Understanding Measurement Systems: Components, Variables, and Errors, Study notes of Physics

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Chapter 1 - Basic Concepts

Measurement System Components

Sensor - Transducer

Signal-conditioning

Output

Feedback-control

Example:

The thermocouple produces a voltage which is proportional to a temperature difference. The A/D converter converts this voltage to a digital value which is fed to the computer. The program converts this voltage to the equivalent temperature. The value may be plotted on the screen, or printed out, or stored.

Sensor/ Transducer

Thermo- couple

Signal Condition- ing (amp, filter…)

A-to-D Converte r

LabView Control

LabView Processin g

Signal Conditioning Output

Extraneous Variable - a variable which is not controlled during a measurement. Ambient temperature is frequently an extraneous variable.

What is the extraneous variable?

Noise - a random variation in the value of the measured signal to a variations in extraneous variables.

Interference - a deterministic variation of the measured signal to extraneous variables. 60 Hz signals from power sources frequently cause interference.

Minimizing the Effect of Extraneous Variables

To find the relationship between a dependent and one independent variable:

1.Hold all other independent variables constant

2.Vary the independent variable in a random order.

3.Replicate the test several times

4.Where possible, check result using a different method

Unfortunately, we frequently do not know what the extraneous variables are! Nevertheless, the above procedure will minimize their effect.

In order to determine the characteristics of the instrument, and to estimate its accuracy it is necessary to perform a calibration. This consists of applying known values of the independent variable and observing the output.

Calibration

Two Types:

1. Static - steady state.
2. Dynamic - during changing input.

The calibration should be performed as nearly as possible to the actual measurement conditions, and should follow the procedures for minimizing the effect of extraneous variables.

Accuracy - how well does the instrument indicate the "true value" of the input? Instrument errors can be classified as either:

Precision errors - random fluctuations in output for repeated applications of the same input.

Bias Errors - consistent inaccuracies in output for the same input.

Types of Instrument Error

TABLE 1.1 Manufacturer's Specifications: Typical Pressure Transducer

Operation

Input Range 0 tp 1000 cm H 20

Excitation ±15% V dc

Output range 0 to 5 V

Performance

Linearity error ±0.5% full scale

Hysteresis error Less than ±0.15% full scale

Sensitivity error ±0.25% of reading

Thermal sensitivity error

±0.02%1oC of reading

Thermal zero drift 0.02%1^0 C full scale

Temperature range 0 to 50 0 C

Standards

Primary Standards - define the size of a unit

Interlaboratory Transfer Standards - maintained by national laboratories such as the US National Institute for Science and Technology

Local Standards - maintained by companies and individual laboratories

Working Instruments - our laboratory thermometers.

TABLE 1.2 Standard Dimensions and Units Unit

Dimension SI US

Primary

Length meter (m) foot (ft)

Mass kilogram (kg) pound-mass (lbm)

Time second (s) second (s)

Force Newton (N) pound-force (lb)

Temperature Kelvin (K) Rankine (R)

Derived

Voltage volt (V) volt (V)

Current ampere (A) ampere (A)

Resistance ohm ( Ω ) ohm ( Ω )

Capacitance farad (F) farad (F)

Inductance henry (H) henry (H)

Pressure pascal (Pa) pound/foot (psf)

Energy joule (J) British thermal unit (BTU)

Power watt (W) foot-pound (ft-lb)