Pressure and Vacuum Standards
Objective: The aim of theVacuum and Pressure Standards Group is to realize, maintain and disseminate the derived SI unit of pressure and vacuum – the pascal (Pa). The Group has been providing apex level calibrations in different pressure regions and maintains traceability of pressure and vacuum measurements at international level. The main objective and activities of the group are to establish, maintain and upgrade national primary standards of vacuum and pressure measurements equivalent to international standards through continuous Research and Development.
Figure 1: Ultrasonic Interfero meter Manometer (UIM) 
Figure 2: Static Expansion System 
Figure 3: Orifice Flow Method (OFM). 
Figure 4: Hydraulic Controlled Clearance Piston Gauge up to 1 GPa 
Figure 5: Pneumatic Piston Gauge up to 40 MPa 
Figure 6: Method of Crossfloating of two different secondary standards 
1. Existing Facilities
Vacuum and Pressure Standards Group is presently located in the TEC building in the National Physical Laboratory campus but has plans to shortly move into a new metrology building coming up in the same campus. The main objective and activities of the group are to establish, maintain and upgrade national primary standards of vacuum and pressure measurements [shown in Figures 1 to 6] compatible to international standards through continuous Research and Development. The following primary and secondary Vacuum and Pressure Standards have been established in the group and are internationally compatible.
Table 1: Different instruments and their ranges

Instruments 
Range 
Vacuum Standards  Barometric Region 
Ultrasonic Interferometer Manometer (UIM) – Primary Standard 
130 kPa to 1 Pa 
Air Piston Gauge – Primary Standard 
up to 3670 kPa 

QUARTZ BOURDON GAUGE (DDR – 6000) – Secondary Standard 
up to 130 kPa 

DPI – 145 – Secondary Standard 
100 kPa, 130 kPa, and 400 kPa 

Capacitance Diaphragm Gauge – Secondary Standard 
13 kPa and 0.13 kPa 

Vacuum Standards – High Vacuum Region 
Static Expansion  Primary Standard 
1000 Pa to 10^{4} Pa 
Orifice Flow Standard  Primary Standard 
0.1 Pa to 10^{6} Pa 

Spinning Rotor Gauge  Transfer Standard 
1 Pa to 10^{4} Pa 

Ionisation gauge  Transfer standard 
10^{2} Pa to 10^{6} Pa 

Extractor gauge  Transfer Standard 
10^{2} Pa to 10^{6} Pa 

Pressure standards  Hydraulic Region 
Controlled Clearance Piston Gauge – Primary Standard 
10 MPa to 1 GPa 
Ruska (USA) model # 2450  NPL28 MPA, 140 MPA and 280 MPA – Secondary Standard 
10 MPa to 280 MPa 

Desgranges Et Huot (France) model # 5306  NPL100 MPN, 200MPN and 500 MPN – Secondary Standard 
10 MPa to 500 MPa 

Desgranges Et Huot (France) model # 5302  NPL100 MPA and 500 MPA – Secondary Standard 
10 MPa to 500 MPa 

Pressure Transducers 
10 MPa to 500 MPa 

Pressure standards  Pneumatic Region 
Controlled Clearance Piston Gauge – Primary Standard 
0.5 MPa to 6 MPa 
Ruska (USA) model # 2465  NPLI4 and NPLI – 110 Secondary Standard 
10 kPa to 4 MPa 

Desgranges Et Huot (France) model # 5502  NPL8 and NPLI20 – Dual Piston gauge  Secondary Standard 
0.8 MPa to 20 MPa 

