Instrumentation: Measurement System

What is a measurement ?

A measurement is an act of assigning a specific value to a physical variable.

What is a measurement System?

A measurement system is a tool used for qualifying the measured variable.

What is a system?

A system is composed of components that work together to accomplish a specific objective.

Control Parameter

Control Parameter: A parameter has an effect on the behavior of the measured variable.

Error

Define an error as the difference between the measured value and the true value.

Dependent Variable

Dependent Variable: A variable that is affected by changes in one or more other variables

Parameter Design Plan Steps?

Determine the test objective Identify the process variables and parameters and a means for their control.

What are the process involved in experimental test plan?

Experimental design involves itself with developing a measured test plan. 1. Parameter Design Plan 2. System & Tolerance Design Plan 3. Data Reduction Design Plan

Extraneous Variables

Extraneous Variables: Variables that are not or cannot be controlled during measurement but that affect the value of the variable measured

True Value:

The exact value of a variable

Hysteresis

Hysteresis: i)A sequential test is an effective diagnostic technique for identifying & quantifying hysteresis error in a measurement system. ii)Hysteresis occurs when the output of a measurement system is dependent on the previous value indicated by the system. iii)Hysteresis is normal for any system & affects the repeatability of the system.

Static Calibration

1. In the static calibration procedure, a known value is input to the system under calibration & the system output is recorded. 2. The values of the variables involved remain constant. They do not vary with time & space 3. In static calibrations, only the magnitudes of the known input & the measured output are important.

Calibration process

1.Static Calibration 2.Dynamic Calibration 3.Static Sensitivity 4.Range 5.Resolution 6.Accuracy & Error

Dynamic Calibration

1.When the variables of interest are time or space dependent. Dynamic variables are time or space dependent in both their magnitude & frequency content. 2.A dynamic calibration determines the relationship between an input of known dynamic behavior & the measurement system output. 3.Dynamic calibrations involve applying either a sinusoidal signal or a step change as the known input signal.

Independent Variable?

Independent Variable: A variable that can be changed independently of other variable.

Interference

Interference:Any uncontrolled influence that causes the signal or test outcome to behave in a manner different from its true behavior. Interference imposes undesirable deterministic trends on the measured value.

Noise:

Noise:is a random variation of the value of the measured signal as a consequence of the variation of the extraneous variables. Noise increases data scatter.

Parameter

Parameter: A parameter is a functional grouping of variables.

Data Reduction Design Plan Steps?

Plan how to analyse, present and use the anticipated data

Random Error

Random Error causes a random variation in measured values found during repeated measurements of a variable.

Random Test

Random Test:A measurement matrix sets a random order to the change in the value of the independent variable.

Range:

Range: A calibration applies known inputs ranging from the minimum to the maximum values for which the measurement system is to be used

Resolution

Resolution: represents the smallest increment in the measured value that can be discerned. It is quantified by the smallest scale increment or least count of the output readout indicator in a measurement system

System & Tolerance Design Plan steps

Select a measured technique, equipment & procedure based on some preconceived tolerance limit for error.

4 stages of General measurement System

Sensor stage, Signal Conditioning Stage (takes the transducer signal & modifies it to a desired magnitude), Output Stage, Feedback Control Stage(Contains a controller that interprets the measured signal & makes a decision regarding the control of the Process.)

Sequential Test

Sequential Test: applies a sequential variation in the input value over the desired input. This is accomplished by increasing (or decreasing) the input value over the full input range.

Static Sensitivity

Static Sensitivity: The slope of a static calibration curve provides the static sensitivity of the measurement system.

Systematic Error

Systematic Error causes an offset between the mean value of the data set and its true value.

Measured Value:

The value of a variable as indicated by a measurement system.

Uncertainty

Uncertainty is a numerical estimate of the possible range of the error in a measurement

Variables?

Variables: are entities that influence the test

Accuracy

refers to the closeness of agreement between the measured value & the true value.

Common Elements of Instrument Errors

•Hysteresis Error •Linearity Error •Sensitivity Error •Zero Shift Error (Null) •Repeatability Error