In the production of the crude oil gathering and transportation system, due to the decrease of pressure, the separation of dissolved gas in the crude oil, and the active agents contained in the crude oil, such as carboxylic acid, colloid, asphaltene and wax, the crude oil in the storage tank produces a lot of foam, which cannot be eliminated in a certain period of time, and has certain influence on the accurate measurement of the station.  Using capacitive sensor for station measurement, can accurately measure the liquid level under foam, and can distinguish air, oil foam, oil, water four media.  

   According to the principle of capacitance sensor, when the dielectric constant of capacitor plate changes, the capacity of capacitor also changes.  Considering the distribution of different media in the oil tank, it can be known that when the same capacitor passes through air, oil foam, oil and water, its capacitance changes due to the filling of different media between the electrode plates, and the change is related to the dielectric constant of the medium.  That is to say, the dielectric environment of the capacitor can be known by measuring the permittivity of the four media in advance.  If the capacitor is combined with a standard dipstick, the position of the different media can be obtained from the dipstick scale, thus completing the measurement of the liquid level under the foam.  

Capacitor design

   As a measuring device, the plates must be able to pass through the detection medium freely, the relative position between the two plates must be fixed, and the plate spacing must be large.  For safety reasons and the conductive properties of sewage, the plates that constitute the circuit must be insulated from the medium.  Due to the measurement accuracy requirements, the capacitor composed of two insulating plates must be reliably fixed on the dipstick, the appropriate position is the dipstick hammer.  

Calibration of dielectric constant

   Capacitors as measuring devices have to pass through different dielectric environments, but by calibrating the dielectric constant of the oil, the influence of oil foam and water on the measurement can be reduced.  If higher accuracy is sought, the permittivity of water can also be measured again, so that combined with the dipstick, the thickness of oil layer can be completely determined.  If the water content in the purification tank is stable, sample can be taken at the outlet of the tank, zero elimination operation can be carried out after the measurement system is connected, and then the measuring capacitor is immersed in the oil sample to read the capacitance indication value as the measurement sample of the purification tank.  For the application in oil tanks with high water content, it is necessary to manually extract oil samples of different liquid level heights, distinguish oil samples on the liquid level from oil samples on water surface, and use measuring capacitors to actually measure the capacitance of the two oil samples as capacitance numerical samples at the beginning and end of oil layer.  

The operation method in practical application

   In actual use, two people should operate, one person holding the dipstick for ruler operation, one person observing and measuring the capacitor drop, and zero elimination operation when the dipstick is closest to the oil foam;  At the same time, the capacitance change was closely observed, and the depth of the dipstick was recorded when the capacitance was consistent with the calibration reference sample.  The difference between the two records indicates the reservoir thickness.