Dynasonics TFXP Series Transit Time Ultrasonic Flow Meter - Manual (Page 101)

Rev. 5/09 -A.5- TFXP
Accuracy
The accuracy of BTU measurements relies on two separate
measurements – mass flow and difference temperature. The BTU
meter can measure volumetric flow to within about 1% of reading
accuracy. For example if the liquid is chilled water in the range
of 40-60 °F, the influences on mass flow accuracy within this
temperature range, with a volumetric flow meter are on the order
of 0.073% potential error.
If a surface mount RTD is being utilized, and the RTDs are installed
properly (mounts them to the pipe with heat sink compound, wraps
them tightly with the wrap tape and then insulates the pipe well) the
absolute readings will be within 0.5 °F of actual. More importantly
though is that absolute temperature measurement is not required to
make BTU measurements, only difference temperature. If both
RTDs are mounted using the same method and similar “R” values of
insulation, the errors between the absolute measurements will be
very similar. The RTDs used in the TFX can deliver difference
temperature accuracies of 0.1 °C or slightly better than 0.2 °F. The
advantage to insertion RTDs is that they respond faster (typically
not an issue in chilled water systems) and they do not need to be
insulated.
Assuming that a typical system has a delta T of 5 °F temperature.
The accuracy of the delta T measurement will be on the order of
0.2/5.0 = 4%. Adding the mass measurement accuracy of 1.073%
to the temperature measurement accuracy of 4%, the heat flow
reading accuracy would be somewhere in the 5% range.
Accuracy
HEAT FLOW ADDENDUM