) containing a liquid sample of the vapor to be generated. By controlling the temperature of the bubbler and the flow rate of the carrier gas, a known amount of the target vapor can be delivered. This concentrated vapor stream can be blended with additional carrier gas to reduce the concentration as required to achieve the desired concentration for sensor calibration (see, e.g., Grate, J. W., Ballantine, D. S., Wohltjen, H.; “An automated vapor-generation and data collection instrument for the evaluation of chemical microsensors”; Sensors & Actuators, 1987, 11(2): 173). However, this methodology is not well suited for field-portable systems since the bubblers are usually physically large and prone to leakage if tipped over. Furthermore, in addition to accurate temperature control of the bubbler and flow control of the carrier gas, heating of the vapor flow path is required to prevent vapor condensation. Therefore, overall, this known methodology is not well suited when a compact or portable system is required.
Another popular method relies on the diffusion of gases and vapors from a concentrated reservoir through an orifice or small diameter tube of precisely known dimensions (see, e.g., Altshuller, A. P., Cohen, I. R., “Application of Diffusion Cells to Production of Known Concentration of Gaseous Hydrocarbons”, Anal. Chem., 1960, 32(7): 802). The diffusion rate of the gas or vapor through the tube is defined by its molecular properties as well as the temperature, pressure, diameter and length of the tube. By holding these parameters constant, the rate of diffusion is constant. Gases and vapors delivered from the diffusion tube can be blended with a stream of pure carrier gas (e.g., air or nitrogen) to provide a known concentration suitable for sensor calibration. However, as with other known methods, this method suffers from the disadvantage that it requires significant time for equilibrium to be achieved. That is, the temperatures, pressures and dilution air-flow rates must all be controlled and stabilized before accurate concentrations can be delivered. Furthermore, the vapor diffusion sources are “open”. This means that tipping of the source can result in the leakage of the calibration liquid contained in the reservoir. Overall, therefore, this methodology is also not well suited for a compact or portable vapor calibration system.
The problem of spillage from the vapor reservoir is mitigated by another popular vapor standard generation technique that relies on the controlled rate of diffusion of vapors through a polymer diffusion barrier. Such “permeation tubes” typically contain a small quantity of liquid sealed in a short length of Teflon™ tube. Liquid vapors are able to permeate the polymer matrix and slowly escape from the tube. The permeation rate depends on temperature and pressure. Like the diffusion tube, the permeation tube requires that all temperatures, pressures and flow rates be stabilized before accurate concentrations can be delivered. For most permeation tubes, this stabilization process can take hours. This methodology, too, therefore, is not well suited for use in a compact or portable vapor calibration system.
A popular alternative method for preparing gas and vapor standards is to use a manual syringe to inject a small quantity of “pure” gas or headspace vapor from a liquid reservoir into a larger vessel (e.g., a plastic bag or larger syringe) and then to fill the larger vessel with a known volume of clean air or nitrogen. If the pure gas or vapor headspace concentration is known, the concentration in the larger vessel can be determined. This method typically affords only modest accuracy, since the volumes delivered by the manual syringes are affected by the skill of the operator.
Yet another approach was described recently by Wohltjen in U.S. Pat. No. 7,484,399, in which a small volume of liquid contained in a small, electrically-heated chamber isolated by a solenoid valve, is used. When the heater is energized, the temperature and pressure in the chamber are increased, thereby permitting a small amount of the vapor to be dispensed through the solenoid valve when it is energized. While compact, rugged, and reliable, the concentrations delivered are not as precisely accurate as may be desired, since the external air-flow rates and temperatures are not controlled.
There are other vapor generation methods that are known to those skilled in the art. However, none of these known methods are well suited to provide accurate vapor concentrations using an apparatus that is very compact, free of high-pressure compressed gases and capable of high accuracy with minimal warm-up and user skill and which is sufficiently robust to facilitate use in a wide variety of contexts.
The present invention provides a new method and apparatus for preparing known concentrations of gases and vapors that can be used to calibrate chemical sensor devices. Therefore, this invention solves the problems identified in the art, as discussed above, and provides a solution for which there has been a long-felt need.
