Deadweight tester
Encyclopedia
A dead weight tester apparatus uses known traceable weights to apply pressure to a fluid for checking the accuracy of readings from a pressure gauge. A dead weight tester (DWT) is a calibration
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
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.
where :
To be able to do accurate measurements, this formula has to be refined.
1. gas operated gas lubricated PCU's
2. gas operated oil lubricated PCU's
3. oil operated oil lubricated PCU's
All three systems have their own specific operational demands. Some points of attention :
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
standard method that uses a piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
cylinder on which a load is placed to make an equilibrium
Equilibrium
Equilibrium is the condition of a system in which competing influences are balanced. The word may refer to:-Biology:* Equilibrioception, the sense of a balance present in human beings and other animals...
with an applied pressure underneath the piston. Deadweight testers are so called primary standard
Primary standard
A primary standard in metrology is a standard that is accurate enough 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 used to calibrate other standards referred to as working...
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 :| p | = | F / A | [Pa] |
where :
| p | : | reference pressure | [Pa] |
| F | : | force applied on piston | [N] |
| A | : | effective area PCU | [m2] |
To be able to do accurate measurements, this formula has to be refined.
Absolute pressure with vacuum reference
| p= |
|
- >(r >N2- >r >a).gl.h+pvac |
gauge pressure
| pe= |
|
- >(r >N2- >r >a).gl.h |
nomenclature
| p | absolute pressure at reference level | [Pa] |
| pe | gauge pressure at reference level | [Pa] |
| m | total true mass load | [kg] |
| gl | local acceleration due to gravity | [N/kg] |
| ra | ambient air density | [kg/m3] |
| rm | average density mass load | [kg/m3] |
| P.d.t | surface tension effect | [N] |
| rN2 | density of pressure medium | [kg/m3] |
| h | reference level offset DWT .. DUT | [m] |
| pvac | vacuum residual pressure | [Pa] |
| A20,0 | PCU effective area at p=0 and 20oC | [m2] |
| ap | linear thermal expansion coefficient piston | [oC-1] |
| ac | linear thermal expansion coefficient cylinder | [oC-1] |
| t | temperature of PCU | [oC] |
| lp | elastic deformation coefficient PCU | [Pa-1] |
| pnom | nominal line pressure | [Pa] |
| pe | gauge pressure applied to PCU | [Pa] |
| p | absolute pressure applied to PCU | [Pa] |
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 :1. gas operated gas lubricated PCU's
2. gas operated oil lubricated PCU's
3. 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 th oil flow is directed to the DUT.
See also
- Blaise PascalBlaise PascalBlaise Pascal , was a French mathematician, physicist, inventor, writer and Catholic philosopher. He was a child prodigy who was educated by his father, a tax collector in Rouen...
- Pascal (unit)Pascal (unit)The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
- Pascal (unit)
- CalibrationCalibrationCalibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
- Force gaugeForce gaugeA force gauge is measuring instrument used across all industries to measure the force during a push or pull test. Applications exist in research and development, laboratory, quality, production and field environment...
- PiezometerPiezometerA piezometer is either a device used to measure static 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 of groundwater at a specific point...
- Pressure measurementPressure measurementMany techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....
- Pressure sensorPressure sensorA pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
- Vacuum engineeringVacuum engineeringVacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure...
External links
- http://www.euramet.org/index.php?id=calibration-guides
- http://www.dhinstruments.com
- http://www.minervaipm.com
- http://hutchinson.belmont.ma.us/tth/in_action.html