Emerson Rosemount 644 with HART Protocol, Reference Manual

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Reference Manual
00809-0200-4728, Rev SA
Operation and Maintenance
July 2018
Operation and Maintenance
RTD or Ohm inputs
The transmitters will accept a variety of RTD configurations, including 2-, 3- or 4-wire. If the transmitter
is mounted remotely from a 3- or 4-wire RTD, it will operate within specifications, without recalibration,
for lead wire resistances of up to 60 ohms per lead (equivalent to 6,000 feet of 20 AWG wire). In this case,
the leads between the RTD and transmitter should be shielded. If using only two leads, both RTD leads
are in series with the sensor element, so significant errors can occur if the lead lengths exceed three feet
of 20 AWG wire (approximately 0.05 °C/ft). For longer runs, attach a third or fourth lead as described
above.
Sensor lead wire resistance effect– RTD input
When using a 4-wire RTD, the effect of lead resistance is eliminated and has no impact on accuracy.
However, a 3-wire sensor will not fully cancel lead resistance error because it cannot compensate for
imbalances in resistance between the lead wires. Using the same type of wire on all three lead wires will
make a 3-wire RTD installation as accurate as possible. A 2-wire sensor will produce the largest error
because it directly adds the lead wire resistance to the sensor resistance. For 2- and 3-wire RTDs, an
additional lead wire resistance error is induced with ambient temperature variations. The table and the
examples shown below help quantify these errors.
Note
For HART transmitters, the use of two grounded thermocouples with a dual option Rosemount 644
Transmitter is not recommended. For applications in which the use of two thermocouples is desired,
connect either two ungrounded thermocouples, one grounded and one ungrounded thermocouple, or
one dual element thermocouple.
4.3.2 Power the transmitter
1. An external power supply is required to operate the transmitter.
2. Remove the housing cover (if applicable).
3. Connect the positive power lead to the “+” terminal. Connect the negative power lead to the “–”
terminal.
– If a transient protector is being used, the power leads will now be connected to the top of the
transient protector unit. See the transient label for indication of “+” and “-” terminal connections.
4. Tighten the terminal screws. When tightening the sensor and power wires, the max torque is 6.5 in-lb
(0.73 N-m).
5. Reattach and tighten the cover (if applicable).
6. Apply power (12–42 Vdc).