The invention provides an apparatus, system and method for providing precise concentrations of vapor for sensor calibration and other applications. The apparatus, and associated method, comprises (a) a constant volume reservoir containing a vapor source comprising a liquid containing the vapor to be generated, such that said liquid in the reservoir is in equilibrium with a headspace volume in the reservoir at a given reservoir temperature; (b) a temperature controller for precisely controlling the temperature of the constant volume reservoir; (c) a source of positive pressure for imparting a precisely controlled pressure to the interior of the constant volume reservoir; and (d) a seal (e.g. a septum, O-rings, gaskets and the like) through which a tube is insertable into the constant volume reservoir, while maintaining a seal to ambient air surrounding the reservoir, such that precisely metered quantities of vapor are dispensed from the reservoir via the tube to a chemical sensor requiring calibration, upon pressurization of the constant volume reservoir to a pressure above atmospheric pressure, by the source of positive pressure.
Accordingly, it is an object of this invention to provide an apparatus, system and method for production and delivery of precisely known concentrations of a vapor to a sensor for calibration thereof.
Other objects, advantages and benefits of the present invention will become apparent to those skilled in the art from a review of the complete disclosure and the appended claims.
The well-known Ideal Gas Law, first described by Clapeyron in 1834, provides a good approximation of the behavior of gases and vapors under various pressures, temperatures and volumes. The law is defined by the equation:
PV=NkT where;
The concentration of a gas or vapor is defined as the number of molecules (N) per unit of volume (V). Thus, to find the concentration of a gas, the Ideal Gas Law is re-arranged as follows:
N/V=P/kT
Therefore, preparation of precise vapor concentrations requires accurate knowledge of the temperature and pressure of the vapor source.
The apparatus and method of the present invention provides a temperature controller, a source of positive pressure and constant volume reservoir so that accurate gas and vapor concentrations can be prepared and dispensed, for example, to a chemical sensor requiring calibration or to a receptacle for the vapor which can be used independently of the apparatus, for example, as a source of calibration gas.
The apparatus 100 according to this invention is shown schematically in
Commercially available 2 cc “crimp-top” glass vials (e.g., Restek P/N 21152) are particularly attractive for this application. The vial preferably contains a “wick” material 101a (e.g. glass wool, cotton, zeolites, polymer foams, charcoal, cardboard, cloth, or other material known in the art or which is hereafter discovered) that is chemically inert to the chemicals in the reservoir and which retains the liquid while allowing vapor from the liquid to fill the remaining headspace in the vial 101 with an equilibrium concentration for the given vial temperature and pressure. In one embodiment according to this invention, the opening of the vial 101b is sealed using a ‘crimped-on” elastomeric septum material 104. Alternatively, of course, those skilled in the art will appreciate that alternatives to a septum, including but not limited to O-rings, gaskets and the like, and combinations thereof, may be employed to achieve this function. The seal, e.g. septum material, 104 may be comprised of a material such as but not limited to silicone, butyl rubber, combinations thereof, or other material or combination of material known in the art or which is hereafter discovered. Commercially available “crimp-caps” or the equivalent having silicone or butyl rubber septa with a PTFE (polytetrafluoroethylene) surface (e.g., Restek P/N 24444) facing the reservoir are particularly preferred for this purpose. The septum material 104 seals the vial while allowing a small hollow needle, tube, or the equivalent 104a to be inserted into the vial while retaining a seal to the ambient air. This arrangement allows small quantities of vapor to be dispensed from the vial 101 when its internal pressure is increased, even slightly, above atmospheric pressure. This is achieved by methods and devices known in the art, including, but not limited to any known source of positive pressure, such as a small pump 105 (including, but not limited to, a diaphragm pump, rotary vane pump, syringe pump, peristaltic pump or the equivalent, now known which is hereafter developed), and the applied pressure is controlled via a pressure sensor 109. A pair of electrically activated solenoid valves 102, 103 allow pressurized scrubbed air from the pump 105 to be directed into the vial 101 or to the outlet 106 via flow restrictor 122 to serve as dilution air or calibration gas.
