molbloc-S® Sonic Nozzle Flow Element is a high precision transfer standard intended for calibrating other transfer standards or process flow meters
The mass flow rate through molbloc-S Sonic Nozzle is directly proportional to the upstream absolute pressure and independent of the downstream or differential pressure. molbloc-S flow ranges are defined by the molbloc's Pressure to Flow Conversion Ratio, KF. KF is expressed in units of sccm/kPa and defines the relationship between mass flow and the absolute upstream pressure delivered to the molbloc-S. molbloc-S calibrations are performed over flow ranges corresponding to either 20 to 200 kPa (3 to 30 psia), 50 to 500 kPa (7 to 70 psia) or 200 kPa to 2 MPa (30 to 300 psia) upstream pressure.
For flow measurements to be valid, the flow at the throat of the molbloc-S venturi nozzle must be critical, i.e. the gas speed is equal to the local speed of sound. Critical flow is achieved when a sufficiently low back pressure ratio, or BPR (molbloc-S downstream absolute pressure divided by upstream absolute pressure) is maintained. The upper limit of acceptable molbloc-S BPR values for critical flow varies between about 0.5 and 0.9 depending on the Reynolds number of the flow.
Since the mass flow through molbloc-S is proportional to the upstream absolute pressure, the flow range for this sonic nozzle calibration device in an application may be limited by the BPR limit. To maximize the range of a molbloc-S element, a vacuum pump can be connected downstream to reduce the downstream pressure while flowing. When the downstream pressure is kept sufficiently low, the upstream pressure, and thus the mass flow rate, can be adjusted all the way down to the minimum value for the molbloc's calibration type without being limited by the BPR. If molbloc-S will be used with atmospheric pressure downstream (no vacuum pump), then the molbloc can only be used over a range of upstream pressures starting at the maximum pressure for its calibration type down to a minimum pressure value at which the BPR becomes equal to the BPR limit at the current Reynolds number. molbox1 terminals include features to measure BPR, automatically alert the operator when the BPR is too high and prevent measurements when flow is not critical.
molbloc/molbox specifications - General
|molbox1+||molbox RFM All specifications same molbox1 unless specified otherwise.|
|Normal Operating Temperature Range:||15 to 35 °C|
|Storage Temperature Range:||-25 to 35 °C|
|Normal Operating Pressure Range:||20 to 200 kPa absolute (3 to 30 psia) or 50 to 500 kPa absolute (7 to 70 psia) upstream, or 200 to 2000 kPa (30 to 300 psia) with molbox1+S only.|
|Overall Flow Range:||<1 to 5 000 slm|
|Inlet Connection: 1E1-S thru 1E2-S 2E2-S thru 2E3-S 5E3-S and 1E4-S||1/4 in. VCR™ 1/2 in. VCR™ 1 in. VCR™ or 1 in. NPT|
|Outlet Connection: 5E1-S thru 2E3-S 5E3-S, 1E4-S||16 mm (3/4 in.) ISO-KF style vacuum flange 40 mm (1 1/2 in.) ISO-KF style vacuum flange|
|Gases Supported:||Nitrogen (N2), Dry Air, Humid Air, Argon (Ar), Carbon Monoxide (CO), Helium (He), Oxygen (O2), Carbon Dioxide (CO2), Carbon Tetrafluoride (CF4), Ethane (C2H6), Ethylene (C2H4), Fluoroform (CHF3), Hexafluoroethane (C2F6), Hydrogen (H2), Methane (CH4), Nitrous Oxide (N2O), Propane (C3H8), Sulfur Hexafluoride (SF6), Butane (C4H10), Octafluorocyclobutane (C4F8), Xenon (Xe)|
|Range:||Dependent upon molbloc size and pressure calibration range (see molbloc-S Ranges table)|
|Required Back Pressure Ratio:||0.4 to 0.9 depending on Reynolds number|
|Calibration Gases:||Standard: Dry air Optional: N2 (to replace standard air calibration or in addition), Ar, H2, He, N2O. Consult factory regarding availability of availability of these gases for specific ranges, or for other gases. *Dry air calibration is required for humid air operation|
|molbloc-S Pressure Calibration Ranges:||Low Pressure (LP): 20 to 200 kPa absolute (3 to 30 psia) upstream Standard Pressure (SP): 50 to 500 kPa absolute (7 to 70 psia) upstream High Pressure (HP): 200 to 2 MPa absolute (30 to 300 psia) upstream * HP calibration usable with molbox1+S terminal only * Flow measurement specifications valid from 10% to 100% of molbloc full scale|
|Resolution:||± 0.0015 % of FS||± 0.01 % of reading|
|Precision:||± 0.06 % of reading||± 0.30 % of reading|
|Predicted Stability (One Year):||± 0.05 % of reading||± 0.2 % of reading|
|Measurement Uncertainty||± 0.2 % of reading Premium calibration: ± 0.125 % of reading||± 0.5 % of reading from 50 to 500 kPa|