Calibration

What are various Level calibrators and How to use it?

What is the purpose of a level transmitter?

  • The purpose of Level transmitters in the process industry or in power plants produces continuous level measurements over the system range other than at a single point and produces an output signal proportional to the level in the vessel.
  • The generated output signal is used to display the depth and to operate control functions.

How are level transmitters calibrated?

  • Set up the guided wave radar level transmitter as shown in the above diagram using a HART communicator, 24 VDC power supply, and the multimeter for mA reading.
  • Check the configuration of lower range at 4mA, 0%, and higher range values at 20 mA, 100%.
  • Input the data as per the datasheet. For example, if lower range value and higher range values are 10 inches and 35 inches. Both of these values must be considered from the bottom of the level transmitter probe.
  • Now fill the transmitter chamber with water till the 0% level mark. Read the level in the transmitter LCD Display or through HART. Using the HART communicator set this condition as 0% level.
  • Read the mA output of the transmitter through a multimeter. For the 0% level, the output must be 4mA. Make some minor adjustments through the HART communicator if required.
  • Now fill the transmitter chamber with water till the 100% level mark. Read the level in the transmitter LCD Display or through HART. Using the HART communicator set this condition as 100% level.
  • Read the mA output of the transmitter through a multimeter. For 100% level, the output must be 20mA. Make some minor adjustments through the HART communicator if required.

What is the Importance of Transmitter Calibrations?

The importance of transmitter calibration is shown below steps.

1. Helps in Maintaining Process Environment safety:

          Transmitter calibration is an important task in every process industry and power plant because if the transmitter produces inaccurate readings there may be a chance of occurrence of accidents and huge revenue losses if there is a massive drift between accurate and current readings.

2. Helps to identify errors in readings:

  • Due to several reasons, the transmitter performance may take a dip.
  • Though top manufactured transmitters are pledged to produce long-term performance.
  • Calibration of instruments will minimize the drift in readings and reduces the harmful impact on productivity and helps the process to deliver the desired efficiency.
  • Additionally, calibration of transmitters advantages in identifying several installation and alignment errors.
  • Additionally, calibration helps identify several installation and alignment errors in these transmitters.  These transmitters are installed at the right location within the equipment to obtain high accuracy in measurements. 

3. Helps in avoiding damage:

  • Instrument downtime is one of the main causes of industrial inactivity.
  • This may be expensive in terms of revenue and wastage of available resources.
  • Onetime calibration of transmitters minimizes this downtime and increases plant productivity.
  • A minor error in calculation increases the gap between desired results and damage.

4. Conformance to Regulations:

  • Maintaining product quality is the most important consideration for any business that wishes to make a huge impact on customers.
  • Calibration or adjustment is the process that helps business organization to match the quality requirements of ISO certification standards.

What are the Types of Level Transmitters?

There are basically seven types of level transmitters, each transmitter works in different ways for various processes.

1. Capacitance Level Transmitters:

  • These Capacitance Level Transmitters use containers as a di-electric medium between electrodes.
  • The energy of the capacitor is directly proportional to the level of liquid.
  • If the liquid level is more then the energy increases and decreases if the liquid level is reduced.
  • With this variation in energy level, the capacitance level transmitters can easily determine the exact level of liquid inside the tank.

2. Hydrostatic Level Transmitters:

  • These hydrostatic level transmitters are known as pressure level transmitters.
  • The liquid level in the container is measured by the pressure of the resting body within the fluid.
  • The volume of the liquid is directly proportional to the force of the liquid.

3. Magnetic Level Transmitters:

  • These magnetic level transmitters utilize a magnetic object suspended in a buoyant float.
  • These restrict a lateral movement of the float,
  • The float movement is computed by various magnetic devices.
  • This method is convenient for continuous measurement owing to the tendency of the float to rise or sink based on the liquid level.

4. Radar Fill Level Transmitters:

  • This Radar Fill level transmitter functions on the radar principle that makes use of radio wave emissions.
  • Generally, these transmitters are fixed at the top of the tank or container.
  • The transmitter emits a radar signal into the liquid and receives the reflected signal as an echo.
  • Based on the time consumed by the signal to return the transmitter investigates the actual level of liquid present inside the tank.

5. Ultrasonic Level Transmitters:

  • In Ultrasonic Level Transmitter an ultrasonic transducer is fixed at the top of the container.
  • The transducer emits an ultrasonic pulse into the liquid, this pulse will hit the surface of the liquid and receives the reflected signal as an echo.
  • Based on the time consumed by the pulse to return the transmitter investigates the actual level of liquid present inside the tank.

6. Guided Microwave Level Transmitters:

  • These transmitters work by transmitting a microwave pulse through a sensor cable or rod.
  • The signal strikes the liquid surface and travels back to the transmitter housing.
  • Depending upon the time consumed by the signal to travel down the sensor and back up again the electronics united inside the transmitter housing determines the level of fluid inside the tank
  • These types of level transmitters are applied in all areas of process in various industrial applications.

