A WEIGHT EXCHANGE SYSTEM FOR AN ABSOLUTE PRESSURE BALANCE Authors: H. Kajastie, J. Manninen, M. Rantanen, K. Riski and S. Semenoja, Centre for Metrology and Accreditation (MIKES), P.O. Box 239, FIN Helsinki, Finland, Telephone: , Speaker: S. Semenoja, Centre for Metrology and Accreditation (MIKES)

The pressure laboratory of MIKES The national laboratory for pressure in Finland Secondary level laboratory The laboratory standards are commercial instruments, covering the pressure range from 5x10 -4 Pa to 500 MPa

Introduction The pressure laboratory of MIKES has designed and built a system for changing a load of a pressure balance while the reference vacuum is maintained The pressure balance is a well known DH Instruments PG7601

Traditional calibrations with PG7601 calibrations are performed using a pressure balance equipped with a bell jar and a vacuum pump each pressure point needs its own weight combination, and the weights can be changed only after removal of the bell jar the reference vacuum must be pumped separately for each point time consuming and not very convenient

DH Instruments PG7601

Automated mass handling system for PG7601 fully automatic full-range (5 kPa – 350 kPa) systems are commercially available an automated system concentrated in most used pressure range (850 hPa – 1050 hPa) can be much simpler, and a self-made weight exchanger is good alternative (= low-cost) for the commercial systems MIKES have some previous experience on developing weight exchangers for mass and density calibrations

General discussion for a weights exchanger centring of the weights the motorised exchange system and its operator generate heat in the environment, which can lead to temperature gradients in the weighing chamber magnetic interaction between the weights, a measurement device and the weight exchange system

…and special discussion for the weights exchanger of the pressure balance a user interface into the vacuum chamber the weighing point (on a piston of the pressure balance) is rotating the piston is floating and it moves vertically during loading/unloading the weights exchange system should be easily removable from the pressure balance

The weights exchange system - The pressure balance the original bell jar was replaced with a larger chamber to fit in a chain of eight weights a lid and the chamber is attached to the balance with help of vacuum like the original bell jar rubber gaskets are used

The chamber with the rubber seals

The weights exchange system - Mechanical construction the aluminium lid a weight set a flexible aluminium KF connection between the weights and a threaded shaft for mechanical feed through a gear box which moves the threaded shaft movement for the gear box by a step motor / a manual crank

The mechanics of the vertical movement

The weights exchange system - The weights the weight set of 8 discs from stainless steel the weights are designed so that rotated weights are stacked and centred with help of chamfer under and top of the weights when the weights are lifted they are chain connected with small hooks The true masses, densities and volumes of the 250 g weights were determined in the mass laboratory of MIKES

The weights

The weights exchange system - The control unit an eurocard rack the step motor a driver for the motor a power source a controlling device PC-connection with an RS 232/485 – interface adapter, option for future

The weight exchange system with the control unit

Testing The operational testing of the new weights exchange system was made by calibrating three barometers with the new system and comparing the results to those obtained by the traditional method on the same day The barometers used in the testing were products of Vaisala, a Finnish company well known for its weather monitoring instruments

Testing The following three instruments were calibrated: –n:o Y of the type PTB 220. This is a new instrument belonging to the MIKES density laboratory –n:o P of the type PTB 200, which is one of the reference standards of the Finnish Meteorological Institute –n:o of the type PTB 200, a working standard of MIKES pressure laboratory since 1994

Testing The instruments P and have been calibrated in the pressure laboratory of MIKES for several times They have been proved to be very stable, the drift is certainly less than 2 Pa / year

The results on Y as averages with uncertainty bars (coverage factor k=2)

The results on P as averages with uncertainty bars (coverage factor k=2)

The results on as averages with uncertainty bars (coverage factor k=2)

The result points for the barometer Y

The result points for the barometer

The temperatures recorded for each measurement point during calibration

The reference vacuum values recorded for each measurement point during calibration

Summary of results Barometer n:oE n –value ranges Y , ,12 P , , , ,17 Obviously the two methods give equivalent results

Conclusions The new weight exchanger system has been used in routine calibrations for barometers The results are not only in a good agreement with the traditional method but they suggest possibilities to decrease the measurement uncertainty The main target, considerable savings in measurement time was achieved

The end. Thank You for Your attention