A typical operating sequence begins by setting the temperature of the vapor reservoir 101 by energizing a thermoelectric heater/cooler 110 attached to the reservoir 101. A temperature sensor 111 attached to the reservoir 101 measures the temperature of the vapor reservoir 101 and signals an electronic control circuit 112, having the function of signal conditioning and providing power drive circuitry, controlled by a programmable microprocessor 113 (for example, but not limited to, an STM32F103 microprocessor commercially available from Digi-Key P/N 497-6066-ND) to increase or decrease the control temperature. The controller 113 compares the temperature measured by the temperature sensor 111 to a desired set-point temperature stored in memory resident in or associated with said microprocessor 113 and power is applied to the thermoelectric element 110 to heat or cool the vapor reservoir 101 as required to maintain the set-point temperature stored in the microprocessor 113 memory. A heat sink, 107 is provided to enable quick cool-down of the reservoir 101 as needed, with a fan 108 provided to ensure even and rapid distribution of heat to and from the reservoir 101. An important advantage of this scheme is that it allows the vapor reservoir 101 to be maintained at sub-ambient temperatures. This reduces the vapor pressure of the test vapor, thereby making it easier to achieve low vapor concentrations. This arrangement also eliminates the possibility of vapor condensation on the valves and connecting tubing, since they are at a warmer (i.e., warmer than the vapor) temperature. The small size of the vapor reservoir 101 affords the advantage of having a very small thermal mass, thereby allowing rapid temperature stabilization and a correspondingly lower need for electrical power to maintain any required set-point temperature. The electronic control circuit 112, having the function of signal conditioning and providing power drive circuitry communicates with each of the internal components via communication channels indicated by arrows 114, and channels 115, between the microprocessor 113 and the signal conditioning and power drive circuitry 112. In order to provide a user interface for controlling and programming purposes, there is provided a keypad 116, with visual display of information being provided on an LCD 117 or the equivalent. A speaker or equivalent means 118 provides audible alerts as and when needed. Power is provided via, for example, a rechargeable battery pack, 119, which, via a voltage regulator 120 (to which external power may be connected) provides power to all other system components.
Once the temperature of the vapor reservoir 101 has been stabilized, the headspace vapor concentration becomes constant. Precise volumes of this headspace vapor can be dispensed by setting the valves 102, 103 to allow a pump 105 (or other source of pressurized gas) to be connected to the reservoir. This action increases the pressure inside the vapor reservoir 101. The valves 102, 103 can then be switched to allow the pressurized vapor reservoir to release a small volume of the headspace vapor via flow restrictor 122 and thence via port 106. By controlling the pressure applied to the reservoir and the time duration that the valves are switched to release the headspace vapor, very precise control of the dispensed headspace volume is achieved.
From the foregoing detailed description of the device, it will be clear to those skilled in the art that the apparatus of this invention dispenses pulses of vapor whose temperature, pressure and volume are precisely known. Under these conditions, the Ideal Gas Law stipulates that the number of vapor molecules is also known.
While some sensors can be calibrated using pulses containing a known number of vapor molecules, most are calibrated using a constant vapor concentration. This invention allows the preparation of constant vapor concentrations by collecting one or more vapor pulses into a reservoir (e.g., a flexible plastic bag or gas-tight syringe) and filling the reservoir with a known volume of clean air. Clean air is obtained by pumping ambient air via ambient air inlet 10 through a scrubber 121 containing sorbents for water vapor (e.g., “Drierite”™, molecular sieves, Calcium Sulfate, silica gel, or any other like moisture absorber now known or which comes to be known hereafter) and sorbents for trace organic vapors (e.g., activated charcoal). The resulting air is relatively free of all molecules except air and can serve as a suitable diluent for the vapor pulse(s). This source of pressurized, clean air can then be metered into the reservoir (e.g., bag or syringe) by using a flow sensor or by simply delivering air at a constant pressure through a constant flow restriction for a known time, to provide a known volume of clean air. Thus, by injecting a known number of molecules of vapor into a reservoir and filling that reservoir with a known volume of clean air, an accurately known vapor concentration can be prepared.