Ruska (USA) model # 2470  NPLI12– Secondary Standard 
0.8 MPa to 17 MPa 

Pressure Transducers 
0.1 MPa to 4 MPa 
2. Highlights of Achievments
The Vacuum and Pressure Standards group, has not only established the above primary standards but has also contributed significantly in the enhancement of knowledge in the field of vacuum and pressure metrology by way of doing some basic and applied research work. Both the groups have published large number of research papers in international journals such as METROLOGIA (BIPM, France); JVST (A), USA; Vacuum (UK); Review of Scientific Instruments (USA), J. Phys. E, Japan. J. Appl. Phys. etc. Some of the research work carried out in the Group in pressure and vacuum metrology has also resulted into award of Ph.Ds.
Important Research outputs (20062012)
2.1 Heydemann and Welch (HW) model:
HW model is based on a thermodynamic fluid flow equation known as NavierStokes (NS) Equation in the laminar flow approximation. There are several limitations of this model equation. This was a first attempt to overcome these limitations through introducing a rigorous statistical analysis. Based on this method, a complete characterization of a Controlled Clearance Piston Gauge (CCPG) was carried out at NIST (USA) in a span of two years. The HW parameters determined from the characterization have also been repeated to within the Type A uncertainty from the two characterizations. The relative combined standard uncertainty of the effective area using the HW model ranges from 23.6×10^{−6} at 20 MPa to 36.8 × 10^{−6} at 200 MPa. Operating at a jacket pressure up to 40% of the system pressure reduces the uncertainty slightly. The effective area from the HW model agrees to within the combined standard uncertainty of a direct comparison to the NIST pressure scale at all conditions from 20 MPa to 200 MPa (Metrologia, 43, 573582 (2006)).
2.2 Experimental study on the effect of phase sensitive detector (PSD) outputs in the uncertainty measurement of UIM
During experimental evaluation of measurement uncertainty of Ultrasonic Interferometer Manometer (UIM), it was observed that there is significant difference between this value and the theoretical value, particularly at low pressure below 1000 Pa. After analyzing the detailed behavior of variation of uncertainty in real operating conditions, it was found that the variation depends significantly on the PSD output parameters, such as imperfection in 90 degree phase difference between two signals, unequal gain in two channels and eccentricity of the circle thus formed by the two signals. After this finding further experimental work was carried out and as a result of which the PSD parameters were properly adjusted to the extent that maximum reduction in uncertainty is obtained. After these adjustments the output signals of PSD in the form of sine and cosine values are measured at different multiple frequencies. The results obtained are reproduced here in Fig. 7 given below, which shows a big improvement in the signals such as perfect 90 degree phase difference and same amplitude of sine and cosine values. This work has improved the measurement uncertainty of UIM in low pressure region.
Figure 7: Phase sensitive detector (PSD) outputs in the uncertainty measurement of UIM
2.3 Characterization and Establishment of Hydraulic Controlled Clearance Primary Pressure Standard up to 1.0 GPa and computation of associated uncertainties
Controlled clearance type piston gauge (CCPG) is an ultimate primary instrument for the hydrostatic pressure measurements and its use as reference or primary standard is well established and internationally accepted inspite of its complicated operations. However, the rheological properties, mainly, the viscosity and density of the pressure transmitting fluids contribute significantly above 500 MPa. Systematic studies were carried out to characterise such a controlled clearance piston gauge, established at NPL, India, in the hydraulic pressure region up to 1000 MPa. During this process, the combination of several working fluids viz. pure J13 and mixture of J13 and aviation turbine fuel (ATF) (one part of J13 and 2 parts of ATF), pure diethyl hexyl sebacate, pure unleaded white gasoline, mixture of white gasoline and sebacate oil were used as pressure transmitting fluids and finally the mixture of white gasoline, J13 and sebacate oil was found suitable to generate the pressure upto 1.0 GPa. Using the mixture of 5 % straight run unleaded white gasoline, 10 % pure J13 and 85 % di ethyl hexyl sebacate, it was possible to generate the hydrostatic pressure up to 1.0 GPa, the ultimate full scale pressure of our primary pressure standard. The detailed studies were carried out on the measurement of piston fall rate as a function of the applied jacket pressure (pj) for each of several loads (50 kg). Following the W & H method, the cube root of the fall rate was plotted as a function of applied pj and extrapolating the linear portion of the curve to zero fall rate provides the values of pz for different loads. From the pz at different loads, the zero clearance between the piston and cylinder was determined. The values of jacket coefficient are computed by analyzing the dependence of effective area and the jacket pressure, pj using a theoretical method as well as an experimental method. The detailed uncertainty budget is prepared at 1.0 GPa using all the possible associated uncertainty contributions. Some of the results are shown in Fig. 8 and Fig. 9.


