Instrumentation test 1 Study

Determine the sensitivity of the instruments that gave the following readings: (a) Load kg 0 2 4 6 8 Deflection mm 0 18 36 54 72 (b) Temperature C 0 10 20 30 40 Voltage mV 0 0.59 1.19 1.80 2.42 (c) Load N 01234 Charge pC 0 3 6 9 12

(a) 9 mm/kg, (b) 0.060 mV/C, (c) 3 pC/N

A capacitive sensor consists of two parallel plates in air, the plates each having an area of 1000 mm2 and separated by a distance of 0.3 mm in air. Determine the displacement sensitivity of the arrangement if the dielectric constant for air is 1.0006.

- 29.7 pF/mm

An iron - constantan thermocouple has a cold junction at 0C and is to be used for the measurement of temperatures between 0C and 400C. What will be the non-linearity error at 100C, as a percentage of the full-scale reading, if a linear relationship is assumed over the full range? Tables give for this thermocouple: 0C, e.m.f. 0.000 mV; 100C, e.m.f. 5.268 mV; 400C, e.m.f. 21.846 mV.

-0.89%

. A Wheatstone bridge has a platinum resistance temperature sensor with a resistance of 120 Ω at 0C in one arm of the bridge. At this temperature the bridge is balanced with each of the other arms being 120 Ω. What will be the output voltage from the bridge for a change in temperature of 20C? The supply voltage to the bridge is 6.0 V and the temperature coefficient of resistance of the platinum is 0.0039 K^-1 .

0.059 V

An electrical resistance strain gauge has a resistance of 120 Ω and a gauge factor of 2.1. What will be the change in resistance of the gauge when it experiences a uniaxial strain of 0.0005 along its length?

0.126 Ω

A platinum resistance thermometer has a resistance of 100.00 Ω at 0C, 138.50 Ω at 100C and 175.83 Ω at 200C. What will be the non-linearity error at 100C if a linear relationship is assumed between 0C and 200C

0.13 Ω

These include changes in: A. Displacement B. Resistance C. Voltage D. Capacitance Select the form of output from above which is concerned with the following sensors: 1. A thermocouple which has an input of a temperature change. 2. A thermistor which has an input of a temperature change. 3. A diaphragm pressure cell which has an input of a change in the pressure difference between its two sides. 4. A LVDT which has an input of a change in displacement. 5. A strain gauge which has an input of a change in length. 6. A Bourdon gauge which has an input of a pressure change.

1. C. 2. B. 3. A. 4. C. 5. B. 6. A.

What will be the feedback resistance required for an inverting amplifier which is to have a voltage gain of 50 and an input resistance of 10 kΩ?

10 kΩ

A differential amplifier is to have a voltage gain of 100 and input resistances of 1 kΩ. What will be the feedback resistance required?

100 kΩ

A platinum resistance coil has a resistance at 0C of 100 Ω. Determine the change in resistance that will occur when the temperature rises to 30C if the temperature coefficient of resistance is 0.0039 K21 .

117 Ω

A chromel - constantan thermocouple has a cold junction at 20C. What will be the thermoelectric e.m.f. when the hot junction is at 200C? Tables give for this thermocouple: 0C, e.m.f. 0.000 mV; 20C, e.m.f. 1.192 mV; 200C, e.m.f. 13.419 mV.

12.227 mV

What is the voltage resolution of an 8-bit DAC when it has a full-scale input of 5 V?

19.6 mV

thermocouple. The input resistances are to be 1 kΩ. What value of feedback resistance is required if there is to be an output of 10 mV for a temperature difference between the thermocouple junctions of 100C with a copper-constantan thermo-couple. The thermocouple can be assumed to give an output of 43 μV/C?

2.33 kΩ

What is the resolution of an analogue-to-digital converter with a word length of 12 bits?

2.442 X 10^-4

A potentiometer with a uniform resistance per unit length of track is to have a track length of 100 mm and used with the output being measured with an instrument of resistance 10 kΩ. Determine the resistance required of the potentiometer if the maximum error is not to exceed 1% of the full-scale reading.

454 Ω

What will be the feedback resistance required for a non-inverting amplifier which is to have a voltage gain of 50 and an input resistance of 10 kΩ?

