388 Energy Project Financing: Resources and Strategies for Success
perature data are required. The main disadvantage of thermocouples is
their weak output signal, making them sensitive to electrical noise and
always requiring amplifiers. Few energy savings determination situa-
tions, except for thermal energy metering, warrant the accuracy and
complexity of thermocouple technology.
Thermistors
These are semiconductor temperature sensors usually consisting of
an oxide of manganese, nickel, cobalt or one of several other types of
materials. One of the primary differences between thermistors and RTDs
is that thermistors have a large resistance change with temperature.
Thermistors are not interchangeable, and their temperature-resistance
relationship is non-linear. They are fragile devices and require the use
of shielded power lines, filters or DC voltage. Like thermocouples, these
devices are infrequently used in savings determination.
Integrated Circuit Temperature Sensors
Certain semiconductor diodes and transistors also exhibit repro-
ducible temperature sensitivities. Such devices are usually ready-made
Integrated Circuit (IC) sensors and can come in various shapes and
sizes. These devices are occasionally found in HVAC applications where
low cost and a strong linear output are required. IC sensors have a
fairly good absolute error, but they require an external power source,
are fragile and are subject to errors due to self-heating.
HUMIDITY
Accurate, affordable and reliable humidity measurement has al-
ways been a difficult and time-consuming task. Equipment to measure
relative humidity is available from several vendors, and installation is
relatively straightforward. However, calibration of humidity sensors
continues to be a major concern (see Chapter 5.4) and should be care-
fully described in the M&V Plan and documented in savings reports.
FLOW
Different types of flow measurement may be used for quantities
such as natural gas, oil, steam, condensate, water, or compressed air.