Omega CNI8, Manual

Page: 13 / 72

9
2.3.3 Two/Three/Four-Wire RTD
The figures below show the input connections and input connector jumpers
(shown in bold lines) required to hookup a 2-, 3- or 4-wire RTD.
Figure 2.6 a) RTD-1000 ohm and b) RTD-100 ohm Wiring Hookup
500 ohm Wiring Hookup
The two-wire connection is simplest method, but does not compensate for
lead-wire temperature change and often requires calibration to cancel lead-wire
resistance offset.
The three-wire connection works best with RTD leads closely equal in
resistance. The device measures the RTD, plus upper and lower lead drop
voltage and the subtracts twice the measured drop in the lower supply current
lead producing excellent lead-resistance cancellation for balanced
measurements.
The four-wire RTD hookup is applicable to unbalanced lead resistance and
enables the device to measure and subtract the lead voltage, which produces
the best lead-resistance cancellation.
When configuring your controller, select RTD type and RTD value in the
Input Type menu (see Part 3).
If the input wires of the meter get disconnected or broken, it will display
+OPN
“Input (+) Open” message except in case of 500/1000 Ω 2-wire
RTD. In this case the display shows
-OPN
“Input (-) Open” message. For
safety purpose you may want to set up your alarm to be triggered when
input is open. See Alarm 1 & 2 chapters for details.
RTD (1000/500
Ω
) 4-Wire
RTD (1000/500
Ω
) 3-Wire
RTD (1000/500
Ω
) 2-Wire
RTD (100
Ω
) 4-Wire
RTD (100
Ω
) 3-Wire
RTD (100
Ω
) 2-Wire