1. Performance characteristics for measurement and instrumentation system
Lecture 3
Performance characteristics for measurement and instrumentation system

2. 3 classifications to define the performance of measurement system: operational, static, dynamic
Operational characteristic
Range
Span
Sensitivity
Resolution
Dead band/threshold

3. Range
-will give the minimum and maximum range
Span
-the difference between the maximum and minimum range
Eg: A specification of a thermometer reads as follows:
Range and subdivision oC to 0.1
Min is –0.50C and max is 40.50C
Span = 410C

4. Sensitivity
-ratio of a change in output to the change in input which causes it at steady-states condition
-Eg. A galvanometer has a sensitivity of 17mm/A
-for a 1A input display, a light spot moving across the scale shows a movement of an index of 17 mm

5. Resolution
The least incremental value of input or output that can be detected
Dead band / Threshold
The largest range of values of a measured variable to which the instrument does not respond

6. RULERS
Normal - straight
Folding ruler

7. VERNIER CALIPERS

8. Digital vernier caliper

9. HOW TO READ A VERNIER Scale reading = 3.70cm
Vernier reading = (0.1/10) x 4 = 0.04
Caliper reading = 3.74cm

10. SCREW GAUGE

11. Static characteristic:
Error
Accuracy
Precision
Repeatability
Linearity
Hysteresis
calibration

12. Error
-Error is the difference between the true value Yn and instrument reading Xn
e = Yn – Xn
-Types of errors: systematic error (bias error) and random error (precision error)
-Bias (systemic) error-consistent and repeatable error
Bias error = average readings – true value

13. -Random error-the lack of repeatability in the output of the measuring system
Random error = reading – average reading

14. Accuracy
-Ability of the system to respond to a true value
-Limit of error of a measuring device under certain operating conditions and can appear is several forms:
1. Measured variable: the accuracy is 0.2 of the measurement
-If the temperature reading = 30.10C, the actual temperature lies between 29.90C and 30.30C
2. Percentage of full scale (FS), accuracy 1% f.s
-If the full scale is 10 A, accuracy = 0.1 A

15. 3. Percentage of instrument span, accuracy 3% span
-If the span for pressure measurement is psi,
accuracy = 0.03 (50-20) = 0.9 psi
4. Given as percentage of actual reading, e.g. for a 2%
-If the true value of the voltmeter is 2 V, accuracy = (2 0.02) = 0.04 V

16. Example 1.1
A temperature sensor has a span of 200C –2500C. A measurement results in a value of 550C for temperature. Specify the error if the accuracy is:
a) 0.5%FS
b) 0.75% of span
c) 0.8% of reading
What is the possible temperature in each case

17. Example 1.2
A temperature sensor has a transfer function of 5 mV/0C with an accuracy of 1%. Find the possible range of the transfer function?

18. Example 1.3
Suppose a reading of 27.5 mV results from the sensor used in previous example. Find the temperature that could provide this reading.

19. System accuracy
Overall accuracy of many elements in a process-control loop to represent a process variable
V V = (K K) (G G)C
V = output voltage
V = uncertainty in output voltage
K,G = nominal transfer functions
K, G = uncertainty in transfer functions
C = dynamic variable

20. K K
G G
V V C
The overall system accuracy as the root-mean-square (rms):

21. Example 1.4
Find the system accuracy of a flow process if the transducer transfer function is 10 mV/(m3/s) ±1.5% and the signal-conditioning system-transfer function is 1mA/mV ±0.05%

22. Precision
-The degree of exactness of which an instrument is designed or intended to performed
-Significant figures convey actual information regarding the magnitude and the measurement precision of a quantity
-The more the significant the figure, the greater the precision of measurement

24. Repeatability
-The ability of the system to display the same output for a series of applications of the same input signal, under the same operating conditions
Linearity
-The output reading of the measurement is linearly proportional to the quantity being measured

25. Hysteresis
-different reading may be obtained if the variable was increasing prior to taking the reading if the variable was decreasing
-causes: friction, mechanical flexure of internal part, electrical capacitance

27. Calibration
-process of checking a measuring system against a standard reading
-purposes:
To ensure readings from an instrument are consistent with other measurements
To determine the accuracy of the instrument readings
To establish the reliability of the instrument i.e. that it can be trusted