Fig. 8: Fall rate data at ten different loads 
Fig. 9: pj as a function of pm 
2.4 Finite Element Method (FEM) for the characterization of a controlled clearance piston gauge
Preliminary study of the behavior of a high performance controlled clearance piston gauge (CCPG) in the pressure range up to 1 GPa through finite elemental method (FEM) is carried out. The FEM analysis provides characterization of a pressure balance in terms of effective area and distortion coefficient of the piston and cylinder, the effect of gap profile between piston and cylinder of this controlled clearance piston gauge, under the influence of applied pressure (p) from 100 MPa to 1000 MPa, on the pressure distortion coefficient (λ) of the assembly. The gap profile is also studied at varying applied jacket pressure (pj) such that pj/p varied from 0.3, 0.4 and 0.5. The di 2ethylhexyl) sebacate is used as pressure transmitting fluid. The piston fall rate values (vf) are also determined as a function of applied jacket pressure (pj). The results thus obtained using FEM are compared with the experimental values.
FEM analysis shows that the clearance h between piston and cylinder decreases as pj. The gap width also increases along the engagement length from top to bottom due to the increase in pressure distribution in the gap profile. The d remains almost unchanged having average value as 2.21 x 10^{6} MPa^{1} with measurement uncertainty 6.4 x 10^{9} MPa^{1} from 400 MPa to 1000 MPa. Though the pressure distortion coefficient, λ is independent of applied pressure, the values of λ are much higher in Free Deformation Mode (FDM) in comparison to controlled clearance mode (CCM). In FDM, the λ varies from minimum 2.95 x 10^{6} MPa^{1} to maximum 5.29 x 10^{6} MPa^{1} having average value as 4.41 x 10^{6} MPa^{1} with measurement uncertainty 2.6 x 10^{7} MPa^{1}. The pressure distortion coefficient λ is not much affected by applied pressure p but it is greatly affected by jacket pressure pj. Consequently, the values of λ are larger in the FDM in comparison to CCM. The fine tuning of the modeling is required to produce consistent results with experimental values, specially the pressure distribution in the gap. The major cause of discrepancy is due to the changing of density and viscosity equations from 300 MPa to 400 MPa. Our future emphasis would be focused on to remove such gaps in the methodology and obtain consistent results.
Fig. 10: (a) Meshed structure of pc assembly (b) meshing around engagement length and (c) image of the distorted pc assembly in CCM mode at p = 1.0 GPa and pj = 0.5 p
2.5 Final report: APMP.SIM.M.PK1c: Bilateral comparison between NIST (USA) and NPLI (India) in the pneumatic pressure region 0.4 MPa to 4.0 MPa
A bilateral comparison of pressure measurement between NIST and NPLI using a piston gauge transfer standard (TS), designated as NPLI4, over the range of nominal applied pressure 0.4 MPa to 4.0 MPa was carried out. The comparison data were analyzed in terms of the effective area [Ap (mm2)] as a function pressure [p (MPa)] of the TS at the abovementioned pressures. We have also estimated the zero pressure effective area [A0 (mm2)] and the pressure distortion coefficient [λ (MPa1)] of the transfer standard. The degree of equivalence between NPLI and NIST is 11.4x10^{6} or better. (Metrologia Technical Supplement 44, 07002 (2007)).
Figure 11: Effective area as a function of pressure as measured by the laboratory standards at NIST and NPLI. Standard uncertainty shown as error bars.
2.6 Final report APMP.SIM.M.PK7: Bilateral comparison between NIST (USA) and NPLI (India) in the hydraulic pressure region 40 MPa to 200 MPa
Another bilateral comparison of pressure measurement between the National Institute of Standards and Technology (NIST), Gaithersburg, USA and the National Physical Laboratory (NPLI), New Delhi, India, over the range of nominal applied pressure 40 MPa to 200 MPa. The comparison data were analyzed in terms of the effective area [Ap (mm2)] as a function of pressure [p (MPa)] of the two transfer standards in the respective pressure ranges of (40 to 80) MPa and (80 to 200) MPa. The degree of equivalence between NPLI and NIST is given as the relative difference in the institutes’ results for effective area of the transfer standards, and is within 7.7x10^{6} in the whole pressure range (40 to 200) MPa. (Metrologia Technical Supplement 43, 07003 (2006)).
2.7 Supplementary Comparison APMP.M.PS2 (bilateral comparison) at a nominal pressure of 0.05 Pa (Collaborative R/D work with NIST (USA)).
