1. What are pressure calibrators?
- Pressure calibrators are exact pressure measuring instruments that can verify and calibrate the pressure value of another pressure measuring device.
- Pressure calibrators include a deadweight tester, multifunction calibrator, or test gauge.
- These pressure calibrators provide a faster and easier way to calibrate and maintain pressure-measuring devices with high accuracy in process industries and power plants.
- The accuracy of pressure calibrators must be greater than the pressure measuring instrument being calibrated.
- The calibration ratio of pressure calibrators with the measuring device must be 4:1.
- This means the pressure calibrator must be 4 times more accurate than the measuring device used in calibrating.
- A pressure calibrator has an additional feature in simplifying the calibration of the instrument.
- A pressure calibrator that measures the output value of a test item may have an internal pump to generate pressure or a precision regulator to control supply pressure to maintain output.
- This pressure calibrating device provides crucial process measurement information to plant control systems.
- The performance of these calibrators plays a significant role in improving plant operations and helps to ensure the proper functioning of the plant’s safety systems.
2. What are the functions of pressure calibrators?
- Pressure calibrators are available in various configurations to fulfill specific requirements.
- Onsite calibrators require portability and functions such as monitoring and recording of voltage, current or digital signals from the instrument being calibrated.
- Calibrators with higher accuracy and precise control of pressures are required in the laboratory environment.
- A selection of hand-held multifunction calibrators, pressure calibrators, or pressure controllers is a better choice.
- Dead weight testers used in a laboratory environment are available in a wide range of accuracies and pressure ranges and have been utilized for many years.
- Deadweight testers produce pressure using the definition of pressure.
- Uncertainty or accuracy measurements associated with dead-weight testers are described as a percentage of the measured value against the percentage of the full scale.
- These pressure calibrators are considered the best choice because these devices don’t require any power supply source for operation.
- These devices are bulky and require skilled technicians.
- For higher calibration levels these higher-end dead-weight testers are considered standard equipment in laboratories around the world.
- Hand-held pressure controllers replace dead weight testers in the field and process applications
3. What are the types of Pressure calibrators?
- The pressure sensors and instruments with high accuracy are most suitable for calibrating other pressure handling devices such as pressure transducers, pressure transmitters, and pressure gauges.
- There are various types of Pressure calibrators available for laboratory, factory, or for field use.
a. Digital Test Gauges:
- Digital test gauges for fitting to the top of a pneumatic or hydraulic calibration hand pump to generate and measure precise pressures.
b. Pneumatic Calibration Hand Pumps:
- Pneumatic calibration hand pumps are used to generate precise air pressure set points manually to calibrate the pressure measuring instruments through a pressure indicator.
c. Pressure Controller Calibrators:
- Pressure calibrators are made with in-built pressure controllers to generate and set the calibration points automatically.
d. Pressure Calibrators with Electrical Measurement:
- Pressure calibrators that include voltage and current measurements for displaying the output signal of the pressure transducer or transmitter under test along with the pressure reading.
e. High-Pressure Calibration Hand Pumps:
- High-pressure pneumatic pumps & hydraulic pumps are used in combination with an accurate pressure gauge or indicator for high-pressure testing or calibrating hydraulic pressure gauges, high-pressure sensors, hydraulic pressure switches, and high-pressure indicators.
- Pressure Range: 0 to 700 Bar
- Master Gauge Connection: 3/8 BSP female test point, gauge thread with ½ BSP female swivel connector.
- Compatible with both oil and water.
- Master gauge mounting point on top of the pump.
- It has an adjustable over-pressure limiter.
- The priming capacity of the pump is unlimited.
f. Low-Pressure Calibration Hand Pumps:
- A low-pressure calibration pump is a hand-operated pressure pump used to calibrate very low air pressures, with very sensitive adjustment of low-pressure set points. These pumps generate pressure from -6 psi (-0.4 bar) to +6 psi (0.4 bar).
- It is Portable with only 3.5 lb
- It consists of a screw press for fine pressure adjustment
- Good stability with specially designed bellows to reduce leakage
- Maintenance is low and durable
- It has an insulated isothermal pressure chamber to reduce the influence of environmental temperature changes.
- The pressure setting is easy and adjusted precisely and quickly through a simple turning of the handle.
- Hand-tight connectors make it easy to connect and disconnect the test pump without using PTFE tape (Teflon) or wrenches.
