M O L B L O C / M O L B O X ™ L O W M A S S F L O W C A L I B R A T I O N S Y S T E M A F M E T C A L
C O N F I G U R A T I O N S
I N S T A L L A T I O N A N D O P E R A T I O N M A N U A L S U P P L E M E N T
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DH Instruments, a Fluke Company
Application Notes
MOLBLOC RANGES
To ensure in-tolerance measurements, the appropriate range molbloc should be chosen to calibrate each
DUT. Remember that a molbloc’s true usable range may be different from its nominal range designation
due to the pressure-dependent calibration performed on the molbloc or due to the gas being used. It is
also important that the molbloc is used at the correct pressure for the stated usable flow range and
measurement specifications to be valid.
A silver label on each AFMETCAL configuration system molbloc displays the calibrated usable flow range and proper inlet
pressure for that molbloc. An example of the label is shown below. The flow range stated on this label, not the nominal range
designation referenced on the black label, should be observed. The arrow on the silver label indicates molbloc flow direction.
Figure 4.
AFMETCAL Configuration molbloc Label
MOLBLOC PRESSURE AND Flow ranges
molbloc ranges are traditionally described according to the flow ranges they would have if they were calibrated in nitrogen for
standard low pressure use. Since the usable range of a molbloc can differ from this nominal value depending on the gas and pressure
used, newer molblocs are defined by a size designation instead to avoid confusion about the molbloc’s true usable range. The size
designations for each molbloc in the AFMETCAL system are included in Table 3 along with their nominal ranges. The black labels
on the back of newer molblocs will display both the molbloc size designation, as part of the model name, and the nominal N2 flow
range. Older molblocs display only the nominal flow range as part of the model name.
To accommodate elevated DUT back pressure applications, molblocs in AFMETCAL configuration systems have special, high
pressure calibrations. The result is that the each molbloc’s usable flow range is shifted upward from its nominal flow range. For
example, a 500 sccm (5E2) molbloc with a high pressure calibration has a usable flow range up to 1 000 sccm (1 slm). For most range
designations, the usable range of the molbloc is double the nominal range as in the previous example. An exception is the 30 slm
(3E4) molbloc, which becomes a 40 slm range when given a high pressure calibration. molblocs are calibrated over their entire usable
range;
usable flow range = calibrated flow range
.
The maximum flow and minimum flow at which molblocs can be most accurately used are both changed by a high pressure
calibration. molbloc flow measurements have an uncertainty of ± 0.2 % of reading, or ± 0.02 % of full scale, whichever is greater (see
note). This means that flow can be measured with ± 0.2 % of reading uncertainty from 10 to 100 % of the molbloc’s range. A high
pressure calibration shifts both the 10 and 100 % value. For example, measurements with a 500 sccm (5E2) molbloc can be made
with ± 0.2 % of reading uncertainty from 100 to 1 000 sccm.
This does not mean that molblocs should not be used below their 10 % value. Below 10 % of a molbloc’s full scale range, uncertainty
is calculated as a percentage of its full scale range (± 0.02 % FS). Since most, if not all, DUTs have uncertainties which are also
calculated as a percent of their full scale range (typically 1 to 5 % FS), a sufficient accuracy ratio will still be maintained as you measure
lower flow points. A general guideline is that the DUT’s full scale range should fall between 10 and 100 % of the molbloc’s usable
(not nominal) full scale range if possible but this is not indispensable in all cases.
When using the 30 slm (3E4) molbloc with a high pressure calibration, the uncertainty specification is
±
0.3 % of
reading when measuring flows between the upper and lower values listed in Table 3. At flows below the lower value,
the uncertainty remains constant and is equal to 0.3 % of the lower value. The only difference between the usable
ranges of molblocs in nitrogen and air is this lower value for 30 slm (3E4) molblocs.
Table 3 lists maximum usable flow rates in each calibration gas for nominal molbloc flow ranges. A bold value indicates that the
maximum flow is limited by the maximum Reynolds number value of 1 200 which is reached before the normal differential pressure
range is reached. In that case, the second value gives the minimum flow for which relative accuracy is ± 0.2 % of the measured value.
Where there is only one value, the minimum flow at which this relative accuracy is valid is 10 % of the indicated value.
Table 3.
molbloc Usable Ranges with High Pressure Calibration