Nominal pressures of 0.05 Pa are generated (i) at the NPLI by their Static Expansion System (SES) using the method of single stage expansion and (ii) at the NIST by their midrange orifice flow standard. NIST (USA) had moved into their newly built Advanced Metrology Laboratory building which has been designed to give high temperature stability and protection from noise and vibration. It was decided by the two laboratories to intercompare their respective standards at a nominal pressure of 0.05 Pa using a pair of spinning rotor gauges as transfer standards, to establish their compatibility. This value of the nominal pressure was chosen since it can be easily generated by their respective standards with high accuracy and it had never been covered in the earlier two comparisons, namely CCM.PK3 and CCM.PK4. This comparison is listed as a supplementary comparison APMP.M.PS2 in the BIPM data base. NPLI served as the pilot lab for the comparison. To minimize the possible effects of rough handling during shipping, the rotors were carefully packed and handcarried between India and the United States and back to India. At each laboratory, multiple measurements of the accommodation coefficients of the two rotors were made using the respective high vacuum standards of NPLI and NIST. In the present comparison the degree of equivalence between the nominal pressure generated by the vacuum primary standards of NPL of India and NIST was tested. Both standards were fully equivalent for the pressure 0.05 Pa (Metrologia, 2008, 45, Technical Supplement, 07003).
2.8 Absolute pressure measurements in gas from 3x10^{6} Pa to 9x10^{4} Pa:
A key comparison of absolute pressure at five national metrology institutes (NMIs) was carried out with a view to determine the degree of equivalence of NMI standards at pressures in the range of 3 × 10^{−6} Pa to 9 × 10^{−4} Pa. The primary standards were dynamic expansion standards at four of the NMIs and a series expansion standard at the fifth NMI. The transfer standard package consisted of two spinning rotor gauges (SRGs) and three Bayard–Alpert ionization gauges. The SRG measurements were used to compare NMIs at 9 × 10^{−4} Pa and to normalize the ionization gauge results at that same pressure. The ionization gauge measurements were used to compare NMIs at the lower pressures. The standards of four of the NMIs including NPLI showed equivalence to the KCRV and each other over the full range of pressures relative to the expanded uncertainties of the comparisons at the k = 2 level (Metrologia, 2010, 47, Tech. Suppl., 07004).
Fig. 12 Summary of results for the degree of equivalence for each NMI with reference to the key comparison reference value.
2.9 Key comparison CCM.PK12 for very low helium flow rates (Leak Rates):
Participated in CCM.PK12 leak comparison in Nov. 2008 Feb. 2009. The goal of this comparison was to compare the national calibration standards and procedures for helium leak rates. Measurements were made on the leak rate of two helium permeation leaks (L1 and L2) for the comparison, one at a molar flow rate of about 4x10^{11} mol/s (1x10^{4} Pa l/s at 23°C), another of about 8x1014 mol/s (2x10^{7} Pa l/s at 23°C). The participants of the key comparison were: 1. Czech Metrological Institute (CMI), Czech Republic, 2. INRIM, Italy, 3. IMT Institute of Metals and Technology, Slovenia, 4. LNE, France, 5. NIM (China), 6. NPL (India), 7. NMIJAIST (Japan), 8. NIST (USA), 9. VNIIM (Russia), 10. ASTAR, Singapore while PTB, Germany was the pilot laboratory. All eleven NMIs measured L1 while only six NMIs measured L2. NPLI showed equivalence with seven other NMIs for L1 (Fig. 13) and with all other five NMIs for L2 (Metrologia, 50, 07001, 2013).
Fig. 13: Artifact of the International key comparison CCM.PK12
Fig. 14 The difference D1j of each laboratory to the reference value with respective uncertainty (k=2) for transfer leak element L1.
3. Traceability of Pressure and Vacuum Standards:
Through a chain of key comparisons either organized by CIPM or APMP and also bilateral comparison with NIST(USA). Also the direct comparison of the various primary and secondary standards within the laboratories in the overlapping pressure regions to establish mutual compatibility. Various Proficiency Tests (PTs) were organized by the group in collaboration with NABL to disseminate the traceability of the laboratory to the NABL accredited laboratories as a part of the service to Nation.
Sr. No 
Class 
Minimum value 
Maximum value 
Units 
Measurement Uncertainty (20092014) (at k=2) 
Measurement Uncertainty (20032008) 
1. 
Absolute Pressure, Gas Medium 
0.001 
130 
kPa 
[(9.2E03)^{2} + (7.2E06p)^{2}]½, p Pressure in Pa 
[(9.2E03)^{2} + (7.2E06p)^{2}]½, p Pressure in Pa 
2. 
Gauge Pressure, Gas Medium 
0.001 
130 
kPa 
[(9.2E03)^{2} + (7.2E06p)^{2}]½, p Pressure in Pa 
 
