Sensor Tube Replacement When Used With A Sensor Head Junction Box - Emerson Rosemount 398R Reference Manual

Maintenance
February 2019
Consult Table 6.1 (above) for temperature-resistance data. Lead resistance is about 0.05 ohm/ft at
25°C. Therefore, 15 ft of cable increases the resistance by about 1.5 ohm. The resistance between
the RTD return and RTD sense leads should be less than 2 ohms. If a connection is open or shorted
and should not be, replace the sensor. If the measured resistance is greatly in error, replace the
sensor.
6.8 Sensor Tube Replacement When Used With A
Sensor Head Junction Box
Replace ment of the retraction versions sensor tube assembly involves the removal and installation
of two sets of male connectors: One at the process end of the sensor, and the other at the junction
box end (See Figure 6-1 and Figure 6-5). Refer to Section 6.2 for proper removal of the sensor from
process.
1.
2.
3.
Note: If stainless steel ferrule was used, male connector body (A) will have to be dis carded with
the sensor tube.
4.
CAUTION
Make sure lubricant does not contact any part of the sensor tip particularly the glass bulb.
5.
6.
7.
8.
36
Remove sensor from process before proceeding. The junction box with attached male
connector must be recovered from the old sensor for reuse. Unscrew the junction box
cover and set aside. Disconnect electrical connections from printed circuit board inside
junction box. Disconnect BNC connector to preamp. Unscrew hex nut (F) from male
connector body (E). Separate junction box from used sensor. Set aside.
Pry off split ferrule from sensor and set aside for reuse. Remove hex nut (F) and set aside
for reuse. Check that the internal O-ring is in place in the male connector body (E)
attached to .the junction box.
Remove hex nut (B) from male connector body (A) at process end of sensor and set aside.
Slide the Teflon ferrule and the male connector off sensor in the direction of junction box
and set.
Discard used O-ring from male connector body (A). Coat new O-ring with a thin film of the
O-ring lubricant provided. Position it in the machined O-ring groove in place of the
discarded O-ring.
Cover the 1" MNPT pipe threads of the male connector body (A) with Teflon tape (not
provided) to protect them from galling during reinstallation.
Pass the wires from the new sensor through the process end male connector (A). Make
sure that the beveled edge of the ferrule faces the process end of the sensor. Snug the hex
nut (B) to keep it in place. Do not tighten down fully on the hex nut at this time.
Pass the wires from the new sensor through the hex nut (F), the split ferrule (from the old
sensor), male connector body (E), O-ring, and through the junction box from the "neck"
opening and out to the printed circuit board in the junction box. Butt the ferrule's beveled
edge and the sensor tube against the junction male connector (E).
Screw the hex nut (F) by hand until the tube is "locked" into the male connector body.
Reference Manual
00809-0100-3098
Maintenance