It will be appreciated from the foregoing description of a first embodiment of the invention that equivalents thereto come within the scope of this invention. Furthermore, it will be appreciated from the foregoing description that additional, alternative embodiments of the invention are useful in various situations. The embodiment described herein above may be considered to be a “1-port” design with a relatively small, e.g. septum-capped reservoir, comprising a single entry port into a bottle, sealed container and the like. That embodiment operates very well when the vapor reservoir is of a small volume. However, a small volume reservoir, of course, can only hold a small volume of the source vapor, thus limiting the useful life of the vapor source before recharging or replacement of the vapor source is required.
When larger vapor sources are needed, an embodiment comprising a 2-port design illustrated in
With reference to
Due to the greater volume capabilities of this embodiment of the invention, and the somewhat greater complexity required to achieve its functions, additional elements are preferably included in this embodiment, for which a direct correlate component is not described in the first embodiment 100 of the invention described herein above. These additional elements are described herein, below, in the course of describing a typical operating sequence of this embodiment of the invention. By contrast, elements shown for the embodiment 100 which are essentially identical to elements employed in the embodiment 200 are not discussed in detail or shown in
In a typical operating sequence, ambient air is drawn into the system via inlet 20, by activation of pump 205, causing the ambient air to be drawn into pneumatic system 21, (which extends throughout the system) and then pass through disposable charcoal scrubber 221. Those skilled in the art, based on the present disclosure, will appreciate that precise location of the pump 205 in relation to the scrubber 221 may be varied from that shown in
Energizing valve 202, (also labeled V1 in
Alternatively, valve 203 (also labeled V2 in
The temperature of the vapor reservoir 201 is set, (as in the embodiment 100), by energizing a thermoelectric heater/cooler 210 attached to the reservoir 201. Temperature sensor 211, attached to the reservoir 201, measures the temperature of the vapor reservoir 201 and signals electronic control circuit 212, having the function of signal conditioning and providing power drive circuitry, controlled by a programmable microprocessor 213 (for example, but not limited to, an STM32F103 microprocessor commercially available from Digi-Key P/N 497-6066-ND), to increase or decrease the control temperature. The controller 213 compares the temperature measured by the temperature sensor 211 to a desired set-point temperature stored in memory resident in or associated with said microprocessor 213 and power is applied to the thermoelectric element 210 to heat or cool the vapor reservoir 201 as required to maintain the set-point temperature stored in the microprocessor 213 memory. Heat sink 207 is provided to enable quick cool-down of reservoir 201 as needed, with fan 208 provided to ensure even and rapid distribution of heat to and from the reservoir 201. In this way, reservoir 201 may be maintained at sub-ambient temperatures. This reduces the vapor pressure of the test vapor, thereby making it easier to achieve low vapor concentrations. This arrangement also eliminates the possibility of vapor condensation on the valves and connecting tubing, since they are at a warmer (i.e., warmer than the vapor) temperature. While having a larger size than in the embodiment 100 described above, the embodiment 200 still has a relatively small vapor reservoir 201 affording the advantage of having a small thermal mass, thereby allowing rapid temperature stabilization and a correspondingly lower need for electrical power to maintain any required set-point temperature. The electronic control circuit 212, having the function of signal conditioning and providing power drive circuitry, communicates with each of the internal components via communication channels indicated by arrows 214, and channels 215, between the microprocessor 213 and the signal conditioning and power drive circuitry 212. In order to provide a user interface for controlling and programming purposes, there is provided a keypad 216, with visual display of information being provided on an LCD 217 or the equivalent. A speaker or equivalent means 218 provides audible alerts as and when needed. Power is provided via, for example, a rechargeable battery pack, 219, which, via a voltage regulator 220 (to which external power may be connected) provides power to all other system components.
