Deadweight tester
Introduction 100pxA '''dead weight tester'''  apparatus uses unknown 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 standards''' 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'' Pawhere : ''p'' : reference pressure Pa ''F'' : force applied on piston N ''A'' : effective area PCU m2To be able to do accurate measurements, this formula has to be refined.===Absolute pressure with vacuum reference=== ''p''=  ''m''.''g''''l''. æè 1- r''a''r''m'' öø +P. ''d''.t -(r''N''2-r''a'').''g''''l''. ''h''+''p''''vac''===Gauge pressure=== ''p''''e''=  ''m''.''g''''l''. æè 1- r''a''r''m'' öø +P. ''d''.t -(r''N''2-r''a'').''g''''l''. ''h''===Nomenclature=== ''p'' absolute pressure at reference level Pa ''p''''e'' gauge pressure at reference level Pa ''m'' total true mass load kg ''g''''l'' local acceleration due to gravity N/kg r''a'' ambient air density kg/m3 r''m'' average density mass load kg/m3 P.''d''.t surface tension effect N r''N''2 density of pressure medium kg/m3 ''h'' reference level offset DWT .. DUT m ''p''''vac'' vacuum residual pressure Pa ''A''20,0 PCU effective area at ''p''=0 and 20''o''''C'' m2 a''p'' linear thermal expansion coefficient piston ''o''C-1 a''c'' linear thermal expansion coefficient cylinder ''o''C-1 ''t'' temperature of PCU ''o''C l''p'' elastic deformation coefficient PCU Pa-1 ''p''''nom'' nominal line pressure Pa ''p''''e'' gauge pressure applied to PCU Pa ''p'' absolute pressure applied to PCU PaPCUPiston Cylinder Unit
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)*Calibration*Force gauge*Piezometer*Pressure measurement*Pressure sensor*Vacuum engineering
References
External links *http://www.euramet.org/index.php?id=calibration-guides