A Low Pressure Standard for NMISA Author: C. Korasie NMISA Private Bag X34, Lynnwoodridge, Pretoria, 0040, South Africa E-mail: [email protected]Phone: 012 841 4936 Fax: 086 716 0988 Abstract In this paper the Furness Rosenberg Standard (FRS) is introduced as a new primary standard for low Pressures at the NMISA. It has been developed to extend the typical range below 1,5 kPa currently achieved by our National Primary Standard. The range 0,000Pa to 7kPa has been realised with this standard with measurement uncertainties of ~ 0,05 Pa at full range of 7 kPa and ~ 0,002 Pa at 10 Pa. In this range a typical gauge to be calibrated would be a capacitance diaphragm gauge. The design of the FRS, as it is commonly known, being such that calibration of capacitance diaphragm gauges are easy and repeatable. The FRS5 will be used for the establishment of the input pressure to the Static Expansion System, which is the standard for high vacuum. This allows for smaller uncertainties to be obtained by the Static Expansion System. The development of the standard, its operation and the measurement uncertainties achieved at the NMISA will be discussed. The preparation for an international CCM Key comparison will be mentioned as well as the international trends and research in the field of low pressure metrology. The aim of which is to provide industry with the necessary capabilities and measurement uncertainties required to compete successfully in the international arena. 1. Introduction The low Pressure range under discussion is 1Pa to 11 kPa. Standards in this range are typically Manometers, Fixed Piston Gauges (FPG’s), FRS4/5 to name but a few. Typical pressure measuring instruments calibrated in this range are capacitance diaphragm gauges, resonance silicon gauges and Bell-type micromanometers and pressure generating instruments such as conical-piston and ball and force-balance piston gauges. A standard such as the FRS has been characterised and established as a primary standard in the range of 1 Pa to 11 kPa in National Metrology Institutes. The FRS5 was acquired by NMISA in 2001. It was originally calibrated at the PTB using a Precision mercury manometer, a Diving-bell manometer, a micromanometer and a capacitive membrane manometer. The Piston and Cylinder diameter was measured and found to be 75, 9710 mm and 76, 0328 mm respectively with an uncertainty of 0,1 μm. In 2010 the FRS5 was disassembled and the piston and cylinder re-measured and found to be 75, 97051 mm and 76, 03039 mm respectively. A roundness profile was also done on both the cylinder and piston an image of which can be seen in fig 1.
Transcript
A Low Pressure Standard for NMISA
Author: C. Korasie
NMISA Private Bag X34, Lynnwoodridge, Pretoria, 0040, South Africa
In this paper the Furness Rosenberg Standard (FRS) is introduced as a new primary standard for low Pressures at the NMISA. It has been developed to extend the typical range below 1,5 kPa currently achieved by our National Primary Standard. The range 0,000Pa to 7kPa has been realised with this standard with measurement uncertainties of ~ 0,05 Pa at full range of 7 kPa and ~ 0,002 Pa at 10 Pa. In this range a typical gauge to be calibrated would be a capacitance diaphragm gauge. The design of the FRS, as it is commonly known, being such that calibration of capacitance diaphragm gauges are easy and repeatable. The FRS5 will be used for the establishment of the input pressure to the Static Expansion System, which is the standard for high vacuum. This allows for smaller uncertainties to be obtained by the Static Expansion System. The development of the standard, its operation and the measurement uncertainties achieved at the NMISA will be discussed. The preparation for an international CCM Key comparison will be mentioned as well as the international trends and research in the field of low pressure metrology. The aim of which is to provide industry with the necessary capabilities and measurement uncertainties required to compete successfully in the international arena.
1. Introduction
The low Pressure range under discussion is 1Pa to 11 kPa. Standards in this range are typically Manometers, Fixed Piston Gauges (FPG’s), FRS4/5 to name but a few. Typical pressure measuring instruments calibrated in this range are capacitance diaphragm gauges, resonance silicon gauges and Bell-type micromanometers and pressure generating instruments such as conical-piston and ball and force-balance piston gauges. A standard such as the FRS has been characterised and established as a primary standard in the range of 1 Pa to 11 kPa in National Metrology Institutes. The FRS5 was acquired by NMISA in 2001. It was originally calibrated at the PTB using a Precision mercury manometer, a Diving-bell manometer, a micromanometer and a capacitive membrane manometer. The Piston and Cylinder diameter was measured and found to be 75, 9710 mm and 76, 0328 mm respectively with an uncertainty of 0,1 µm. In 2010 the FRS5 was disassembled and the piston and cylinder re-measured and found to be 75, 97051 mm and 76, 03039 mm respectively. A roundness profile was also done on both the cylinder and piston an image of which can be seen in fig 1.
