Capacitance level transmitter working principle
Capacitance type level measurement point as well as continues level measurement . A capacitor is formed when a level sensing electrode is installed in a vessel. The metal rod of the electrode acts as one plate of the capacitor and the tank wall (or reference electrode in a non-metallic vessel) acts as the other plate. As level rises, the air or gas normally surrounding the electrode is displaced by material having a different dielectric constant. A change in the value of the capacitor takes place because the dielectric between the plates has changed. RF (radio frequency) capacitance instruments detect this change and convert it into a relay actuation or a proportional output signal.
Capacitance level trasmitter dielectric constant is a numerical value on a scale of 1 to 100 which relates to the ability of the dielectric (material between the plates) to store an electrostatic charge. The dielectric constant of a material is determined in an actual test cell. In actual practice, capacitance change is produced indifferent ways depending on the material being measured and the level electrode selection. However, the basic principle always applies. If a higher dielectric material replaces a lower one, the total capacitance output of the system will increase.
If the electrode is made larger (effectively increasing the surface area) the capacitance output increases; if the distance between measuring electrode and reference decreases,
then the capacitance output decreases.
Design Pressure of capacitance Trasmitter :- Routinely to 1000 PSI (7 MPa), others to 5000 PSI (35 MPa), specialized applicationsto 20,000 PSI (140 MPa).
Design Temperature of capacitance :- 500 ° F (260 ° C) maximum with insulated sensors; 1000 ° F (540 ° C) bare metal, sealedto 200 PSI (30 kPa); 2000 ° F (1100 ° C) bare metal at atmospheric pressure.
Capacitance level trasmitter Span :- 2 to 3 in. (50 to 75 mm) of insulating liquid to 1000 ft. (300 m) for immersion probes and 0.1 in. (2.5 mm) to 10 in. (250 mm) with proximity sensors.
Capacitance level Transmitter Price :- $200 to $800 for single-point controls; $500 to 1500 for two-wire level transmitters;all with type 316 SS and TFE wetted parts; increased cost with exotic metals, hermeticseals, flange mounting, longer insertion length, digital output, dielectric compensation, extended press and temp, and longer inactives.
Capacitance level sensor advantages and disadvantages
Advantage of capacitance level transmitter:-
- Proven and tested measuring principle – robust and safe
- Capacitance level transmitter is simplicity, relatively low cost, corrosion resistance, and a lack of moving parts.
- A capacitor tolerates a variety of process conditions, such as variable density, high temperatures (1000 °F, 540°C), high pressures (5000 psi, 345 bar), viscous products, slurries, foams, and pastes.
- Capacitance level transmitter used to measure point or with multiple points.
- it can measure continuous level in both solids and liquids.
- Capacitance level transmitter Easy commissioning.
- The sensitivity of capacitive transducer is high.
- The capacitive transducer is useful for small system.
- It has good frequency response.
- It requires small power to operate.
- The loading effect is less due to high input impedance.
Disadvantage of capacitance level transmitter
- For a capacitor, a change in dielectric creates errors in the reading.
- Calibration of a capacitor can be difficult since one cannot “bench calibrate”
- Capacitance level transmitter is Changing vapor space can affect the output.
- The capacitive transducers are temperature sensitive.
- It gives non linear behavior.
- The output impedance depends upon the frequency used.
- The capacitance may get changed by dust particle and moisture which produce error.
Read Next :-
Capacitance level transmitter Technical Specification :
|1.||TAG NO.||LT- 501|
|4.||a. AREA CLASSIFICATION||Zone-2, Gr. IIC, T4|
|b. EXECUTION||IS, Exib IIC T4 as per CENELEC|
|5.||TYPE||2 WIRE CAPACITANCE TYPE|
|7.||OPERATING/ DESIGN PRESSURE||0.34 / FV & 2.8 Kg/cm2 a|
|8.||OPERATING / DESIGN TEMPERATURE||102 / 135 DEG.C|
|9.||a. VISCOCITY (UPPER/LOWER)||0.012/ 1.16 cP|
|b. DENSITY (UPPER/ LOWER)||0.2/ 1195 kg/ m3|
|10.||PROBE MATERIAL||SS 316 L|
|12.||TOTAL PROBE LENGTH||1650 mm FROM FLANGE FACE (NOTE-2)|
|13.||ENCLOSURE CERTIFICATION||EXPLOSION PROOF|
|14.||PROBE HEAD MATERIAL||DIE CAST ALUMINIUM|
|15.||PROCESS CONNECTION||2″ 300# RF|
|16.||FLANGE MATERIAL||SS 316 L|
|19.||INSTRUMENT CALIBRATED RANGE||1200 mm|
|20.||ENCLOSURE TYPE||Cast Al. Dual Compatment Housing IP 65|
|22.||CABLE GLAND ENTRY / MATERIAL||1/2″ NPTF, SS DOUBLE COMPRESSION|
|23.||TRANSMITTER OUTPUT||4 ~ 20 mA Hart Communication|
|24.||TRANSMITTER OPERATING VOLTAGE||24 V DC 2 WIRE LOOP POWERED|
|25.||LINEARITY||± 0.25 % of Reading|
|26.||REPEATABILITY OF THE INSTRUMENTS||± 0.1 % of Reading|
|28.||SYSTEM ACCURACY||± 0.5 % of Reading|
|29.||TEMPERATURE EFFECT||± 0.063%/oC|
|32.||INTERCONNECTION CABLE||NOT APPLICABLE|
capacitance level transmitter inspection and test plan
This is a sample capacitance level transmitter inspection and test plan. You might need to change it to meet your purchase order specifications.
|Capacitance level transmitter Inspection and Test Plan Task||Vendor||TPI Inspection||Client|
|Capacitance level transmitter test report||H||R||R|
|Capacitance level transmitter Calibration test||H||W||R|
|Capacitance level transmitter Performance test including hystersis||H||W||R|
|Capacitance level transmitter Insulation resistance test||H||W||R|
|Capacitance level transmitter Final visual / Dimension Inspection||H||H||R|
|Capacitance level transmitter Documentation review prior to release(Final activity of Technical Inspection)||H||H||R|
|Capacitance level transmitter Pre-shipment Inspection||H||H|
H: Hold Point implies that relevant production activities shall not proceed until the continuation of work is permitted by Purchaser. (Notification req’d)
W: Witness point implies that Purchaser intends to witness the designated inspection feature. If Purchaser decides not to witness the relevant feature, production can proceed without permission of Purchaser. (Notification req’d)
S: Witness, but spot check basis. Initial operation will be witness point and subsequent operation will be witnessed at discretion of Purchaser considering the results of previous inspection. (Notification not req’d : Randam Inspection)
R: Review inspection & test records (Notification not req’d)
Capacitive Type Level Transmitter Manufacturer
|Capacitive Type Level Transmitter Manufacturers / Vendors||Country||Website||Phone No.|
|ENDRESS & HAUSER||GERMANY||www.endress.com||+49 71562090|
|KROHNE MESSTECHNIK GMBH & Co. KG||GERMANY||www.krohne.de||+49 2033010|
|YOKOGAWA ELECTRIC CORP||U.A.E||+973 17358100|