Appendix B 391
2% and 5% of full scale and command prices somewhat less than the
standard transit time-effect ultrasonic devices. Meter cost is independent
of pipe size. Ultrasonic meters can have low installation costs since they
do not require shutting down systems to cut pipes for installation.
Magnetic flow meters
They measure the disturbance that a moving liquid causes in a
strong magnetic field. Magnetic flow meters are usually more expensive
than other types of meters. They have advantages of high accuracy and
no moving parts. Accuracy of magnetic flow meters are in the 1-2%
range of actual flow.
PRESSURE
Mechanical methods of measuring pressure have been known for
a very long time. U-tube manometers were among the first pressure
indicators. But manometers are large, cumbersome, and not well suited
for integration into automatic control loops. Therefore, manometers are
usually found in the laboratory or used as local indicators. Depending
on the reference pressure used, they can indicate absolute, gauge, or
differential pressure. Things to keep in mind while selecting pressure
measurement devices are: accuracy, pressure range, temperature effects,
outputs (millivolt, voltage or current signal) and application environ-
ment
Modem pressure transmitters have come from the differential
pressure transducers used in flow meters. They are used in building
energy management systems and are capable of measuring pressures
with the necessary accuracy for proper building pressurization and air
flow control.
THERMAL ENERGY
The measurement of thermal energy flow requires the measure-
ment of flow and some temperature difference. For example, the cool-
ing provided by a chiller is recorded in Btu and is a calculated value
determined by measuring chilled water flow and the temperature dif-
ference between the chilled water supply and return lines. An energy