P
AGE
11
2.5.2 SENSOR PLACEMENT (Thermocouple or RTD)
If the temperature probe is to be sufnected to corrosive or abrasive conditions, it should be
protected by the appropriate thermowell. The probe should be positioned 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 follow-
ing 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
TABLE 2
Temperature Error in
°
F per 1000 feet of Leadwire
AWG
Thermocouple Type:
No.
J
K
T
R
S
E
B
N
C
10
.61
1.54
.69
1.84
1.91
1.04
12.60
2.65
2.27
12
.97
2.41
1.09
2.97
2.96
1.64
19.80
4.21
3.66
14
1.57
3.86
1.75
4.81
4.76
2.63
31.50
6.69
5.74
16
2.47
6.09
2.77
7.47
7.52
4.14
49.95
10.64
9.10
18
4.00
9.90
4.50
12.17
12.28
6.72
79.95
10.64
9.10
20
6.43
15.51
7.06
19.43
19.59
10.61
126.90
26.89
23.24
24
15.80
39.44
17.83
48.89
49.13
26.70
320.85
68.03
58.75
Example:
An MIC is to be located in a control room 660 feet away from the process. Using 16 AWG,
type J thermocouple, how much error is induced?
Terr = TLe * L
TLe = 2.47 (
°
F/1000 ft) from Table 2
Terr = 2.47 (
°
F/1000 ft) * 660 ft
Terr = 1.6
°
F