Personal Air Sampling System

The field of the invention is personal air sampling devices which can be worn or carried by workers to monitor air quality during a work shift.

Personal air sampling devices are carried by an individual as he performs his normal duties. At the end of a work shift, an air contaminant collection device in the air sampler is removed and inspected to determine the amount of airborne particulate matter or gases to which the individual was exposed. Prior air sampling instruments contain small battery operated air pumps which continuously draw air through the contaminant collection device during the work shift.

The accuracy of prior air sampling devices is dependent on the volume of sampled air delivered by the pump. In early devices a rotameter was calibrated at the beginning and end of a work shift and an average air flow rate was approximated for the entire shift. The volume of sampled air was then calculated using this average air flow rate. Such prior systems were not accurate and, when employed as part of a continuous monitoring program, considerable manpower is required to calibrate the devices and perform the calculations.

More recently, attempts have been made to increase the accuracy of personal air sampling devices. As disclosed in U.S. Pat. No. 4,080,832 for example, a specially designed air pump is used which draws a fixed volume of air through the filter during each stroke. The strokes are counted and employed to calculate the total volume of the air sample during the work shift. In other commercially available devices, the pump is controlled to produce a constant flow rate throughout the sampling time period.

The present invention is a personal air sampling device in which an air velocity sensor is employed to continuously monitor the air velocity in a passageway containing the contaminant collection device, and means is provided for periodically reading the output of the air velocity sensor and calculating the sample air flow rate. By integrating the measured flow rates over time, the total air volume is calculated and available at the end of the sampling time period.

A general object of the invention is to measure directly the total volume of air sampled during a given time period. The air velocity is directly measured using a thermal anemometer. This measurement is performed repeatedly during the sample period at a rate in excess of the response time of the instrument to changes in velocity. As a result, continuous monitoring of actual velocity is effectively achieved. The velocity numbers are multiplied by the cross sectional area of the passageway to arrive at the volume of air per unit time and these numbers are added to provide a total volume of air flow during the sample time period. Although the measured velocity numbers can be used as a feedback signal to control the operation of the air pump, accuracy is assured by actually calculating the air volume flowing through the filter during the sample time period.

Another object of the invention is to provide an air sampling device which employs microelectronics to enhance the measurement accuracy. A programmed microprocessor is employed to input velocity numbers from the thermal anemometer and to perform the calculations required to provide the up-to-date air volume number. This number as well as other data, including elapsed time, may be read out through a display which is driven by the microprocessor system. The microprocessor may also be coupled to drive the air pump and programmed to maintain a reasonably constant air flow through the filter throughout the sample period.

A more specific object of the invention is to provide a reliable, accurate and relatively low cost means for measuring air flow. A constant power thermal anemometer is employed and its output is linearized by the microprocessor system using a second order interpolation technique. The computing power of the microprocessor thus enables the use of a velocity sensor which is highly accurate at the low rates associated with personal air sampling devices.

The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.

FIG. 1 is a schematic diagram of the personal air sampling system of the present invention.

FIGS. 2A-2D are flow charts of the programs executed by the microprocessor which forms part of the air sampling system of FIG. 1, and

FIG. 3 is a partial view with parts cut away of an alternative embodiment of the housing and air passageway.

Referring particularly to FIG. 1, the air sampling system includes a housing 1 which defines a passageway having an air intake 2 and an exhaust opening 3. A cover 4 is received in the housing 1 at the air intake 2 and it entraps a disc-shaped air contaminant collection device 5 in place immediately behind the cover 4. An air pump 6 is disposed in the passageway toward the exhaust 3 and a thermal anemometer 7 is mounted in the passageway behind the collection device 5. The air pump 6 is operated by a small d.c. motor which is controlled by a power amplifier 8 to provide a relatively constant flow of air through the housing 1. The power amplifier 8 is in turn driven by an operational amplifier 9 which receives a speed command signal from a digital to analog converter circuit 10.

