Equipment known as a dead weight tester is used for pressure calibration by measuring the force applied to a specific area and dividing that force by the area to calculate pressure.
The equation for determining pressure in dead weight testers is: pressure = force ÷ area.
Important Considerations for Bench Calibration Accuracy
Effective test system calibration must account for local gravity variations, especially in regions where gravity affects piston behavior. Ambient factors like temperature and air buoyancy can also impact accuracy. Horizontal levelling avoids friction errors, while regular maintenance of pistons and weights prevents wear-related inaccuracies. These factors significantly enhance the reliability of calibration systems.
Range
The first consideration is the pressure range. One of the benefits of a DWT is that it can cover a broad range of instruments. As a percent-of-reading device, its accuracy improves at lower pressures, but typically only above 10% of the full scale.
When selecting a dead weight tester, ensure the maximum required pressure is within its range. It’s also wise to choose a higher full-scale range to accommodate future needs, as the accuracy is proportional to the reading.
Some pneumatic devices can measure below atmospheric pressure (vacuum mode). For these, a DWT can be equipped with a vacuum piston, allowing for negative pressure calibration.
Media
The medium used (air, gas, oil, or water) is typically chosen based on the pressure range. Gas is ideal for low-pressure and clean environments, as it minimizes contamination and is unaffected by head height. At higher pressures, however, oil or water is preferred.
Gas is generally limited to around 2000 psi due to safety concerns and the need for expensive gas boosters. Oil is usually preferred over water due to better lubrication properties.
Pressure Generation
A DWT system includes both pressure generation and measurement. To generate pressure, a priming or propelling pump is used. For example, Nagman’s P3100 and P200 Series include a built-in priming pump, which can also be used for degassing the system.
Vacuum generation (negative gauge) follows a similar principle. Hand pumps suffice for low vacuum needs, while external pumps are better for deeper vacuums.
Weight Sets
Dead weight testers use a piston and calibrated weight masses to apply precise pressure. Weight sets come in various units (e.g., psi, MPa, inH2O) and are matched to specific testers. Additional or conversion weight sets can be used to support multiple measurement units with a single tester.
Precision Alignment
Accurate pressure calibration depends on precise alignment. Small misalignments can introduce significant errors. Always place the tester on a flat, stable surface and use a spirit level to verify vertical alignment. Periodically check for tilt or position shifts during calibration.
Accuracy
Several factors affect DWT accuracy. Local gravity variations significantly influence minimum pressure increments. During manufacturing, Nagman calibrates the weights to match your local gravity for optimal performance.
Standard and high-accuracy options are available, with software to assist in pressure calculations. For maximum precision, a piston gauge—designed for lower uncertainty—can be used in place of a DWT.
Also Read: Why is a Dead Weight Tester used?
Conclusion
Choosing the right dead weight tester depends on the devices to be calibrated, required pressure range, accuracy needs, medium selection, and calibration environment. Dead weight testers offer flexible configurations tailored to various industrial and laboratory applications while delivering unmatched precision.
