Friday, April 26, 2013
These are the most accurate pressure gages, so they are used mostly for calibration of other pressure gages or for measurements, when high accuracy is required (for scientific purposes). The accuracy achieved may vary from ±0.01 to ±0.02% of the measuring pressure (these instruments are used for calibration of dead-weight pressure gages), those with the accuracy of ±0.05% are used for calibration of other types of pressure gages. The range of measured pressures varies from 0.1 to 250 MPa. Figure 1 shows a schematic of a dead-weight pressure gauge.
Figure 1. Dead-weight pressure gauge.
A cylindrical piston 1 is placed inside a stainless-steel cylinder 2. The measuring pressure is supplied through the vent 8 to the fluid 4 of this gauge. To avoid contact of a measured media with the fluid in the gauge, “U”-shape separating tubes (made of stainless steel with a thick wall) are used. These tubes are filled by one half of their volume with mercury. The measuring pressure spreads throughout the fluid in the dead-weight gauge system. Transformer mineral oil and castor oil are used for measurements of low (up to 6 MPa) and high (up to 250 MPa) pressures, respectively. The measuring pressure by acting on the piston develops a force, which tends to bring the piston upwards. The gravitational force developed by calibrated weights 3 can balance this force and the piston itself. This force acts downwards. The balance should be achieved for a certain position of the piston against a pointer 9 of the stainless-steel cylinder. A manual piston pump 5 is used to achieve approximate force balance (to increase pressure in the system), whereas a wheel-type piston pump 6 serves for accurate balancing. A Bourdon-type pressure gauge 7 is used for visual reading of pressure, but not for pressure measurements in this case.
Thus measuring pressure can be evaluated using the formula:
mp - mass of the piston, kg;
P - measuring pressure, Pa;
mw - mass of calibrated weights, kg;
gloc - local gravitational acceleration,m/s2;
Sp - cross-section area of the piston, m2.
In reality, this formula is more comprehensive in order to achieve high accuracy. Therefore, several corrections should be introduced, namely:
• the correction for the variation of piston cross-section area with variation of its temperature;
• the correction which takes into account the difference between local gravitational accelerations of the place where this dead-weight pressure gauge was calibrated and where it is used for pressure measurements;
• the buoyancy-type correction takes into account the weight of the air displaced by the piston and calibrated weights;
• we need to reduce friction of the piston inside the cylinder by spinning the weight platform with the piston to keep the piston floating;
• head of the oil should be constant in every measurement, this corresponds to a certain position of the piston in the cylinder.
Article Source:: Dr. Alexander Badalyan, University of South Australia
Basics of Instrumentation & Control
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