The thermal anemometer 7 generates a current to an operational amplifier 11 which has a magnitude that is a function of the air velocity through the passageway. A temperature sensor 12 is mounted to the housing 1 and is connected to a second operational amplifier 13 to provide a current which is proportional to the ambient temperature. The outputs of the operational amplifiers 11 and 13 connect to respective inputs on a two channel analog multiplexer circuit 14. The analog multiplexer 14 is controlled by a select line 15 to input either the anemometer signal or the ambient temperature signal to an analog to digital converter 16 which forms part of a microprocessor system. As will be described in detail below, the microprocessor system operates to periodically read the anemometer signal, correct it to account for the current ambient temperature and linearize it to provide an accurate indication of air velocity in the housing passageway.

The microprocessor system is structured around an 8-bit bi-directional data bus 17, an 8-bit address bus 18 and a control bus 19. These buses connect to an 8-bit microprocessor manfactured by Zilog, Inc. and sold commercially as the model Z-80. The microprocessor 20 is driven by a two megahertz clock 21 and it operates in response to a set of instructions stored in a read-only memory (ROM) 22. The digital to analog converter 10 and the analog to digital converter 16 also connect to the data bus 17 and the microprocessor 20 operates in response to selected instructions to input 8-bit bytes of data from the converter 16 and to output 8-bit bytes of data to the converter 10. This data is stored in a random access memory (RAM) 23 along with other input/output data and along with data which results from calculations performed by the microprocessor 20.

A set of data input switches 24 and a four-digit display 25 are connected to the microprocessor system through a parallel input/output circuit (PIO) 26. The PIO 26 is initialized during power-up such that eight of its terminals A0-A7 are outputs. Terminals B0-B6 are employed as inputs from the switches 24, and B7 is an output to the multiplexer control line 15.

The switches 24 include a 4-bit air flow select switch and three single bit display select switches. The air flow select switch is manually set to one of 10 settings which establish the preset air flow rate in the passageway. The display select switches may be operated to selectively display the actual air flow rate at the moment the switch is closed, the elapsed time since the sample period was begun, and the total air volume which has passed through the collection device 5 during the sample period. The display 25 includes four seven-segment display devices which are enabled by a digit driver circuit 27 and selected by a seven-segment decoder/driver circuit 28.

The elements of the microprocessor system are enabled when selected addresses are generated on the address bus 18. These addresses are decoded by an I/O port select circuit 29 and a memory select circuit 30 to enable the PIO 26, D/A converter 10, A/D converter 16, a Z-80 CTC 31 or the read-only memory 22 or RAM 23. Table A is a list of the hexadecimal addresses of these elements.

A MEMR control line 32 becomes active to operate the memory select circuit 30 and an IORQ control line 33 becomes active to operate the I/O port select circuit 29. Thus, although the I/O ports occupy the same address space as the ROM 22, the ROM 22 is enabled only during memory read instructions and the I/O ports are enabled only during I/O instructions.

Commercially available devices are used throughout the system and for a detailed description of each device, reference is made to Appendix A.

The microprocessor system is primarily interrupt driven. Referring to FIG. 2A, when a power-on reset switch is activated the system is vectored to an initialization routine. Other interrupt vectors, a stack pointer, CTC ports and PIO ports are then initialized and the elapsed time and air volume numbers are set to zero. The maskable interrupts are then enabled and the system halts to await the occurrence of an interrupt.

The CTC 31 is initialized to provide three real time clocks. The first clock generates an interrupt every one millisecond, the second generates an interrupt every 100 milliseconds and the third generates an interrupt every six seconds. As shown in FIG. 2B, when the six second clock generates an interrupt the system executes a set of instructions which updates the elapsed time by adding six seconds to the total. The system then returns to the background program which, as indicated above, is in the halt condition.

