A dead weight tester apparatus uses unknown traceable weights to apply pressure to a fluid for checking the accuracy of readings from a
pressure gauge
Pressure measurement is the measurement of an applied force by a fluid ( liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pres ...
. A dead weight tester (DWT) is a
calibration
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of kno ...
standard method that uses a
piston cylinder on which a load is placed to make an
equilibrium with an applied pressure underneath the piston. Deadweight testers are so called
primary standard A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are ...
s which means that the pressure measured by a deadweight tester is defined through other quantities: length, mass and time.
Typically deadweight testers are used in calibration laboratories to calibrate pressure transfer standards like electronic pressure measuring devices.
Formula
The formula on which the design of a DWT is based basically is expressed as follows :
where :
To be able to do accurate measurements, this formula has to be refined.
Absolute pressure with vacuum reference
Gauge pressure
Nomenclature
Piston cylinder design
In general there are three different kind of DWT's divided by the medium which is measured and the lubricant which is used for its measuring element :
# gas operated gas lubricated PCU's
# gas operated oil lubricated PCU's
# oil operated oil lubricated PCU's
All three systems have their own specific operational demands. Some points of attention :
Gas-gas
Make sure that the PCU is clean. This is a very important issue as the PCU's operation is sensitive to contamination. Also when connecting a DUT, make sure that the DUT does not introduce contamination in the measuring system.
Gas-oil
Lubricant of the PCU 'leaks' in the gas-circuit of the DWT. For this reason there is a small reservoir incorporated in the system. Before commencing a calibration it is a good practice to purge this reservoir. If the reservoir is full, oil will be introduced in critical tubing and will cause an uncontrollable oil-head.
Oil-oil
When connecting an oil filled DUT on an oil DWT make sure that the DUT oil will not contaminate the DWT oil. If in doubt. Incorporate a small volume between DUT and DWT and manipulate pressure in such a matter that the oil flow is directed to the DUT.
For high accuracy measurement, friction can be lowered by rotation of the piston.
See also
*
Blaise Pascal
**
Pascal (unit)
The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is define ...
*
Calibration
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of kno ...
*
Force gauge
A force gauge (also called a force meter) is a measuring instrument used to measure forces. Applications exist in research and development, laboratory, quality, production and field environment. There are two kinds of force gauges today: mechanical ...
*
Piezometer
A piezometer is either a device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity, or a device which measures the pressure (more precisely, the piezometric head) of groundwa ...
*
Pressure measurement
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressu ...
*
Pressure sensor
*
Vacuum engineering
Vacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure. The most widespread applications of vacuum technology are:
* Pyrolytic chromium carbide coatin ...
References
{{reflist
External links
*http://www.euramet.org/index.php?id=calibration-guides
Measuring instruments