Analysing the effect of gas press's little wave in measuring result of velocity of ultra-sound
7th Asia-Pacific Conference on Medical and Biological Engnieering, 2008-4-22 ~ 2008-4-25, pp 174-178, 2008
As one of common measuring technology, ultrasonic measuring technology was used in many science and technology fields with. Such as: length and distance measuring, environmental surveillance, velocity of flow measuring, gas surveillance, and so on. Expanding with the application area, there are higher and higher demand about *Fund item : Imbursed item by supporting plan of the national important task (2006BA103A00)
As known, air surveillance, for example: aerometry, is a very complex task involved of ultrasonics, aerodynamics, hydromechanics, electronics, material science, refrigeration science, computer application and other science subjects. Even in the condition of linear acoustics, it's very difficult to measure the velocity of sound in gas accurately. There are very close relationship with gas's press, temperature, humidity, velocity of flow, specific heat, chemical composition, and so on. That is the reason why in normal ultrasonic measuring method, the condition is always invariable, for example, invariable pressure, humidity, even temperature. The point is to simplify the measuring devices, optimize the compute function, and economize the spending on software and hardware if the result met our pre-design.
Pointing to gas press whether in perfect state or practice state, its value has been regarded as a constant or changed very slowly physical parameter, so its affection is neglected often. This disposal is fit for measuring in atmosphere reasonably. But involved in human's aerometry such as expiring gas survey, there are always some unable controlled influence factors in process of aerometry. It is one of worthy paying attention problem how extent these factors' influence will be severity to.
Measured in a smart environment, if surveyed gas comes under the fast changing outside force, the gas press will be changed synchronously. And this change will effect the result of ultrasonic measure. So in practical survey, if gas press is changed very quickly and its waving range can not be neglected, the difference would be there between theoretic compute and result of fact experiment. It must be taken into account when we need accurate measuring values under the fast changing environment. In this processing, the author discovered a special affection about the survey result arose by gas press changing.
Phase difference theory is that velocity of sound was gained by comparing the time of ultra sound sending and the time of sound receiving. In this paper, the changing of sound's velocity is very little, so using this method is rather reasonable to computing it. Delta t is the time of passing the length L. The writer designed a sample and hold electrical circuit in order to convert the value of Delta t to linear value of voltage Delta VH. The value of Delta VH and velocity of sound C will change synchronously when the value of Delta t changed. So changing of Delta VH will be expressed the time change Delta t.
The author discovered the relationship between gas press and velocity of sound using ultrasonic sensors in the smart environment. By engineering method, a new point of view was brought forward that is "There is a differential coefficient relationship between the changing velocity of gas press and the velocity of sound, while the gas press is waved around the standard atmosphere", a first-order differential equation was proposed which described this phenomenon, and a tentative explanation was given. The function will be valuable to some extent in future working and studying. This function need to be proved its correctness through a lot of physical experiments.
Ultrasonic; velocity of sound; gas press; differential coefficient arithmetic; method of phase difference