As shown in FIG. 2C, when the 100 millisecond clock interrupts the system the incremental volume of air passing through the housing 1 during the current 100 millisecond iteration is calculated and added to the total air volume number. More specifically, the ambient air temperature signal is first input through the A/D converter 16 as indicated at block 35 and the air temperature is calculated using a look-up table and an interpolation routine as indicated by process block 36. The air velocity signal from the thermal anemometer is then input as indicated at block 37 and an equivalent temperature is calculated using the same look-up table and interpolation routine as indicated at process block 38. The difference between the ambient temperature and the thermal anemometer equivalent temperature is indicative of the cooling effect of the air flow through the filter 5, and this difference is calculated next at process block 39. Air velocity, or flow rate, is then calculated from this temperature difference using a look-up table and an interpolation routine as indicated at process block 40. An incremental air flow volume is then calculated at process block 41 by multiplying this air velocity number times the cross sectional area of the passageway and by the 100 millisecond time increment. The resulting incremental air flow volume is added to the total air flow number for display to the user when requested. Any fluctuations in air flow through the filter 5 occur over very long time periods as compared to the 100 millisecond time increment employed in the preferred embodiment. As a result, the accumulation of incremental air flow volumes is a highly accurate measurement of the actual total air flow over any given time period.

As indicated at block 42, the 4-bit flow rate switch data is input next during each 100 millisecond interrupt through the PIO 26. The preselected flow rate is then calculated using a look-up table as indicated at process block 43. The preselected flow rate is then compared with the actual flow rate number as determined by the thermal anemometer, and a flow rate error is calculated as indicated at process block 44. If there is no significant error in the flow rate, as determined at decision block 45, the system returns from the interrupt to the halt state. Otherwise, a motor speed command is calculated at process block 46 and the command is output through the D/A converter 10 to change the air pump speed. The thermal anemometer is thus used as a feedback device to provide closed loop control of the air pump 6. This control loop is closed every 100 milliseconds to provide virtually continuous air flow control.

The one millisecond real time clock generates an interrupt which vectors the system to a display driver routine. Referring to FIG. 2D, this routine first executes an instruction indicated by block 50 to input the status of bits 0-3 of the PIO A port which indicate the last display digit to be updated. The display digits are then turned off as indicated by block 51 and the status of the data input switches 24 is input as indicated by block 52. All but bits 4, 5 and 6 are masked off as indicated by process block 53 and a test is made at decision block 54 to determine if any one of the three display push buttons has been depressed. If no display is requested, the routine returns to the background program. Otherwise, a pointer to the appropriate data (air flow rate, total elapsed time or total air volume) is formed as indicated by process block 55. One digit of the four-digit BCD number is output to the display 25 during each 1 millisecond interrupt and all four digits are thus output in sequence during each 4 millisecond time interval. As a result, the display 25 provides a visual indication of one of three selected numbers when the appropriate push button switch 24 is depressed.

A complete assembly language listing of the above described programs is provided in Appendix B. In addition, reference is made to an article in the February 1978 issue of Computer Design by Thomas A. Seim entitled "Numerical Interpolation For Microprocessor-Based Systems" for a detailed description of the interpolation subroutine used in the present invention.

It should be apparent to those skilled in the art that both the circuitry and the programs employed in the preferred embodiment can be varied considerably without departing from the spirit of the invention. Indeed, it is contemplated that a single, non-programmable large scale integrated circuit may be employed in a mass-produced implementation of the present invention.

Also, the construction of the housing and the manner in which the air contaminant collection device is connected to receive air flowing through the air passageway can be varied considerably from that shown in the first preferred embodiment. Referring particularly to FIG. 3 for example, the housing 1 may be connected to a flexible tube 60 by a connector 61 which is received at the air intake end of the housing 1. The housing 1 may be attached to the user's belt, or placed in a pocket, and the tube 60 extends upward to a filter holder 62 which attaches to the user's shirt lapel. The filter holder 62 contains a disc-shaped air contaminant collection device 63 which is held in place by a cover 64, and sample air flows into air intake openings 65 formed in the cover 64. The air passageway through which sample air is pumped is thus physically extended by the tube 60 to allow the air intake to be positioned near the user's head while enabling the bulk of the apparatus to be conveniently and comfortably carried. There are numerous other types of air contaminant collection devices which may be employed in lieu of those shown and described herein and the invention is not limited by the type or construction of the air contaminant collection device which is used.