3. 
Absolute Pressure, Gas Medium 
6.5 
360 
kPa 
[(1.4E01)^{2} + (12E06p)^{2}]½, p Pressure in Pa 
 
4. 
Gauge Pressure, Gas Medium 
20 
360 
kPa 
12E06p, p Pressure in Pa 
 
5. 
Gauge pressure gas medium 
0.036 
4 
MPa 
22E06p, p in MPa 
52E06p, p in MPa 
6. 
Gauge pressure gas medium 
4 
8 
MPa 
26E06p, p in MPa 

7. 
Gauge pressure gas medium 
8 
12 
MPa 
32E06p, p in MPa 

8. 
Gauge pressure gas medium 
12 
20 
MPa 
33E06p, p in MPa 
 
10. 
Gauge pressure gas medium 
20 
40 
MPa 
36E06p, p in MPa 
 
11. 
Differential pressure (Dp), gas medium 
30 
150 
kPa 
[(10)^{2}+(2.5E06 *pl)^{2}+ (2.9 E05*Dp)^{2}]^{1/2}, where pl in Pa and Dp in Pa. 
1E04p, p in kPa 
12. 
Gauge pressure, liquid medium 
500 
1000 
MPa 
250E06p, p in MPa 
 
13. 
Gauge pressure, liquid medium 
200 
500 
MPa 
135E06p, p in MPa 
133E06p, p in MPa 
14. 
Gauge pressure liquid medium 
100 
200 
MPa 
50E06p, p in MPa 

15. 
Gauge pressure liquid medium 
50 
100 
MPa 
48E06p, p in MPa 

16. 
Gauge pressure liquid medium 
0.1 
50 
MPa 
45E06p, p in MPa 

17. 
Absolute pressure, gas medium, vacuum 
0.05 
10 
Pa 
4E03p, p pressure in Pa 
4E03p, p pressure in Pa 
18. 
Absolute pressure, gas medium, vacuum 
3.00E06 
0.1 
Pa 
2E02p, p pressure in Pa 
2E02p, p pressure in Pa 
4. Specific important information for users
4.1 Calibration facilities
 Vacuum standards: Facilities available for the calibration of different types of vacuum gauges, Airpiston gauges, Dead Weight Tester, Quartz Bourdon Gauges, Pirani, Thermocouple gauges, Penning gauges, different dial gauges, ionization gauges, Spinning rotor gauges, capacitance diaphragm gauges, inductive diaphragm gauges etc.
 Pressure Standards: Facilities available for the calibration of different types of piston gauges and dial gauges, Dead Weight Tester, pressure transducer/transmitters, differential pressure transducer/transmitters.
4.2 Consultancy services
 Characterization of the Airpiston gauges and other piston gauges,
 Establishment of the calibration laboratories
 Software packages