Fluke 45 user manual Thermal Voltages

45

Users Manual

Table 4-4. Typical Measurement Intervals (in Seconds) for Dual Display Measurements

Meas.

Range

All AC or All DC Type

Slow Med Fast

AC & DC Types Mixed

Slow Med Fast

V

W

A

B

e

GR

300mV

3V

30V

300V

1000V

All

All

All

300Ω

3kΩ

30 kΩ

300kΩ

3MΩ

30 MΩ

300 MΩ

N/A

0.70

0.70

0.80

0.80

0.80

1.80

0.70

1.50

0.70

0.80

0.90

1.40

1.30

1.90

2.10

0.70

0.50

0.50

0.60

0.60

0.60

1.60

0.50

1.30

0.50

0.60

0.70

1.20

1.10

1.70

1.90

0.50

0.10

0.10

0.10

0.10

0.10

0.50

0.10

0.50

0.10

0.10

0.10

0.10

0.10

0.10

0.10

0.10

1.10

1.00

1.10

1.00

1.00

2.30

090

1.80

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

0.90

0.80

0.90

0.80

0.80

2.10

0.70

1.60

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

0.10

0.10

0.10

0.10

0.10

0.50

0.10

0.50

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

FREQ* N/A 0.50 0.50 0.30 0.70 0.70

* Frequency greater than 150 Hz. Refer to frequency measurement rates in Table 3-6.

0.30

Thermal Voltages

Thermal voltages are the thermovoltaic potentials generated at the junction between dissimilar metals. Thermal voltages typically occur at binding posts and can be greater than 1

∝V. When making low-level tic measurements, thermal voltages can present an additional source of error.

Thermal voltages can also cause problems in the low ohms ranges. Some low-value resistors are constructed with dissimilar metals. Just handling such resistors can cause thermal voltages large enough to introduce measurement errors.

Use the following techniques to reduce the effect of thermal voltages:

1.Use similar metals for connections wherever possible (e.g., copper-to-copper, gold- to-gold, etc.).

2.Use tight connections.

3.Use clean connections (especially free of grease and dirt).

4.Use caution when handling the circuit under test.

5.Wait for the circuit to reach thermal equilibrium. (Thermal voltages are generated only where there is a temperature gradient.)

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