Figure 1 .The roundness profile of the FRS piston
The roundness measurements for both the cylinder and piston are not as accurate as original measurements due to the use of less accurate measurement instruments. This however does not impact the measurement of the FRS as a pressure calibration will validate the measurement. The diameter of the piston cylinder unit has not changed within the last ten years which gives us confidence in the measurement capability of the FRS. 2. The Design
The piston and cylinder is manufactured from invar, a low expansion nickel-iron alloy which gives an area expansion coefficient of 4 ppm or less. The FRS is a non-rotating large area piston mounted in a parallelogram suspension system with flexible hinges to eliminate the friction between the piston and cylinder which is overcome in a standard configuration by the rotating piston. This is attached to an electronic balance capable of measuring to 3,2 kg which allows the FRS to measure up to a maximum of 7 kPa. The balance has an internal calibration mass which was set using the “extern cal” method to a predetermined mass of 1175,727g. This allows the balance to read out lbs equivalent to Pa. Thus a resolution of 0,000002 kPa is capable for the range of 0,000 Pa to 2,2 kPa and 0,00002 kPa for the range 2,2 to 7 kPa is achievable. In Figure 2, it can be seen that the FRS Piston cylinder suspension is mounted in such a way that the piston is connected to a turbomolecular pump above and beneath. The gap between the piston and cylinder is nominally 25-30 micron and requires a considerable amount of gas
flow. The system was designed to manage on a 30l/s hybrid turbo-molecular pump and a triple drag pump backed up by a rotary pump. It is housed in a custom designed casing providing the optimum path from the first intermediate port located between the turbo and first drag section and the pump at the cylinder. An additional pump is mounted to the top plate to facilitate the evacuation of the connecting pipe to the connection manifold. It also reduces the time to return to zero pressure to under 30seconds.This brings us to the measurement principle.
Figure 2.The FRS5 low pressure standard
Figure 3.FRS4 original design
Combining the pressure and conduction curves with the overall diagram of the absolute pressure shows that in theory with a modified stator assembly a reference pressure of 1x10-3
Pa up to a pressure of 2 kPa and less and less than 1x10-2 Pa up to 7 kPa can be maintained. If the figure below is carefully studied one can see that the FRS typically creates a static pressure which is measured at the connection ports
Figure 6.Port connections to the FRS[6]
The constant flow of gas from above and being drawn away below the piston and from the sides of the cylinder creates this static pressure that allows the low pressure measurement to be done. One can also relate the conductance diagram to the diagram above and see that at the connection ports the flow create 4. Calibration of the FRS
The piston was dimensionally characterised, and the effective area confirmed by calibration against a known pressure standard. Recent calibrations have been done against a calibrated capacitive membrane manometer and the results prove satisfactory. The balance proved to be due for an upgrade as many NMI’s have done in the last few years as the technology has increased to such an extent that the resolution and stability has increased. Calibration of the electronic balance can be done externally by the mounting of the weight pan supplied with nominal weights. This was done by the mass laboratory of NMISA, but
this proved futile as there was too much draft in the room. This can be eliminated by calibrating the balance under vacuum and with the internal weight only.
Figure 7.Piston cylinder assembly.
