Isotech TTI-7-R, Operator'S Handbook Manual

Page: 31 / 120

Consequently more than one equation is required for resistance to
temperature conversion. Calibration data for the PRTs takes the
form of Callendar van Dusen coefficients.
ISOTECH NA Inc. provides a range of proven PRTs especially for
use with the TTI-7-R , as well as offering a service to provide
customised PRTs to meet individual customers' requirements.
High "alpha" PRTs: The best possible system accuracy is
achieved using high "alpha" ( Ω ) PRTs, or more correctly, PRTs
using high Ω (high purity) platinum wire.
Low "alpha" PRTs: Low Ω PRTs contain a higher level of impurities
in the platinum resistance wire used. This affects the resistance
value at a given temperature (the temperature coefficient). As
impurities already exist in the platinum resistance wire, additional
contamination has a reduced effect and hence low
Ω PRTs are more
immune to contamination and are therefore better for industrial
applications. To ensure a robust PRT, the detector within the PRT is
contained within materials, which can themselves be the source of
contamination at elevated temperatures. The PRTs supplied by
ISOTECH NA Inc. have been optimized for the temperature ranges
for which they are specified and, when calibrated, are temperature
cycled to enhance stability in use.
PRTs which are used outside their design and/or calibration
temperature range, especially at higher temperatures, risk
irreversible alteration to their calibration either by induced thermal
stresses or by contamination.
Measuring Temperature
Page 4-5