User's Manual

9
Reference Manual
00809-0300-4248, Rev AA
Section 2: Configuration
July 2014
Configuration
2.3 Sensor connections
The 248 Wireless is compatible with a number of RTD and thermocouple sensor types.
Figure 2-1 shows the correct input connections to the sensor terminals on the transmitter. To
ensure a proper sensor connection, anchor the sensor lead wires into the appropriate
compression terminals and tighten the screws.
Thermocouple or Millivolts inputs
The thermocouple can be connected directly to the transmitter. Use appropriate thermocouple
extension wire if mounting the transmitter remotely from the sensor.
RTD or Ohm inputs
The transmitters will accept a variety of RTD or ohmic configurations, including 2-wire, 3-wire or
4-wire connections. If the transmitter is mounted remotely from a 3-wire or 4-wire RTD, it will
operate within specifications, without recalibration, for lead wire resistances of up to 5 ohms
per lead (equivalent to 500 feet of 20 AWG wire). In this case, the leads between the RTD and
transmitter should be shielded. If using a 2-wire connection, 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 to achieve a
3-wire or 4-wire connection as described above.
Effect-RTD input
Since the lead wires are part of the RTD circuit, the lead wire resistance needs to be
compensated for to achieve the best accuracy. This becomes especially critical in applications
where long sensor and/or lead wires are used. There are three lead wire configurations
commonly available. In a two-wire configuration there can be no compensation for lead wire
resistance since the lead wires are in series with the element and appear to the transmitter as
part of the sensor's resistance causing inherent accuracy degradation. In a three-wire
configuration, compensation is accomplished using the third wire with the assumption that it
will be the same resistance as the other two wires and the same compensation is applied to all
three wires. A four-wire design is ideal because the lead wire resistance is inconsequential to the
measurement. It uses a measurement technique where a very small constant current of about
150 micro amps is applied to the sensor through two leads and the voltage developed across
the sensor is measured over the other two wires with a high-impedance and high resolution
measuring circuit. In accordance with Ohm's Law, the high impedance virtually eliminates any
current flow in the voltage measurement leads and therefore the resistance of the leads is not a
factor.
Table 2-1. Examples of Approximate Basic Error
Sensor input Approximate basic error
4-wire RTD Negligible
(1)
(1) Independent of lead wire resistance up to 5  per lead.
3-wire RTD Error in reading is equivalent to unbalanced lead wire resistance
(2)
(2) Unbalanced lead wire resistance is the maximum resistance differences between any two leads.
2-wire RTD Error in reading is equivalent to total lead wire resistance