7. Liquid Level Transmitters:

  • These transmitters are invented to sense and measure the liquid levels in storage tanks, and transport tanks.
  • These transmitters are utilized to detect the interface between two different liquids such as oil and water.
  • The liquid level can be sensed by measuring the head pressure of the liquid through pressure transmitters.

Auto Calibration of DP Level Transmitter:

  • Advanced software functionality of DP-level transmitters eliminates or eradicates the time requirements.
  • Configuration of the Level transmitter is time-consuming.
  • For traditional transmitters, the calculation is complex to determine the proper range of values.

Application:

Based on the type of application, the vessel may be open to referencing atmosphere or closed under some blanket pressure. The following factors must be considered for using smart or conventional products.

  • The specific gravity of the process and capillary fill or sealing liquid for impulse tubing.
  • Precise location of 0% and 100%.
  • Height of capillary or impulse piping in the vertical orientation.
  • The definite attitude of the transmitter is fixed to the container.
  • The vertical distance between the flanges.

Elevation:

  • This is generally considered for closed vessels or containers.
  • Low-side wet leg or remote seal is used to reference blanket pressure.
  • A negative force is developed at the low-pressure side of the transmitter which is equal to vertical height times the specific gravity of the fill fluid

Span:

  • It is the vertical distance of the process times the specific gravity.
  • 10.5 X 0.9 = 9.45 mH2O (31.5 inH2O).

Suppression: 

  • It is the positive pressure developed on the high-pressure side due to a zero point above the level transmitter.
  • Suppression is generally occupied in an open vessel referencing atmosphere.
  • The positive pressure developed is equal to the vertical distance between the 0% point and the transmitter times the specific gravity of the liquid.
  • Elevation or suppression distance never complies with the Piping and Instrumentation Diagram because existing piping or remote seal capillaries do not form precise angles in the field.

Calibration range:

  • The calibration range is calculated at 0% and 100% considering both positive and negative pressures.

From the figure

  • H1= 15 Feet or 4.49 meter
  • H2 =  35 Feet or 10.5 meter
  • L = 50 Feet or 15 meter
  • Specific Gravity of Capillary fill  SG 1 = 0.8
  • Specific Gravity of TankSG 2 = 0.9

Calibration range for 0%:

At LRV 0 % = (H1X SG 1) – (LX SG 1) or (H1-L) X SG 1

                   LRV 0%       = (H1 – L) x SG 1

                                      = (4.5 – 15) * 0.8

                                      = -10.5 * 0.8

                                      = -8.4 mH2O (-28 inH2O)

Calibration range for 100%:

At 100% level (URV) = (H1 + H2) x Specific Gravity

At URV 100 % = (H1 X SG 1) + (H2 X SG 2) – (L X SG 1)

URV 100% = (H1X SG 1) + (H2 X SG 2) – (L X SG 1)

 = (4.49 x 0.8) + (10.5 x 0.9) – (15 x 0.8)

 = 3.59 + 9.45 – 12

 = 1.05 mH2O (3.5 inH2O).

The calibration range for 0% and 100% is:

-8.4 mH20 to 1.05 mH2O (-28 inH2O to 3.5 inH2O)

What are the Different types of level transmitters?

Below shown are different types of level transmitters

  • Capacitance type level transmitter
  • Hydrostatic level transmitter
  • Magnetostrictive level transmitter
  • Radar level transmitter
  • Electromechanical level transmitter
  • Ultrasonic level transmitter

Calibration of Level Transmitter:

1.  Remove the housing cover of the transmitter on the FIELD TERMINALS side.

2.  Connect the power cable to the positive and negative terminals of the 24VDCpower supply.

3.  Connect the power cable to +ve lead to “+” and –velead to the “-” terminal of the transmitter from the 24VDC power supply

4.  Connect the multimeter at TEST terminals of the transmitters i.e. positive lead to “+” test terminal and negative lead to ‘’ terminal

5.  Connect the DP calibrator to the LP side of the transmitter through pressure tubing and the HP side open to the atmosphere.

6.  Switch on the 24V DC power supply.

7.  With zero input pressure applied to the transmitter check the mA o/p of the transmitter. If it is not 20 mA. Press the zero buttons until the transmitter reads 20 mA DC.

8.  Apply pressure corresponding to the range of the transmitter and check the mA o/p of the transmitter. If it is not 4mADC. Press the span button until the transmitter reads 4mA DC.

9.  Release the input pressure and check the transmitter mA reading.

10.  Check for remaining 8mA, 12mA and 16 mA DC outputs by applying 25%,50%, and 75% range pressures to the transmitter.

11.  After completing calibration release input pressure and disconnect the power supply.

12.  Switch off the power supply and finally close the housing cover of the transmitter on the FIELD TERMINAL’s side.

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