Agilent Technologies 34970A manual Measurement Fundamentals

Chapter 8 Tutorial

Measurement Fundamentals

RTD Measurements An RTD is constructed of a metal (typically platinum) that changes resistance with a change in temperature in a precisely known way. The internal DMM measures the resistance of the RTD and then calculates the equivalent temperature.

An RTD has the highest stability of the temperature transducers.

The output from an RTD is also very linear. This makes an RTD a good choice for high-accuracy, long-term measurements. The 34970A supports RTDs with α = 0.00385 (DIN / IEC 751) using ITS-90 software conversions and α = 0.00391 using IPTS-68 software conversions. “PT100” is a special label that is sometimes used to refer to an RTD with α = 0.00385 and R0 = 100Ω .

The resistance of an RTD is nominal at 0 °C and is referred to as R0. The 34970A can measure RTDs with R0 values from 49Ω to 2.1 kΩ .

You can measure RTDs using a 2-wire or 4-wire measurement method. The 4-wire method (with offset compensation) provides the most accurate way to measure small resistances. Connection lead resistance is automatically removed using the 4-wire method.

Thermistor Measurements A thermistor is constructed of materials that non-linearly changes resistance with changes in temperature. The internal DMM measures the resistance of the thermistor and then calculates the equivalent temperature.

Thermistors have a higher sensitivity than thermocouples or RTDs. This makes a thermistor a good choice when measuring very small changes in temperature. Thermistors are, however, very non-linear, especially at high temperatures and function best below 100 °C.

Because of their high resistance, thermistors can be measured using a 2-wire measurement method. The internal DMM supports

2.2 kΩ (44004), 5 kΩ (44007), and 10 kΩ (44006) thermistors.

The thermistor conversion routines used by the 34970A are compatible with the International Temperature Scale of 1990 (ITS-90).

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