IntroductionThis is an introduction to some of the physical principles that underly sensors in instrument systems. It is not intended to be definitive, or very detailed, but to give the reader an idea of what is readily achievable with the various systems. I'd welcome corrections and suggestions for improvements.This HTML document supported modulePHY3128.ResistanceElectrical resistance is the easiest electrical property to measure precisely over a wide range at moderate cost. A simple digital multimeter costing a few tens of dollars can measure resistances in the range 10 ohm to 10 megohm with a precision of about 1% using a two-wire technique (circuit1).

Circuit 1. Two-wire resistance measurement,RX= (V/I)RL1RL2.

The precision of the two-wire method is limited by uncertainties in the values of the lead resistances RL1 and RL2.

Circuits 2 & 3. Three-wire resistance measurement methods.

Providing the leads are well-matched, three-wire techniques can be used. Circuit2 employs two matched current sources, I1 and I2, to eliminate the effects of lead resistance providing RL1=RL2. Circuit3 is an AC-bridge that is in-balance when RX=RY providing RL1=RL3. If alock-in amplifieris used as a null-detector, determination of RX with an extremely low excitation current is possible.

Circuit 4. Four-wire 'Kelvin' resistance measurement,RX=V/I.

The 4-Wire 'Kelvin' method (circuit4) is used in difficult cases when lead resistances vary, RX is very small, or when very high accuracy is required. The method is immune to the influence of lead resistance and is limited by the quality of the constant current source and voltage measurement. Thermoelectric voltages can be eliminated by averaging two measurements with the polarity of the excitation current reversed.See also: AC Resistance Bridge. Direct Current & Low Frequency Masurements(National Physical Laboratory) Application Note 43 - Bridge Circuits(Linear Technology, 1990)Resistive Temperature DetectorsResistance Temperature Detectors (RTD) exploit the fact that the electrical resistivity of metals and alloys varies in a reproducible way with temperature. Platinum, with a temperature coefficient of about 0.0039 K1, is the most popular material used in this application. An RTD consists of a coil of wire, or a thin-film, with four-wire electrical connections supported in a way that is a compromise between robustness and thermal time-constant. RTDs have excellent accuracy (e.g.0.025K at room temperature) over a wide temperature range. At cryogenic temperatures the resistance of metals becomes constant, and it is usual to use a sample of doped-semiconductor as the sensing element. When using RTDs, it is always important to check that the measured resistance is independent of excitation current in order to avoid errors caused by self-heating.See also: Measuring Temperature with RTDs - A Tutorial(National Instruments) Germanium Resistance Thermometersfor use at cryogenic temperaturesStrain GaugesAt constant temperature, the resistanceRof a metal or semiconductor element of areaA,lengthl,resistivity,is