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PRESSURE CALIBRATION

Deadweight tester
Pressure balances

LR-Cal

Deadweight tester also called pressure balances, piston manometers or pressure test stands with weighted pistons, are primary standards for the measurand pressure. They include both a calibration pressure source and a reference and are therefore self-sufficient.

  • Rigid basements with integrated manual test pressure generation (spindle pump)

  • Extremely high reproducibility

  • Measurement uncertainties mathematically calculable

  • Accuracy data in % of the measured value (not of the FS full scale value)

  • Basic mechanical-physical principle

LR-Cal LDW-P pneumatic
0.03...2 to 0.4...100 bar + vacuum
10...1,600 bis 100...16,000 psi + vacuum
Accuracy ±0.008/0.015%
of reading

LR-Cal LDW-H hydraulic
1...120 to 1...1,400 bar
10...1,600 to 10...20,000 psi
Accuracy ±0.006/0.015%
of reading
also double-piston versions

LR-Cal LDW-HK hydraulic
1...120 to 10...1,200 bar
10...1,600 to 100...16,000 psi
Accuracy ±0.025/0.05%
of reading
Compact design

LR-Cal CPB5000-HP high pressure
25...4,000 to 25...5,000 bar
365...60,000 to 365...70,000 psi
Accuracy ±0.020/0.025%
of reading

LR-Cal CPB5600-DP differential Δp
2 to 100 bar pneumatic
30 to 1,500 psi pneumatic
60 to 1,000 bar hydraulic
1,000 to 14,500 psi hydraulic
Accuracy ±0.008/0.015%
of reading
Differential pressure at true static pressure (measuring range = static pressure + differential pressure)

Why calibrate with a deadweight tester / pressure balance?

Why calibrate with a deadweight tester / pressure balance?

  • Pressure balances are primary standards for pressure measurement.
  • Pressure balances are the most accurate measuring devices with very high long-term stability.
  • Long-term stability is guaranteed as the masses do not change their original weight (except in case of mechanical damage).
  • Since the measuring accuracy of pressure balances is calculated from the measured value and not from the measuring range, a large pressure range can be covered with consistently high measuring accuracy using only one piston-cylinder system.
  • Another advantage over electronic precision instruments is the fact that no power supply is required (purely mechanical instruments).
  • Pneumatic pressure balances do NOT require an external pressure supply for measuring ranges up to 10 bar (self-sufficient systems).
  • With hydraulic pressure balances NO external pressure supply required (self-sufficient systems).

Druck ist der Quotient aus Kraft und Fläche

Features of LR-Cal deadweight tester / pressure balances

Features of LR-Cal deadweight tester / pressure balances

Pressure test stands with weighted piston (pressure balances / deadweight tester) are the most accurate devices available for calibrating mechanical or electronic pressure measuring instruments. An extremely small overall measurement uncertainty as well as a large long-term stability (recalibration recommended only every 5 years) are ensured by the direct measurement of the pressure according to its definition as quotient of force and area, as well as by the use of particularly high-quality materials.

The pressure is adjusted via an integrated pump (pneumatic models above 10 bar require an external pressure supply, e.g. nitrogen bottle with pressure reducer). A very precisely adjustable spindle pump with internally running precision spindle is installed for fine adjustment.

As soon as the measuring system is in a floating state, there is a balance of forces between the pressure and the mass supports. Due to the excellent workmanship of the system, this pressure remains stable for several minutes, so that the pressure values for comparative measurement can be read off without any problems or even longer adjustment work can be carried out on the test specimen.

Principle of a pressure balance

Both the piston and the cylinder are made of tungsten carbide (depending on the model). Tungsten carbide has very low coefficients of pressure and temperature expansion compared to other materials, which results in very good linearity of the effective piston cross-sectional area and high accuracy.

Piston and cylinder are very well protected in a solid stainless steel housing against contact, impact or contamination from the outside. At the same time, an overpressure safety device is integrated to prevent the piston-cylinder system from being pushed out vertically in the event of disk weights being removed under pressure.

Schematic: piston-cylinder system of a LR-Cal pressure balance

Depending on the model, the target weights are placed on a bell or base weight plate. For lower starting pressures, a lighter aluminum plate can be used instead of the bell on some models.

Set of masses of a LR-Cal deadweight tester

The overall design of the piston-cylinder unit and the extremely precise manufacture of the piston and cylinder stand for excellent running characteristics with a high rotational duration and low sink rates and for very high long-term stability. The recommended recalibration cycle is therefore 5 years, depending on the operating conditions. The piston-cylinder system connection is an M30 x 2 male thread as standard. Optionally, a quick-release fastener is available for easy piston (= measuring range) change without tools.

A factory calibration certificate is included as standard with all LR-Cal pressure balances. DAkkS certification is available as an option.

The accuracy of pressure balances is characterized by the deviation span. This is made up of the sum of the systematic measurement deviations and the measurement uncertainty contributions. An MS Excel spreadsheet for automated correction calculations is available as a free download on the Internet: dwt-corrections.zip

 

 

 

Measuring range and usable measuring range for pressure balances

Measuring range and usable measuring range for pressure balances

The respective starting values are theoretical values and correspond to the pressure value generated by the piston (due to its own weight). Additional masses should be added to optimize the running properties.

The accuracy is related to the measured value from 10% of the measuring range. In the lower range, a fixed error applies, related to 10% of the range.

The measurement uncertainties apply under reference conditions (ambient temperature 20°C, air pressure 1013 mbar, relative humidity 40%). Correction calculations may have to be performed. For this purpose, we provide an MS-Excel file for free download:
dwt-corrections.zip