P
AGE
10
FIGURE 2-3
2.2.2 SENSOR PLACEMENT (Thermocouple or RTD)
Two wire RTD's should be used only with lead lengths less then 10 feet.
If the temperature probe is to be subjected to corrosive or abrasive conditions, it should be
protected by the appropriate thermowell. The probe should be positoned to reflect true
process temperature:
In liquid media - the most agitated area.
In air - the best circulated area.
THERMOCOUPLE LEAD RESISTANCE
Thermocouple lead length can affect instrument accuracy since the size (gauge) and the
length of the wire affect lead resistance.
To determine the temperature error resulting from the lead length resistance, use the following
equation:
Terr = TLe * L
where;
TLe = value from appropriate table below
L = length of leadwire in thousands of feet
TABLE 1
Temperature error in
°
C per 1000 feet of Leadwire
AWG
Thermocouple Type:
No.
J
K
T
R
S
E
B
N
C
10
.34
.85
.38
1.02
1.06
.58
7.00
1.47
1.26
12
.54
1.34
.61
1.65
1.65
.91
11.00
2.34
2.03
14
.87
2.15
.97
2.67
2.65
1.46
17.50
3.72
3.19
16
1.37
3.38
1.54
4.15
4.18
2.30
27.75
5.91
5.05
18
2.22
5.50
2.50
6.76
6.82
3.73
44.25
9.40
8.13
20
3.57
8.62
3.92
10.80
10.88
5.89
70.50
14.94
12.91
24
8.78
21.91
9.91
27.16
27.29
14.83
178.25
37.80
32.64
Inductive
Load
R
C
MOV