The calculation of the balance display must be adjusted to assign the indication to defined conditions, thus the internal weight which is made of stainless steel is used for this purpose. Thus the indication given by Rcal is generated for a weight mcal .. Where mcal =11175,727g Forces acting at the moment of calibration is thus:
���� = ��, ∗
� ��∗������, ����, �� �∗� ��
� �� (1)
Where Rz,0 =R-R0 The mass value results from:
� = ��, ∗ � ��� ��
∗������, ��
��, �� �������,����
�� � ∗ � �������
(2)
When pressure is being measured the balance thus indicates the pressure calculated by the formula:
� = � !"#∗$�%&'%()∗&*�+,°./ (3)
Thus:
� = ���� = ��, ∗� ��∗������, ��
��, �� �∗� ��� ��
∗ �"#∗$�%&'%()∗&*�+,°./ (4)
So for the numerical values of the balance reading are to be identical with the numerical values of the pressure the following conditions must be complied with:
1 =� ��∗������, ��
��, �� �∗� ��� ��
∗ �"#
(5)
This means that either mcal or Rcal must be matched to realise equation 5. The pressure would then be � = ��,, ∗ 11 + &3 + 4) ∗ &5 − 20°9: Due to the custom design of the FRS5 it allows for the zero drift to be determined either by direct reading of the balance’s zero (which is readable only after each measurement value can be started at the same reference vacuum) or reading of the indication of the mass difference at “zero check”. This is achieved by applying the internal calibration weight or the piston on the balance and checking the difference of the forces exerted. This is done at the measurement pressure, thus the difference is interpreted as the zero drift. This zero drift is also temperature dependent but these were plausible with the design of the system to be measured and were no mathematical models available for this effect. 5. Results
The results obtained during the first calibration at the PTB are as follows: Table 1: Effective area in the absolute pressure range [2]
Table 4: Subsequent results between the FRS5 and 100 Torr Baratron(PS-E-81)
Difference between FRS5 & PS-E-81 STDEV
Pa Pa Pa
-0.0079 -0.0128 0.0622 0.041958
-0.0153 -0.0374 -0.0439 0.014992
-0.0276 -0.0354 -0.0352 0.004447
-0.0245 -0.0884 -0.0446 0.032674
-0.1135 -0.1101 -0.1071 0.003202
-0.1990 -0.1957 -0.2051 0.004769
-1.0378 -1.0290 -1.0307 0.004668
-2.0603 -2.0671 -2.0628 0.003439
-2.9927 -2.9802 -3.0018 0.010845
-3.8835 -3.8879 -3.8829 0.00273
-4.7986 -4.7641 -4.7702 0.018412
-5.6326 -5.5762 -5.5758 0.032679
-6.3950 -6.3576 -6.3658 0.019658
-7.1111 -7.1221 -6.9044 0.122637
-8.4453 -8.4192 -8.4563 0.019055
-9.0114 -9.0105 -8.9749 0.020818
-9.4844 -9.4607 -9.4521 0.016728
The following model is used for the calculation of the measurement uncertainties:
� = &;� + <= + <>?> − �,@ ∗ A ∗ BC ∗ D) + �EF>�
Where B = ���G ∗ H ∗ I/((���G − �,��G))
D = 1 + K ∗ (5 − L) These quantities are defined as follows: p- the absolute pressure of the FRS5 in Pa R- the reading on the balance in lb Ub- uncertainty of the balance in lb Usyst- uncertainty of the absolute pressure reading on the balance in lb R0- the zero reading in lb f- the conversion factor in g/lb x- the “calibration” setting of the balance
A- Effective area of the piston in m2 c- the correction for temperature prest- the reference pressure in Pa mcal- calibration mass in kg b- the buoyancy correction g- gravity in m/s2 Rcal- the reading of the calibration balance in g R0cal- the zero reading of the “calibration” balance k- the thermal expansion coefficient ϑ- the temperature FRS5 d- 20°C 6. Discussion
The FRS5 was evaluated against a low pressure standard, PS-E-81, which is traceable to the PTB. The measurements would ensure confidence in the pressure generation of the FRS5 and validate the dimensional calibration of the FRS5 Piston. The FRS5 showed satisfactory stability throughout the measurement process with standard deviations being acceptable, since uncertainties of ± 0,002 Pa at 10 Pa and ±0,05 Pa from 1 kPa to 7 kPa is expected. The FRS5 has not been in operation for a few years. Recent measurements will thus require validation. The pressure laboratory has received the opportunity to participate in a CCM Low pressure inter-comparison for the first time and will be using this platform to validate the measurement uncertainties of the FRS5.The inter-comparison will be using a “pressure transducer package” as transfer standard. This was built by NIST, the NMI of the United States of America, currently the inter-comparison is underway and NMISA has been given the slot in April 2013. This has also given NMISA the opportunity to familiarise itself with the measurement protocol in order to be ready and be as efficient as possible when the TS has arrived.
7. Acknowledgment
I would like to acknowledge the NMISA and the DTi for the funding which enabled the measurements to be done as well as Dr K Jousten who was kind enough to provide guidance in understanding the FRS5.
8. Conclusion
The establishment of the FRS5 will pave the way for the development of a primary vacuum standard as it will provide the necessary input pressures for the initial volume of the static expansion system at a lower uncertainty than was previously possible. Further research is being done on the characterisation of the FRS5 as was done with the FRS5 at the PTB. The need for accurate Low pressure measurements is indicated by industrial applications from the semiconductor industry, photovoltaic, lightning industry to food packaging to name just a few examples. 9. References
