Reference Manual
00809-0100-4825, Rev BB
August 2005
2-11
Rosemount 248
Examples of Approximate Lead Wire Resistance Effect Calculations
Given:
•
Pt100 4-wire RTD: No lead wire resistance effect.
•
Pt100 3-wire RTD:
Lead wire imbalance seen by the transmitter = 0.5
Ω
•
Pt100 2-wire RTD:
Lead wire resistance seen by the transmitter = 150 m × 2 wires ×
0.025
Ω
/m = 7.5
Ω
POWER SUPPLY
To communicate with a transmitter, an 18.1 V dc minimum power supply is
required. The power supplied to the transmitter should not drop below the
transmitter lift-off voltage (see Figure 2-8). If the power drops below the lift-off
voltage while the transmitter is being configured, the transmitter may interpret
the configuration information incorrectly.
The dc power supply should provide power with less than 2 percent ripple.
The total resistance load is the sum of the resistance of the signal leads and
the load resistance of any controller, indicator, or related pieces of equipment
in the loop. Note that the resistance of intrinsic safety barriers, if used, must
be included.
Total cable length:
150 m
Imbalance of the lead wires at 20 °C:
0.5
Ω
Resistance/length (18 AWG Cu):
0.025
Ω
/m °C
Temperature coefficient of Cu (
α
Cu
):
0.039
Ω
/
Ω
°C
Temperature coefficient of Pt(
α
Pt
):
0.00385
Ω
/
Ω
°C
Change in Ambient Temperature (
Δ
T
amb
):
25 °C
RTD Resistance at 0 °C (R
o
):
100
Ω
(for Pt 100 RTD)
Basic Error
Imbalance of Lead Wires
α
Pt
R
o
×
(
)
------------------------------------------------------------------
=
Error due to amb. temp. variation
α
Cu
(
)
Δ
T
amb
(
)
×
Imbalance of Lead Wires
(
)
×
α
Pt
(
)
R
o
(
)
×
-------------------------------------------------------------------------------------------------------------------------
=
Basic error
0.5
Ω
0.00385
Ω
/
Ω
°
C
(
)
100
Ω
(
)
×
----------------------------------------------------------------------------------
1.3
°
C
=
=
Error due to amb. temp. var. of
25 °C
±
0.0039
Ω
/
Ω
°
C
(
)
25
°
C
(
)
×
0.5
Ω
(
)
×
0.00385
Ω
/
Ω
°
C
(
)
100
Ω
(
)
×
-------------------------------------------------------------------------------------------------------
0.13
°
·
C
±
=
=
Basic Error
Lead Wire Resistance
α
Pt
R
o
×
(
)
----------------------------------------------------------
=
Error due to amb. temp. variation
α
Cu
(
)
Δ
T
amb
(
)
×
Lead Wire Resistance
(
)
×
α
Pt
(
)
R
o
(
)
×
-----------------------------------------------------------------------------------------------------------------
=
Basic error
7.5
Ω
0.00385
Ω
/
Ω
°
C
(
)
100
Ω
(
)
×
----------------------------------------------------------------------------------
19.5
°
C
=
=
Error due to amb. temp. var. of
25 °C
±
0.0039
Ω
/
Ω
°
C
(
)
25
°
C
(
)
×
7.5
Ω
(
)
×
0.00385
Ω
/
Ω
°
C
(
)
100
Ω
(
)
×
-------------------------------------------------------------------------------------------------------
1.9
°
C
±
=
=
Summary of Contents for 248
Page 2: ......
Page 4: ......
Page 46: ...Reference Manual 00809 0100 4825 Rev BB August 2005 Rosemount 248 4 8 ...
Page 64: ...Reference Manual 00809 0100 4825 Rev BB August 2005 Rosemount 248 A 18 ...
Page 74: ...Reference Manual 00809 0100 4825 Rev BB August 2005 Rosemount 248 B 10 ...
Page 77: ...Reference Manual 00809 0100 4825 Rev BB August 2005 0 1 Rosemount 248 NOTES ...
Page 78: ...Reference Manual 00809 0100 4825 Rev BB August 2005 Rosemount 248 0 2 NOTES ...
Page 79: ...Reference Manual 00809 0100 4825 Rev BB August 2005 0 3 Rosemount 248 NOTES ...
