CAPACITIVE LIQUID LEVEL TRANSDUCER

Transcription

1 CAPACITIVE LIQUID LEVEL TRANSDUCER Task: Measure and graph steady state characteristic of a capacitive liquid level transducer C = f(h) or C = f(v) and user characteristic h = f(c) or V = f(c). Description of experiment: The experiment is composed of a glass tank filled with water. The capacitor electrodes will be submerged into the tank according to the marked scale on the glass tank. The electrodes are connected to a digital multimeter measuring capacity. In our experiment we will change the position of electrodes (and by this change the depth how they are submerged). In a real application, the electrodes would of course be fixed and the liquid level would change. As we consider water a conductive liquid, one electrode is insulated. Note: After the experiment is finished remove electrodes from the water! Capacitive liquid level transducer principle: The principle is the change of capacity between the submerged electrodes. In our case the electrodes are cylindrical parallel wires. The total capacity of such a capacitor is given by equation. π π. 0. C = C + C + C = C +.( H h) +. h = k k. h D ln d D d 1 D ln d D d 1 C 0 - constant capacity of electrode fixtures, C 1 - variable capacity of surfaced part of the electrode, C - variable capacity of submerged part of the electrode, 0 - vacuum permittivity 0 = 8, F/m, 1 - relative permittivity of material between the surfaced parts of electrodes, air 1 = 1,00054, - relative permittivity of material between the submerged parts of electrodes, water =80,1 (0 C) d - electrode diameter, D - distance between electrode's axes, h - measured height, H - tank and electrode height. References [1] Electroquasistatic Fields in the Presence of Perfect Conductors (online )

2 LIQUID LEVEL TRANSDURES IN OPEN TANKS Task: Measure user characteristics of 4 industrial liquid level transducers - bubbler, ultrasonic sensor, capacitive sensor and hydrostatic pressure sensor. From the measured data plot separate charts. First for bubbler, measured for three different flows and one the theoretical dependence for flow 0 l/h. In the second chart plot dependencies for all the other sensors. Draw a block diagram of this task. Note: The pressure transducer at the bubbler has range (0-7,5) kpa, this corresponds to current output (4-0) ma (linear dependence). Description of control application The control application was created in a ControlWeb 000 system. Filling switch - "Vypínač plnění" This button starts the filling of the tanks. The filling stops at the upper level limit. Measuring switch - "Vypínač měření" This button starts the measuring mode. When the tanks are filled, the filling stops at one of seven pre-programmed liquid levels where the sensor values can be stored. Waiting switch - "Vypínač čekání" This button is used to continue in a stopped experiment when a liquid level was measured with the measuring switch and we would like to go on to the next preprogrammed water level. It is necessary to hold this switch until the waiting signal - "signál čekání" turns off. Mode selector - "Volba režimu" This switch selects measuring or filling mode. Write for other sensors - "Zápis pro ostatní snímače" This button writes into a file measured data from ultrasonic, capacitive and hydrostatic pressure sensors. At the same time the data is written into the onscreen table. Write for bubbler - "Zápis pro pneumatický stavoznak" This button writes into file measured data from bubbler. In this case it is first required to set manually the required flow, then select the corresponding flow on the flow selector and then to write the measured data into the file afterwards. Flow selector - "Průtok" This selector select what value of flow is set and where should it be written. The selector does not set the flow!!! It has to be set manually. Save button - "Uložení naměřených hodnot" This button saves all the aquired data into a file with extension.wk1 on the desktop.

3 EXPERIMENT PROCEDURE 1. Check hardware connections (power supplies for pump, valves and sensors are on).. In the ControlWeb 000 system start the application hladina.cw (with a link on the desktop, eventually from directory c:\hladina). 3. With left switches under each indicator prepare valves and pump. 4. With switch vypínač plnění fill the tanks to approximately 0 cm and equalize manually water level in the tanks by opening the manual release valve. 5. Empty carefully the tanks so that on the bottom there is a small amount of water (this will ensure equal filling of the tanks). 6. With switch volba režimu change mode from fill - plnit to measure - měřit. 7. Save data from ultrasonic, capacitive and hydrostatic pressure sensors with button zápis pro ostatní čidla. 8. Start the measurement with button vypínač měření. The tanks will automatically filled to 10 cm. If the filling stops prematurely, more water can be added by holding down the button vypínač čekání. If more water is filled then desired, the water has to be emptied manually with the release valve. 9. Save data from ultrasonic, capacitive and hydrostatic pressure sensors with button zápis pro ostatní čidla. 10. For the bubbler save the data with the following approach: a) manually set air flow to 40 l/h and set the flow switch on the screen to the b) manually set air flow to 80 l/h and set the flow switch on the screen to the c) manually set air flow to 10 l/h and set the flow switch on the screen to the d) before you start the next measurement, set the flow back to 40 l/h and the flow switch to the same position. 11. By holding down the button vypínač čekání restart the filling. Hold down the button until the signal signál čekání is not off. 1. Repeat steps 8-11 for other liquid levels. 13. With button uložit naměřené hodnoty save all measured data into a file. The file is on the desktop named záznam.wk1. It can loaded into a spreadsheet program. 14. Turn off vypínač měření and empty the tanks.

4 BUBBLER Task: Measure steady state characteristics of a bubbler p = f(h) for different air flows Q 1, Q Q 3. Plot the measured dependencies into a common graph together with the theoretical dependency for flow Q 0 = 0 l.h -1. Used instruments: Bubbler Control valve Tank with liquid (water) - cylindrical glass probe - rotameter (0 to 60) l.h -1 - pressure difference sensor (inclined tube manometer filed with distilled water) Block diagram h bubbler p ~Q h ρ K ρ N l p = f(h) In the laboratory task, there is a manual flow controller with rotameter as a flow meter. Note: For practical reasons, we simulate the change of liquid level by moving the glass probe of the bubbler. The liquid level remains constant, we submerge the probe into the liquid. For a more precise reading it is better to first set the desired height h when flow is Q 0 = 0 l/h and only after this to set the desired air flow Q 1, Q, Q 3. Measure in 10 points of the whole range (step 30 mm). Note that the scale begins 0 mm from the begin of the probe. In the conclusion explain why it is necessary to maintain a constant flow Q and why it is not possible to use the same measured steady state characteristics for other instrument arrangements.

5 Used equations: for Q 0 = l/h p = ρ K.g.h for Q = Q 1, Q, Q 3 0 p sk = ρ K.g.h + p Q where p Q = f(q, α, ρ K, h, losses, ) from inclined tube manometer: p sk = e.ρ N.g.l filling: distilled water ρ N = 1000 kg:m -3 tank filled with: water with density ρ K (from table) Dependency of water density ρ [kg.m -3] on temperature t [ C]: t [ C] ρ [kg.m -3 ] 999,0 997,8 995,4 99,0