Once the temperature of the vapor reservoir 201 has been stabilized, the headspace vapor concentration becomes constant. Precise volumes of this headspace vapor are dispensed by setting the valves 202, 203 such that pressurized vapor reservoir 201 releases a precise volume of the headspace vapor via pneumatic line 21 originating in the reservoir 201 at port 204b. When valve 202 (V1) in this embodiment is opened, vapor is conducted via pneumatic line 21 to valve 203 (V2) which can be opened to vent 23, or to coupling 309 via further coupling 310 and out of port 206. Coupling 310 optionally includes a removable plug 311. This port facilitates the connection of the device to more than one output at any given time. Port 206 may be connected to an external device or to, e.g. a bag 312 or other receptacle for dispensed vapor collection. By controlling the pressure applied to the reservoir and the time duration that the valves are switched to release the headspace vapor, very precise control of the dispensed headspace volume via port 206 is achieved.
In the embodiment 200, a further refinement may optionally be included by incorporating, beyond coupling 309, a further valve 307 (V3 in
A further unique feature of the device according to this invention is that it affords self-correction of the vapor concentration as the reservoir is depleted. There is a fixed amount of the vapor initially available in reservoir that becomes slowly depleted every time a vapor pulse is dispensed. Because the initial mass of vapor in the bottle is known and the mass of vapor dispensed with each valve (V1) (102, 202) pulse is also known, the new headspace concentration is optionally continuously recalculated. Utilizing the “corrected” vapor headspace concentration, the valve pulse duration is slowly increased to allow a constant mass of vapor to be dispensed, even though the headspace concentration gradually falls with use. This process is readily monitored and controlled by the microprocessor and greatly extends the useful life of the vapor reservoir before re-filling is required. Specifically, the mass of vapor initially present in the vial is known (e.g., 100 mg) and the mass of vapor dispensed with each pulse is known (e.g., 100 ng). Knowledge of these two quantities allows the remaining mass in the vial to be re-calculated after each vapor pulse by simply subtracting the mass of vapor pulses delivered from the starting mass in the vial. Since the volume of the vial is a constant, then the concentration of vapor in the vial (i.e., the mass/volume) is subject to real time, iterative recalculation by the microprocessor 213. This revised concentration is utilized by the microprocessor to re-calculate the timing of the valve pulse to assure that the same vapor mass is delivered. In a typical application (e.g., 100 mg starting vapor mass and 100 ng mass delivered in each vapor pulse), the injection valve timing is increased by approximately 0.0001% (i.e., 100 ng/100 mg) with each pulse in order to maintain a constant mass delivery. Based on this disclosure, those skilled in the art are able to define algorithms, software code and equivalents thereof for inclusion in the microprocessor 213 memory to achieve this result.
In light of the foregoing disclosure relating to embodiment 200 of this invention, those skilled in the art will appreciate that alternate embodiments according to this invention accommodate excellent portability and smallness of footprint (embodiment 100 and equivalents thereof,
It will further be appreciated that an embodiment of the device according to this invention optionally includes an additional valve, pump and vacuum sensor that can be used to pre-condition sample bags for use with the device.
It will yet further be appreciated that an embodiment according to this invention provides a running tally of the amount of vapor dispensed and adjusts (i.e., increases) the injection valve pulse duration to provide a constant mass in each pulse even though the reservoir concentration drops with time.
Having generally described this invention above, the following additional description ensures detailed disclosure which enables those skilled in the art to practice this invention to the full extent of the claims appended to this disclosure.
Embodiment 100 of the device of this invention is small (e.g., <40 cubic inches with approximate dimensions of 2.5″×3.5″×4.5″) and uses relatively little electrical power (e.g., 6 Volts DC @ 250 mA (average)) making it suitable as a hand-held, battery-powered calibration device, or as a calibration device that can be built into the measurement sensor system.
The device utilizes a small vapor reservoir such as a common 2 cc crimp-top septum vial (see
The active control of temperature, pressure and delivery volume allows this system to maintain high accuracy over a range of ambient environmental conditions.
