Emerson Rosemount 644 Reference Manual page 76

Electrical Installation
March 2021
Thermocouple or millivolt inputs
The thermocouple can be connected directly to the transmitter. Use appropriate
thermocouple extension wire if mounting the transmitter remotely from the sensor. Make
millivolt inputs connections with copper wire. Use shielding for long runs of wire.
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
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
Procedure
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
4. Tighten the terminal screws. When tightening the sensor and power wires, the
5. Reattach and tighten the cover (if applicable).
6. Apply power (12–42 Vdc).
76
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.
maximum torque is 6.5 in-lb (0.73 N-m).
Reference Manual
00809-0200-4728
Emerson.com/Rosemount