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4.4.4 Thermal Electromotive Force (EMF) Compensation
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Input Setup
Q
Input
(A, B, C, or D)
Q
Autorange
Q
(Off or On)
Input Setup
Q
Input
(A, B, C, or D)
Q
Range
Q
(See table below)
Default: On
Interface Command:
INTYPE
Range and sensor power
Sensor Type
Sensor
Excitation
Available
Range
Settings
Maximum Sensor Power
Current Excitation
PTC RTD (Platinum)
n/a
10
)
10 µW
1 mA
30
)
30 µW
300 µA
100
)
100 µW
100 µA
300
)
300 µW
30 µA
1 k
)
1 mW
10 µA
3 k
)
3 mW
3 µA
10 k
)
10 mW
1 µA
NTC RTD (Cernox)
10 mV
10
)
10 µW
1 mA
30
)
2.7 µW
300 µA
100
)
1 µW
100 µA
300
)
270 nW
30 µA
1 k
)
100 nW
10 µA
3 k
)
27 nW
3 µA
10 k
)
10 nW
1 µA
30 k
)
2.7 nW
300 nA
100 k
)
1 nW
100 nA
300 k
)
270 fW
30 nA
1 mV
10
)
100 nW
100 µA
30
)
27 nW
30 µA
100
)
10 nW
10 µA
300
)
2.7 nW
3 µA
1 k
)
1 nW
1 µA
3 k
)
270 fW
300 nA
4.4.4 Thermal
Electromotive Force
(EMF) Compensation
To keep power low and avoid sensor self heating, the sensor excitation is kept low.
There are two major problems that occur when measuring the resulting small DC
voltages. The first is external noise entering the measurement through the sensor
leads, which is discussed with sensor setup. The second is the presence of thermal
EMF voltages, or thermocouple voltages, in the lead wiring. Thermal EMF voltages
appear when there is a temperature gradient across a piece of voltage lead. Thermal
EMF voltages must exist because the sensor is almost never the same temperature as
the instrument. To minimize them, use careful wiring, make sure the voltage leads are
symmetrical in the type of metal used and how they are joined, and keep unnecessary
heat sources away from the leads. Even in a well-designed system thermal EMF volt-
ages can be an appreciable part of a low voltage sensor measurement.
The Model 350 can help with a thermal compensation algorithm. The instrument will
automatically reverse the polarity of the current source every other reading. The
average of the positive and negative sensor readings will cancel the thermal EMF
voltage that is present in the same polarity, regardless of current direction. This cor-
rection algorithm is enabled by default for RTD sensor types, but can be turned off
using the Current Reversal parameter.
TABLE 4-8
Summary of Contents for 350
Page 4: ...Model 350 Temperature Controller ...
Page 6: ......
Page 14: ...Model 350 Temperature Controller ...
Page 28: ...14 cHAPTER 1 Introduction Model 350 Temperature Controller ...
Page 53: ...Examples of PID control 2 13 4 ManualOutput 39 www lakeshore com FIGURE 2 4 ...
Page 58: ...44 cHAPTER 2 Cooling System Design and Temperature Control Model 350 Temperature Controller ...
Page 118: ...104 cHAPTER 5 Advanced Operation Model 350 Temperature Controller ...
Page 178: ...164 cHAPTER 7 Options and Accessories Model 350 Temperature Controller ...
Page 192: ...178 cHAPTER 8 Service Model 350 Temperature Controller ...