Deadweight Testers
Deadweight Testers use traceable weights to apply pressure to a fluid such as air, water, or oil to calibrate pressure gauges, transducers, transmitters, and portable calibrators. Due to their fundamental method of pressure measurement using calibrated piston-cylinders and masses, they offer unmatched measurement stability and reliability. DWT’s are considered primary standards.
Deadweight testers also inherently regulate a stable test pressure once the piston is floated, solving a problem that operators of some manual pressure calibrators encounter.
Deadweight testers also measure accurately over a wide range of pressure. The uncertainty of deadweight tester measurements is a percent of the measured value (% of reading). By including multiple piston-cylinders, a single Fluke Pressurements Deadweight Tester can calibrate units under test with full scale ranges that vary by a 100:1 ratio or more.
If you think deadweight testers are too complicated or expensive, then this whitepaper from Fluke calibration is a must read:
Performing Precise Pressure Calibrations May Cost Less Than You Think
Deadweight Tester Selection Considerations
- Pressure range. Consider the highest and lowest pressure that needs to be generated. DWT’s cover from vacuum to 60,000 psi (400 MPa). The performance is less than ideal when the pressure being generated is below 10% of the full scale. Above 10% of full scale accuracy is percent of reading. Applications that require both low and high hydraulic pressures can be handled using dual piston units. These units are provided with both a low range and high range piston/cylinder. Switching between the low range and high range on a Pressurements DWT is as simple as removing the masses from one piston and placing them on the other. No valves need to be switched.
- Pressure media. Pneumatic or hydraulic (water or oil). Pneumatic, or gas, instruments are ideal for lower pressure ranges. Gas is preferred whenever cleanliness is required. In addition, using gas reduces the impact of head height corrections. However, at higher pressures it is necessary to use water or oil.
The usefulness of gas as a media is limited to approximately 2 000 psi (14 MPa). There are two reasons for this. First, there is more risk of explosion at high gas pressures and that is a safety concern. Second, generating high gas pressure will require expensive intensifiers or gas boosters. Using oil or water eliminates these issues. Since water is not a very good lubricant, oil is preferable when allowed.
One advantage of liquids is they are incompressible. This allows a small change in the volume of the system (through a screw pump) to result in large changes in pressure. The most common approach is to use a mineral oil or water as the media. Oil is ideal in that it assists in lubricating the piston and cylinder. The downside to using oil as a media is it now introduces the device under test to possible contamination. An option is a liquid-to-liquid separator, which uses one liquid in the device and another in the DWT.
- Pressure generation options. On-board hand pumps are offered on Fluke Calibration Pressurements deadweight testers to generate vacuum or air pressure, or to prime higher pressure hydraulic systems.
- Weight increments. Purchase the necessary weights for the desired ranges. The calibration weights need to be trimmed for the local gravity, so providing the final location of the instrument is critical for accuracy.
- Accuracy. DWT are inherently a percent of reading device. There is a lower breakpoint, normally 10% of full scale, where the specification ceases to be percent of reading.