Dead Weight Pressure Calibrators

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A Dead Weight Pressure Calibrator is a calibration standard that uses physical principles to generate a wide range of known pressures.

Dead Weight Pressure Calibrators use calibrated weights with a piston/cylinder assembly and a fluid medium to calibrate a number of pressure measuring instruments such as mechanical and digital pressure gauges, pressure transducers and pressure transmitters.

They work in accordance with the basic principle that P= F/A, where P = the Pressure applied from the fluid medium, F = the Force of the calibrated weights and A = the cross-sectional Area of the piston/cylinder assembly. Because the mass of the weights and the area of the piston/cylinder are known to a very high accuracy, they have been used by calibration laboratories for many years to provide a highly accurate ‘primary’ reference standard for pressure calibration purposes.

Physical Construction

A Dead Weight Pressure Calibrator typically consists of a base, piston/cylinder assembly and a set of weights.

The base contains the transfer fluid which is usually a hydraulic oil or a compressed gas such as air or nitrogen. A screw wheel or a pressure regulator on the base is used to generate and control the pressure. Many models will have a built-in hand pump to prime the system. A small level on the base ensures that the calibrator is level before use.

Calibrators typically contain either a single-piston or a dual-piston system with pistons of different areas to extend the range of the pressures that can be generated. The pistons and cylinders are usually manufactured from high quality tungsten carbide which provides a low thermal coefficient which results in small measurement uncertainty contributions and excellent long-term stability. The piston/cylinder assembly is designed to fit together with a clearance of no more than a few thousands of an inch to create a minimum amount of friction to limit any additional error.

The weight set contains various weight values to generate the full range of pressures when placed on the piston/cylinder assembly. The weights are usually made of stainless steel. Each weight is stamped with a serial number of the instrument, identification number for the individual weight and the nominal pressure value when used on the appropriate piston.

Operating Principle

The Dead Weight Pressure Calibrator works by connecting the device being calibrated to the output pressure port of the calibrator. The weights required for the pressure to be generated are then placed on the appropriate piston/cylinder platform. Most calibrator models will include software that will calculate the combination of weights needed for any pressure value and the exact pressure that is generated.

The screw wheel or regulator is adjusted to increase or decrease the pressure in the system. When the system pressure is equal to the applied force of the weights, the piston and weights will lift from the base so that they are floating on the cylinder. The piston must then be set spinning slightly to reduce the friction between the piston and cylinder. At this point, the system is in equilibrium and the upward pressure of the fluid is balanced against the downward force of the weights. The pressure of the device being calibrated can then be measured and compared with the nominal pressure value generated by the calibrator.

Correction Factors

There are several factors that can have a significant effect on the accuracy of the Dead Weight Calibrator. The main factor is the force of the local gravity which acts as an added force to the weights of the system. The gravitational constant varies depending on the geographical location where the calibrator is being used. In most cases, the calibrator can be calibrated to the local gravity, or a correction factor can be applied to account for the difference.

Other factors that have an effect on the pressures obtained include friction losses between the piston and cylinder, proper leveling, air buoyancy, and thermal expansion. In many calibrator systems, the included software or attached specialized sensors can take the specific environmental factors into account and provide accurate corrections for them to provide the most accurate pressure measurement.

Although highly accurate digital pressure equipment has become available in recent years, the Dead Weight Pressure Calibrator is still a widely used pressure standard in the calibration industry and due to its fundamental design, it will likely remain as a primary pressure source for many years to come.

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