Emerson Rosemount 644 Reference Manual page 60

Hardware Installation
March 2021
Figure 3-1: Head Mount Transmitter Connection Head Temperature Rise vs.
Extension Length
Example
The maximum permissible housing temperature rise (T) can be calculated by subtracting
the maximum ambient temperature (A) from the transmitter's ambient temperature
specification limit (S). For instance, if A = 40 °C.
T = S - A T = 85 °C – 40 °C T = 45 °C
For a process temperature of 540 °C (1004 °F), an extension length of 3.6-in. (91.4 mm)
yields a housing temperature rise (R) of 22 °C (72 °F), providing a safety margin of 23 °C
(73 °F). A 6.0-in. (152.4 mm) extension length (R = 10 °C [50 °F]) offers a higher safety
margin (35 °C [95 °F]) and reduces temperature-effect errors but would probably require
extra transmitter support. Gauge the requirements for individual applications along this
scale. If a thermowell with lagging is used, the extension length may be reduced by the
length of the lagging.
60
60 (108)
50 (90)
40 (72)
30 (54)
22
20 (36)
10 (18)
0
3 4 5
3.6
Extension Length (in.)
Reference Manual
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