The instant invention relates generally to fluid sampling devices and more specifically to the problem of taking a representative sample of a fluid stream to obtain information on the constituents of the fluid in the stream. 
One well known application is in the field of air pollution sampling wherein a flow proportional or isokinetic sample is required in order to obtain accurate particulate size and concentration.  Specifically, this invention relates to the novel use of two nearly identical thermal flow sensors, operated in a differential manner, wherein one sensor is used to sense the velocity of the fluid stream and the second sensor is used to sense the flow-rate of the sampled stream.  Since the two sensors are of identical construction, and operated identically under all conditions, the difference in the rate of loss of thermal energy, from the two sensors is used in conjunction with a sensing and control circuit to change the sample flow-rate by means of a pump, valve or other means, and thereby force the velocity of the fluid passing the flow sensing probe to be equal to the velocity of the fluid stream, which is by definition, isokinetic sampling. 
The applicant's experience and expertise in the fields of fluid mechanics, aerosal physics, and instrumentation have demonstrated to him that the current state-of-the art in isokinetic sampling is replete with problems and disadvantages.  Prior art devices have been used in which independent absolute measurements are made of fluid velocity and the sample flow-rate by means of conventional instruments such a as pitot velocity probe and an orifice type flow meter.  With these devices, additional data must be taken on the temperature, pressure, and composition of the fluid stream before isokinetic sampling is accomplished.  One of the most apparent difficulties in source or stack sampling is that the velocity sensing probe is exposed to the "wet" fluid stream, whereas the sample flow-rate meter is usually placed outside of the stack and downstream of a filter and a condensing device used to remove water vapor.  Hence the flow-rate meter senses the "dry" fluid conditions and numerous calculations and corrections need to be made in order to obtain the corrected readings such that an accurate isokinetic sample is taken.  In the situations in which the fluid composition is unknown, the temperature is rapidly changing or the velocity is rapidly changing, the prior art methods are wholly inadequate. 
Therefore, there is a strong felt yet unfulfilled need for the device according to the instant application which fully and automatically corrects for all the aforementioned fluid parameters and enables the user to obtain a valid sample in a much more accurate and efficient manner. 
The prior art of which applicant is aware that would appear to be germane to the patent process encompasses the following citations:
Nelson, U.S. Pat.  No.  3,469,453; Schneider, U.S. Pat.  No.  3,643,508; Bellinga, U.S. Pat.  No.  3,950,136; McCorkle, U.S. Pat.  No.  3,965,747; Werner, U.S. Pat.  No.  4,154,088; Wright, et al.  U.S. Pat.  No.  4,297,871. 
The patent to McCorkle is of interest since he teaches the use of a gas sampling system and method therefore in which a vacuum pressure regulator is disposed in a gas sampling train to simplify isokinetic sampling at selected locations of a sampling nozzle in a gas conducting flue.  As shown in this patent, a gas sampling apparatus includes a sampling nozzle having interposed therebetween a condenser, a filter, and a control valve which thereafter communicates with the regulator, thus forming the invention. 
Similarly, the patent to Werner teaches the use of an apparatus for measuring the particulate matter content of a gas which includes a temperature control device for permitting application to hot gases and cold wet gases.  The apparatus includes an exhaust for coupling with a duct through which gas passes, associated filters, heat exchange passages therein, and a conduit for coupling a heat exchange medium to the exhaust tube passage and a controllable heating element for heating the heat exchange medium in the conduit between a source and the exhaust tube in response to readings taken by plural thermometers disposed within the environment.  A plurality of other measuring devices including monometers are included which show differences in static pressure between the outer surface of the nozzle and the interior of the nozzle.  The remaining references further delineate the state-of-the-art as known to applicant. 
The device according to the instant application is easily distinguished from prior art devices in that both the velocity sensor and the flow-rate sensor are exposed to the same fluid conditions and therefore, no additional measurements and corrections are required.  The instant device operates in a differential manner, thus eliminating the disadvantages of prior art devices.  The instant device also incorporates automatic fluid temperature compensation such that it will operate over a wide range of operating conditions without adjustment or corrections.  The instant device is routinely used with conventional sampling systems which incorporate a pump, condenser, filters, etc.