4.Specifications Calibration Hand Pumps:
- Media: Air.
- Pressure Resolution: 0.01 Pa (0.0001 millibar).
- Adapters: Stainless steel
- Body: Stainless steel or Aluminum
- Seals: Buna-N
- Connection: Hand Tight connectors are used for both the test gauge and reference gauge.
- Test Gauge and Reference Gauge Connection: ¼ National Pipe Taper female, ¼ British Standard Pipe female.
- Height: 145 mm.
- Base: 245 mm x 165 mm.
- Weight 1.6 kg.
5. What is the difference between calibration and verification?
- The difference between calibration and verification is that the verification is done by a public authority, but sometimes, these may be legal requirements. But calibration is the uniform checking of measuring instruments for proper functioning. A traceable reference measuring instrument is utilized for this purpose to confirm national standards.
6. What is the difference between calibration and adjustment?
- If the deviation of the calibrated instrument is too large then the instrument needs to be reset. This process is known as adjustment.
7. How to calibrate a pressure gauge using a dead weight tester?
- Dirty and chemically contaminated gauges must be first cleaned before they are fitted on the deadweight tester.
- Fit the pressure gauge to be calibrated on the union connector of the deadweight tester with the necessary adapters required.
- Open the oil chamber side valve and close the pressure gauge side valve.
- Rotate the screw pump handle in the clockwise direction to remove air from the system. Check the level of oil inside the cup. If it is low then fill the oil cup.
- Repeat this procedure approximately 3 to 4 times until air bubbles come out.
- After confirming that no air is present inside the oil, rotate the handle in an anticlockwise direction fully and close the oil chamber side valve.
- Open the pressure gauge side valve.
- Check the zero reading on the pressure gauge without applying any pressure to it.
- If it shows the error, remove the pointer with the help of a pointer puller and fix the pointer exactly at zero reading on the scale.
- Put dead weight in the weight carrier corresponding to the range of the pressure gauge and rotate the pump handle clockwise till the weight balances. The added deadweight must be within the PG range.
- While rotating the screw pump handle clockwise, keep on rotating the weight carrier till it lifts to the black or red band marks on the carrier stem.
- Also, observe the pressure gauge readings.
- The readings on the pressure gauge must be equal to the added weights on the carrier. If not, do the span setting. It is carried out by adjusting the pointer travel with adjustable link liver adjustments.
- Carry the procedure for some more reps (2 to 3 reps) by adding some more dead weights on the carrier.
- Finally, release the pressure by rotating the handle in an anticlockwise direction till the pointer reads zero on the pressure gauge.
- After confirmation of zero on the pressure gauge, remove the dead weights one by one slowly, and finally, remove the pressure gauge from the dead weight tester.
- After the calibration is over the dead weight tester must be set to the following condition.
- Release valve: open
- Screw pump: fully loose by turning in an anticlockwise direction.
- Union connector: plugged
- Weights: remove from the instrument weight carrier.
8. What are the factors affecting Pressure Calibration and Corrections?
a. Surface Tension:
- A surface tension correction is made with oil-lubricated piston-cylinder systems
- The surface tension of the fluid must be defeated to “free” the piston.
- On larger-diameter pistons this effect is high and on smaller-diameter pistons this effect is low.
- The surface tension correction is calculated using the following formula:
b. Local Gravity:
- Local gravity varies across the entire surface of the earth,
- The lowest acceleration due to gravity is 9.7639 m/s2.
- The highest acceleration due to gravity is 9.8337 m/s2.
- In pressure calculation for an instrument, the local gravity is considered and a gravity correction may be applied.
- Most industrial deadweight testers are calibrated to standard gravity of 9.80665 m/s2.
9. What is the need for the Calibration of instruments?
- Calibration of instruments confirms standardization, safety, and efficiency. It is required to know the pressure of a particular process or environmental condition, and then the instrument we are using to determine pressure information should be calibrated for accurate indication within the tolerance. Unless calibrated, it is very difficult to obtain accurate information and may lead to accidental issues.
10. Where Pressure Calibration of the instrument is performed?
- All pressure devices such as pressure gauges, pressure indicators, transmitters, or transducers are required to calibrate uniformly.
- Pressure calibration of instruments can be performed in a laboratory environment, a test bench, or in the field.