Appendix B 387
Resistance Temperature Detectors or RTDs
These are common equipment for measuring air and water tempera-
ture in the energy management field. They are among the most accurate,
reproducible, stable and sensitive thermal elements available. An RTD
measures the change in electrical resistance in special materials.
RTDs are economical and readily available in configuration pack-
ages to measure indoor and outdoor air temperatures as well as fluid
temperatures in chilled water or heating systems. Considering overall
performance, the most popular RTI)s are 100 and 1,000 Ohm platinum
devices in various packaging including ceramic chips, flexible strips and
thermowell installations.
Depending on application, two, three and four-wire RTI)s are avail-
able. Required accuracy, distance, and routing between the RTD and
the data logging device can determine the specific type of RTD for a
project. Four-wire RTI)s offer a level of precision seldom required in the
energy savings determination, and are most commonly found in high
precision services or in the laboratory. Three-wire RTDs compensate
for applications where an RTD requires a long wire lead, exposed to
varying ambient conditions. The wires of identical length and material
exhibit similar resistance-temperature characteristics and can be used to
cancel the effect of the long leads in an appropriately designed bridge
circuit. Two-wire RTI)s must be field-calibrated to compensate for lead
length and should not have lead wires exposed to conditions that vary
significantly from those being measured.
Installation of RTI)s is relatively simple with the advantage that
conventional copper lead wire can be used as opposed to the more ex-
pensive thermocouple wire. Most metering equipment allows for direct
connection of RTDs by providing internal signal conditioning and the
ability to establish offsets and calibration coefficients.
Thermocouples
They measure temperature using two dissimilar metals, joined
together at one end, which produce a small unique voltage at a given
temperature that is measured and interpreted by a thermocouple ther-
mometer. Thermocouples are available in different combinations of met-
als, each with a different temperature range. Apart from temperature
range, consider chemical abrasion and vibration resistance and instal-
lation requirements while selecting a thermocouple.
In general, thermocouples are used when reasonably accurate tem-