molbloc and molbloc-S are trademarks, registered and otherwise, of
DH Instruments, Inc.
VCR is a registered trademark of the Swagelok Company.
Document 560048
030527
DH Instruments, Inc.
4765 East Beautiful Lane
Phoenix AZ 85044-5318
USA
Tel 602.431.9100
Fax 602.431.9559
www.dhinstruments.com
laboratory in which the calibration takes place can be
measured independently and used for this purpose.
The gas temperature measurement made by the
molbloc-S can be selected for use by either the
molbox or by COMPASS in order to calculate the
FTS flow rate. However, the molbloc-S pressure
measurement should NOT be used in this manner
since the upstream absolute pressure at the
molbloc-S is not the same pressure as the inlet
pressure to the FTS due to the flow control valve
in the hardware setup.
DETERMINATION OF CALIBRATION
COEFFICIENTS
Once the calibration is complete, the data obtained
can be used to recalculate the FTS values of slope (m)
and offset (b). A simple spreadsheet can be produced
to make the following calculations:
FTS indication:
amb
amb
P
T
P
∆
IND
•
=
FTS flow rate:
b
P
T
P
∆
m
Q
amb
amb
FTS
+
•
•
=
Where:
∆
P = Difference between ambient pressure
and FTS pressure tap
[in. H
2
O]
T
amb
= Ambient air temperature in Kelvin
[°C + 273.15]
P
amb
= Ambient air pressure in Atm
[mmHg/760]
m =
Slope
coefficient
[marked on FTS label]
b =
Offset
coefficient
[marked on FTS label]
Once sufficient calibration data has been obtained, the
“as received” % of reading error can be calculated at
each point as follows:
(
)
100
Q
Q
Q
ERR
%
REF
REF
FTS
•
−
=
The “slope” function in MS Excel can be used to
determine the new “m” slope coefficient. The format of
this function would take the form similar to:
=slope(ref Q data range:FTS indication data range)
The “intercept” function in MS Excel can be used to
determine the new “b” offset coefficient. The format of
this function would take the form similar to:
=intercept(ref Q data range:FTS indication data range)
The predicted “as left” % of reading error can be
determined in the same manner using the new
coefficients of m and b, and recalculating the FTS
indication and volumetric flow rate.
