Calibration, Troubleshooting and Installation of Capacitance Type Level Transmitter

Principle of capacitance type level measurement

• Capacitance type level measurement is based on the principle of change in the capacitance of the capacitor.
• An insulated electrode behaves like one plate of the capacitor and the tank wall behaves like another plate
• A reference electrode in a non-metallic vessel can be used instead of a tank wall to function like another plate.
• An insulated electrode and the tank wall form the capacitor whose capacitance depends upon the level of fluid present in the tank.
• Capacitance is directly proportional to tank level, such that the higher the tank level capacitance is maximum, and vice versa.
• Generally, a capacitor comprises two insulated electrode plates isolated by a small thickness of an insulator such as solid, liquid, gas, or vacuum.
• This insulator in other words is known as a dielectric.
• The dielectric constant and capacitance vary with an increase in level because the change in capacitance is directly proportional to the level of the liquid.
• AC bridges are used to measure the change in capacitance.

• The capacitance is given by

Where:

C = capacitance in Pico-farads (pF)

E = a constant called absolute permittivity of free space

K = relative dielectric constant of the insulating material

A = effective area of the conductors

d = distance between the conductors

LRV & URV adjustment of capacitance type level transmitter

Calibration of capacitance type transmitter is carried at the site with process medium taking into consideration the actual value of its dielectric constant under operating conditions.

Level Transmitter Calibration Procedure

• Set up the connection with the level transmitter, HART Communicator, 24VDC Power supply source, and multimeter as shown in the diagram.
• Inspect the configuration at the lower range and high range values at 0%, 4mA, and 100%, 20mA.
• Ensure the data entered is as per the datasheet provided. For example, the Lower Range Value is 10 inches, & the Higher Range Value is 35 inches.
• Both LRV & URV are measured from the bottom of the level transmitter probe.
• Fill the level transmitter chamber with water as a process fluid up to the 0% level. See the level in the transmitter Display and in the HART communicator display as 0% level.
• Observe the level transmitter reading through a multimeter in mA scale.
• Use the HART communicator, if required to adjust the output of the transmitter on the multimeter so that the output must be 4 mA.
• Fill the level transmitter chamber with water as a process fluid up to 100% level. See the level in the transmitter Display and in the HART communicator display as 100% level.
• Observe the level transmitter through a multimeter in mA scale.
• Use the HART communicator if required to adjust the output of the transmitter on the multimeter so that the output must be 20 mA.

Calibration of capacitance type level transmitter

A. Steps to calibrate smart capacitance type transmitter

• Isolate the level transmitter from the process tank.
• Bleed out the trapped pressure present inside and check the zero reading in the transmitter.
• Connect the multimeter to the level transmitter.
• Turn on the 24 VDC power supply and observe the condition of the LED for ON and OFF.
• Verify zero reading in the level transmitter, if not release the pressure.
• Connect the HART communicator along with the multimeter to the level transmitter.
• Set the multimeter to mA by rotating the knob, and linking the probe to the appropriate socket for mA reading.
• Fill the water used as process fluid to 0% level, and note down the readings.
• Fill the same fluid (water) up to 25%, 50%, 75%, and 100% in both ascending and descending orders and note down the readings.
• Adjust for zero and span error, if there is any error.
• For ZERO calibration drain the fluid to 0%, and check the multimeter reading if it is not 0, then go to the sensor trim option followed by zero trim in the HART communicator to trim the sensor to 4mA automatically.
• For SPAN calibration fill the fluid to 100%, just wait for some period, and go to the sensor trim option followed by span trim in the HART communicator to trim the sensor to 20mA automatically.
• After performing ZERO & SPAN trimming, now verify the readings for 0%, 25%, 50%, 75%, and 100%.

B. Steps to calibrate non-smart capacitance type transmitter

• Perform the same steps 1 to 9 mentioned in smart capacitance type transmitter. If zero and span calibration is having any considerable error in readings then follow below steps.
• For ZERO calibration fully drain the fluid to 0% and connect a multimeter in series with a level transmitter and rotate the zero screw or pot or press the zero key until the multimeter displays 4mA.
• For SPAN calibration fill the chamber with fluid to 100% of range to get 20 mA now rotate the span screw or pot or press the span key until the multimeter  displays 20mA
• After performing ZERO & SPAN trimming, now verify the readings for 0%, 25%, 50%, 75%, and 100%.

Troubleshooting of Capacitance Type Level Transmitter

Fault or Defect:

1. Output is not accurate:

2. Fluctuation in output current:

Installation of capacitance-type level transmitter

• The transmitter is installed vertically from the top.
• Select a suitable location for the probe on the tank where turbulence is minimum
• Probe should be located such that the direct flow of material on the probe is avoided.
• Ensure that the process connection of the level transmitter matches with those on the tank.
• Sensing electrode should be parallel to the reference electrode or tank wall.
• In case of outdoor installation, an Electronic instrument must be safeguarded from sunlight by jacketing it with sheets.
• During the installation of the flexible probe, insert it carefully inside the tank as rough handling may lead to damage to the probe and its insulation.