The septum-topped vapor reservoir is sealed and eliminates problems with leakage, making the system robust and amenable for transportation or use in a wide variety of contexts.
The device according to this invention does not use compressed gas cylinders and thus can be transported easily without safety concerns.
By programming higher pump pressures, or longer injection times, or multiple injection pulses, it is possible to deliver a broad range of vapor concentrations, rather than being restricted for use at a single concentration.
By holding the vapor reservoir at a temperature that is cooler than ambient temperature, problems associated with vapor condensation downstream of the reservoir are reduced or eliminated.
Embodiment 200 is slightly larger than embodiment 200, as defined, primarily, on the size of the included vapor reservoir and operates substantially similarly to that of embodiment 100, with
It will therefore be appreciated that the device of this invention accommodates a plurality of alternate embodiments and uses, as follows:
1) Use of a variety of vapor source materials: The vapor reservoir can contain pure chemicals that are liquids at room temperature (e.g., ethanol, benzene, methyl salicylate, dimethylmethylphosphonate, and the like) or solids that have a significant vapor pressure at room temperature (e.g., para-dichlorobenzene). Likewise the source can contain mixtures of liquids to allow simultaneous calibration of multiple vapors (e.g., benzene, toluene, xylene). It is also possible to use liquids (e.g., water) that are in equilibrium with a volatile gas (e.g., ethylene oxide, ammonia, chlorine, hydrogen sulfide, and the like). Furthermore it is possible to use dilute solutions of an organic component (e.g., 2,6-diisopropylphenol) in a solvent (e.g., water) to allow very low concentrations of the organic vapor to be delivered.
2) The vapor reservoir vial can contain liquid by itself or an inert wick material (e.g., glass wool) along with the liquid that can prevent the liquid from pooling.
3) A wide variety of materials can be used to construct the device as long as the materials in contact with the liquids and vapors do not significantly absorb the vapors and are not degrade by the vapors. Preferred materials include glass; non-porous plastics; metals such as stainless steel, brass, aluminum and any metal with surface plating that is non-reactive such as nickel, chromium and gold.
4) Temperature measurements can be performed by any convenient electronic device, (e.g., thermocouple, thermistor, RTD, semiconductor temp sensor, and the like).
5) The measurement of pressure can be performed by any convenient electronic device, (e.g., silicon strain gauges, MEMS piezoresistors, MEMS capacitors, and the like).
6) The measurement of volume can be performed by any convenient method. Preferred methods include using a calibrated electronic flowmeter, and a clock (contained in the microprocessor) to determine when the desired volume has been delivered. Alternatively the volume can be determined by using a supply of air at a constant pressure that is delivered through a constant flow restriction. This scheme produces a constant flow rate that can be timed by the microprocessor clock to establish when the desired volume has been delivered.
While the foregoing disclosure provides a thorough, enabling written description of this invention, without detracting from the generality thereof, the following additional, exemplary, support is provided to ensure that those skilled in the art are fully able to practice the disclosed and claimed invention, and equivalents thereof. The scope of this invention is not restricted by this exemplary support and reference should be had to the appended claims and their equivalents for an appreciation of the full scope of the invention.
The operation of the method and system according to this invention, which utilizes an embodiment of the device according to this invention is demonstrated using an aqueous solution of 2-butanone (i.e., 100 μL 2-butanone in 10 mL distilled water) applied to a glass-fiber wick (Johns-Manville) in the 2-port (embodiment 200,
The repeatability of the device was verified by preparing 10 consecutive 1-liter samples of 2-butanone vapor into a 1-liter Tedlar bag. Each measurement was conducted from a “cold-start”, i.e., the instrument was initially unpowered and at room temperature before being turned-on. The system then re-established the reservoir temperature and carrier flow rate before beginning the vapor delivery to the 1-liter bag. The PID readings for the 10 consecutive bags of vapor are illustrated in
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US14/43697 | 6/23/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61838683 | Jun 2013 | US |
