Non-Contact Capacitive Sensor
The conventional contact type drum level sensing techniques may suffer from the corrosion effect of the liquid incontact. In the present paper a novel non-contact capacitance type drum levelmeasurement technique has been described. Its theory of operation and preliminary experimental analysis at room temperature and pressure have been reported. Keywords :
Boiler drum level; Modified Desanty bridge; Non-contact capacitive
In any thermal power plant or steam generation plant, the boiler drumlevel is one of the most important parameters to be measured andcontrolled for the safe running of the plant without any disruption.In the conventional differential pressure type and hydrastep conductivitytype techniques1 −2 which are being used in the plants, one ofthe major problems may be due to the scale formation on the sensingdia phragm or the sensing conductor tips by the water in contact.Hence, anumber of non-contact techniques have been developed bydifferent investigators. In the optical spectrum type technique1, 3, 4,two monochromatic light beams at different wavelengths are passedthrough water and steam portions of a guage glass and the differencein absorption of these two light beams is taken as a measure of thedrum level by an optical sensor based measuring circuit. The moltenmetal level in blast furnace is measured by a non-contact inductivepick-up technique1. In the present paper, a novel non-contact capacitancetype technique has been used to measure the drum level. In thistechnique, a short circuited non-inductive two layer winding surroundingthe guage glass tube of the boiler drum is used as the sensingprobe of the level. It has been shown that the capacitance betweenthe probe and the boiler drum body has a linear relationship with thelevel. This capacitance has been measured by a modified De-Sautybridge circuit using a sinusoidal excitation signal. This signal is thenconverted into a dc voltage signal by an IC based signal conditionercircuit and is finally displayed in a PC monitor through a dataacquisition system (DAS) using icon based Labtech NoteBookPro-Software. The preliminary experimental results of the observedmonitor display in actual level are reported in the present paper forthe boiler drum at room temperature. It is found from this experimentalstudy that the linearity and repeatability of the proposedsensor is quite satisfactory.
METHOD OF APPROACH
The non-contact level sensing probe is in the form of a two layernon-inductively wound coil surrounding the gauge glass of the drum.The two ends of the coil are shorted to form one measuring lead wirewhereas the other measuring lead wire is taken from the body of thegrounded boiler drum as shown in Figure 1
Figure 1 Non-contact capacitance type drum level measurement
Since the drum water has appreciable conductivity, the watercolumn in the gauge glass tube is taken as one electrode and thesurrounding short-circuited non-inductive coil is taken as anotherelectrode of a cylindrical capacitor with the glass material as thedielectric.Let it be assumed that the coil is made from a PVC or enamelinsulated single strand copper wireof internal conductor of radiusr with insulation thickness t as shown in Figure 2.
Figure 2 Cross section of boiler drum level sensor
If it is assumed that a single turn coil of conductor radius r forms asmall cylinder of height 2r, then a cylindrical capacitor will beformed between the coil and the inner conducting liquid column ofdiameter d which is equal to the inner diameter of the gauge glasstube. The capacitance formed between this coil andinner liquidcolumn is given by
C1 4 r 1 1ln [D d] 1 2ln [D 2 t D]-----------------1 where D is the outer diameter of the gauge glass, is the permittivityof the sensing cylinder glass and is the permittivity...