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National Instruments Corp.
11
FP-TB-1/2/3 Operating Instructions
to be at different temperatures, and so the resulting measurement
creates not only errors in absolute accuracy but also in the relative
accuracy between channels. The accuracy specifications for the
FP-TC-120 include the errors caused by a 0.2 °C (0.36 °F)
gradient. The actual gradient you can expect to encounter depends
on the terminal base you use and the details of your installation.
The following sections provide guidelines for estimating and
minimizing thermal gradients.
Estimating Thermal Gradients with the FP-TB-3
Isothermal Terminal Base
The FP-TB-3 is designed with
isothermal
construction to keep
the terminals at the same temperature. It is the terminal
base recommended for the best accuracy of thermocouple
measurements. Adjacent FieldPoint modules (either network
modules or I/O modules) create a thermal gradient across the
terminals of the FP-TB-3, which you can estimate by dividing
the larger of the amounts of heat dissipated by each of the adjacent
modules by 20 W/°C (11 W/°F). For example, if the FP-TB-3 is
between an analog input module dissipating 0.35 W and a discrete
output module dissipating 3 W, the thermal gradient would be
3 W
÷
20 W/ °C = 0.15 °C.
Estimating Thermal Gradients with the FP-TB-1
or FP-TB-2 Terminal Bases
The lack of isothermal construction in the FP-TB-1 and FP-TB-2
terminal bases makes them more susceptible to errors caused by
thermal gradients. These terminal bases are recommended for use
with thermocouple measurements only if such errors are
acceptable or if they can be minimized, as described in the
Minimizing Thermal Gradients
section. Adjacent FieldPoint
modules (either network modules or I/O modules) create a thermal
gradient across the terminals of the FP-TB-1, which you can
estimate by dividing the larger of the amounts of heat dissipated by
each of the adjacent modules by 1 W/°C (0.6 W/°F). For example,
if the FP-TB-1/2 is between an analog input module dissipating
0.35 W and a discrete output module dissipating 3 W, the thermal
gradient would be 3 W
÷
1 W/°C = 3 °C. The typical thermal
gradient created by the FP-TC-120 mounted on an FP-TB-1
(neglecting any adjacent modules) is about 0.2 °C.
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