Communications: directive radio wave systems and devices (e.g. – Determining distance – Material level within container
Patent
1995-06-20
1997-01-14
Eldred, J. Woodrow
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Material level within container
340612, 73290R, G01S 1308
Patent
active
055944493
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a tank-contents level measuring assembly comprising a horn antenna which transmits short electromagnetic waves, microwaves, through an opening in the tank toward the surface of contents of the tank, which electromagnetic waves are reflected at the surface and received by the same horn antenna. From the received echo waves, an echo function representing the echo amplitudes as a function of the distance is then formed for each measurement. The transit time of the microwave pulse is determined from this echo function, and the distance between the horn antenna and the surface of the tank contents is determined from this transit time.
various microwave tank-contents level measuring techniques are known which permit short distances to be measured by means of reflected waves. The most frequently used systems are pulse radar and frequency-modulated continuous-wave (FM-CW) radar. While in pulse radar, short microwave pulses are periodically radiated, their transit time is measured, and distance is determined therefrom, in FM-CW radar, a continuous microwave is transmitted which is periodically linearly frequency-modulated. The frequency of each received echo signal therefore differs from the frequency of the transmitted signal by an amount which depends on the transit time of the echo signal.
If such microwave level measuring assemblies are used in process control systems, such as in the chemical industry, exact measurements must be possible even under difficult measuring and ambient conditions. These difficult conditions are determined, for example, by high and/or constantly varying temperatures, high and/or varying pressures, and similar conditions, particularly by explosive or corrosive or toxic tank contents, i.e., media to be measured.
As is apparent from German Patent 41 00 922, these difficulties are obviated in the prior art by spatially separating the level measuring assembly so that the transmitting and receiving portion is located outside the dangerous interior of the tank and only the antenna is necessarily mounted inside the tank. The two parts are interconnected by a waveguide passed through the tank wall. To separate the transmitting and receiving portion from the interior of the tank, a waveguide window of quartz glass is disposed in the waveguide. The quartz glass is chosen to have a low dielectric loss factor, which is favorable to the transmission of the microwaves.
Such a separation of the measuring assembly, however, involves an increased amount of design complexity. In addition, matching devices of a material with a medium dielectric constant, such as Teflon, must be provided on both sides of the glass body, but this restricts the range of application, such as the temperature range and the chemical resistance. This increased complexity and the restrictions on the range of application would be avoidable if the complete level measuring assembly could be mounted outside the interior of the tank containing the explosive, corrosive, or toxic media to be measured. This, however, is possible only if the tank is made of a plastic material having a high transmission coefficient with respect to short electromagnetic waves, such as GFK, PVC, PD with .epsilon..sub.r <7.
Frequently, however, the tanks used in process engineering are of metal. Their metallic surfaces reflect electromagnetic waves, so that measurements through the closed tank wall are impossible.
The invention starts from the fact that such tanks very frequently have openings through which the interior of the tank has heretofore been visually monitored by operating personnel. These openings are so designed and so closed with a glass plate of suitable thickness that isolation in accordance with explosion protection regulations is provided between the interior of the tank and the ambient atmosphere, affording multiple safely. The problem could be solved by mounting the microwave level measuring assembly above such an inspection window so that the transmitted wave is directed through the window directly towar
REFERENCES:
patent: 4566321 (1986-01-01), Zacchio
patent: 4670754 (1987-06-01), Zacchio
patent: 5115218 (1992-05-01), Jean
patent: 5279156 (1994-01-01), Van Der Pol
patent: 5351036 (1994-09-01), Brown et al.
patent: 5365178 (1994-11-01), Van Der Pol
Eldred J. Woodrow
Endress & Hauser GmbH & Co.
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