Omega Vehicle Security OMP-MODL - Manual (Page 65)

3... I NTERFACE M ODULES
U SING THE OMP-MODL 3-41
additive lead wire resistance can be ignored (eg in thermistor or Kohm resistance
measurements). However, in applications of RTDs or lower resistance ranges this
lead wire resistance can add up to substantial measurement errors... especially if
long runs or lighter gauge lead wire is used. For example, in a 100 ohm RTD, 0.4
ohms of lead wire resistance would translates to a reading error of 1 Deg C.
To minimize these lead wire induced errors, the MLIM-4 supports 3-wire and 4-wire
connection methods. Connection diagrams and descriptions for each of the wiring
methods follow.
2-Wire Configuration
The 2-wire configuration is easiest to use and allows for utilization of all four
input channels of the MLIM-4 as individual channels. All three input types,
RTD, thermistor, and resistance can be measured with the 2-wire technique.
For short runs, heavier gauge lead wires and/or higher resistance
measurements, the 2-wire technique will provide excellent performance with
minimal error.
Calculating Lead Wire Effects
To calculate resistance errors induced by lead wires in a 2-wire
configuration:
1. Estimate the total length of the lead wire to be used.
2. Multiply this length by the resistance per foot of the wire
to be used. Complete wire tables are available from wire
manufacturers and in many electronic reference books.
For general reference, an abbreviated table is included
below.
Note that wire resistances are typically given per 1000
foot.
3. Assess the effects of this resistance on the required
accuracy. For RTD applications, tables are available
from the manufacturer that correlate RTD element
resistance to degrees over the usable range. As a
general guideline, a 100 ohm RTD will have a 1 Degree
C change for every 0.36 ohms, a 1000 ohm RTD will
have a 1 degree C change for every 3.6 ohms (hence the
increasing popularity of the 1000 ohm RTD).
Wire Gauge ohms per 1000 ft
@ 25C (77F)
ohms per 1000 ft
@ 65C (149F)
26 41.6 48
24 26.2 30.2
22 16.5 19.0
20 10.4 11.9
18 6.5 7.5
16 4.1 4.7
Table 5: Typical Copper Wire resistance
2-Wire
Config