60
Reference Manual
00809-0200-4728, Rev RA
Section 4: Operation and Maintenance
April 2015
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-wire, 3-wire or 4-wire. If
the transmitter is mounted remotely from a 3-wire or 4-wire RTD, it will operate within specifi-
cations, 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.-lbs (0.73 N-m).
5.
Reattach and tighten the cover (if applicable).
6.
Apply power (12 – 42 Vdc).
Summary of Contents for Rosemount 644
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