490 kΩ

Calibration of a voltmeter gave the following data. Determine the maximum hysteresis error as a percentage of the full-scale range. Increasing input: Standard mV 0 1.0 2.0 3.0 4.0 Voltmeter mV 0 1.0 1.9 2.9 4.0 Decreasing input: Standard mV 4.0 3.0 2.0 1.0 0 Voltmeter mV 4.0 3.0 2.1 1.1 0

5%

A diaphragm pressure gauge employs four strain gauges to monitor the displacement of the diaphragm. A differential pressure applied to the diaphragm results in two of the gauges on one side of the diaphragm being subject to a tensile strain of 1.0 X 10^-5 and the two on the other side a compressive strain of 1.0 X 10^-5 . The gauges have a gauge factor of 2.1 and resistance 120 Ω and are connected in the bridge with the gauges giving subject to the tensile strains in arms 1 and 3 and those subject to compressive strain in arms 2 and 4 (Figure 2.57). If the supply voltage for the bridge is 10 V, what will be the voltage output from the bridge?

5.25 X 10^-5 V

An operational amplifier circuit is required to produce an output that ranges from 0 to -5 V when the input goes from 0 to 100 mV. By what factor is the resistance in the feedback arm greater than that in the input?

50

. A capacitive sensor consists of two parallel plates in air, the plates being 50 mm square and separated by a distance of 1 mm. A sheet of dielectric material of thickness 1 mm and 50 mm square can slide between the plates. The dielectric constant of the material is 4 and that for air may be assumed to be 1. Determine the capacitance of the sensor when the sheet has been displaced so that only half of it is between the capacitor plates

55.3 pF

A sensor gives a maximum analogue output of 5 V. What word length is required for an analogue-to-digital converter if there is to be a resolution of 10 mV?

9

. Decide whether each of these statements is True (T) or False (F). Sensors in a measurement system have: i. An input of the variable being measured. ii. An output of a signal in a form suitable for further processing in the measurement system. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

A

Decide whether each of these statements is True (T) or False (F). A data presentation element which has an input which results in a pointer moving across a scale is an example of: i. An analogue form of display. ii. An indicator form of display. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

A

Decide whether each of these statements is True (T) or False (F). A float sensor for the determination of the level of water in a container is cylindrical with a mass 1.0 kg, cross-sectional area 20 cm2 and a length of 0.5 m. It floats vertically in the water and presses upwards against a beam attached to its upward end. i. The maximum force that can act on the beam is 9.8 N. ii. The minimum force that can act on the beam is 8.8 N. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

A

The resolution of an analogue-to-digital converter with a word length of 8 bits and an analogue signal input range of 10 V is: A. 39 mV B. 625 mV C. 1.25 V D. 5 V

A

The signal conditioner element in a measurement system: A. Gives an output signal dependent on the temperature. B. Changes the temperature signal to a current signal. C. Takes the output from the sensor and makes it bigger. D. Gives an output display.

A

Show that the output voltage for a Wheatstone bridge with a single strain gauge in one arm of the bridge and the other arms all having the same resistance as that of the unstrained strain gauge is 1 /4VsGε, where Vs is the supply voltage to the bridge, G the gauge factor of the strain gauge and ε the strain acting on the gauge.

As in the question

A copper-constantan thermocouple is to be used to measure temperatures between 0 and 200C. The e.m.f. at 0C is 0 mV, at 100C it is 4.277 mV and at 200C it is 9.286 mV. If a linear relationship is assumed between e.m.f. and temperature over the full range, the non-linearity error at 100C is: A. -3.9C B. - 7.9C C. + 3.9C D. + 7.9C

B

An incremental shaft encoder gives an output which is a direct measure of: A. The absolute angular position of a shaft. B. The change in angular rotation of a shaft. C. The diameter of the shaft. D. The change in diameter of the shaft.

B

Decide whether each of these statements is True (T) or False (F). A highly reliable measurement system is one where there is a high chance that the system will: i. Have a high mean time between failures. ii. Have a high probability of failure. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

B

Decide whether each of these statements is True (T) or False (F). A measurement system which has a lack of repeatability is one where there could be: i. Random fluctuations in the values given by repeated measurements of the same variable. ii. Fluctuations in the values obtained by repeating measurements over a number of samples. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

B

Decide whether each of these statements is True (T) or False (F). In selecting a temperature sensor for monitoring a rapidly changing temperature, it is vital that the sensor has: i. A small thermal capacity. ii. High linearity. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

B

Decide whether each of these statements is True (T) or False (F). The discrepancy between the measured value of the current in an electrical circuit and the value before the measurement system, an ammeter, was inserted in the circuit is bigger the larger: i. The resistance of the meter. ii. The resistance of the circuit. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

B

. A pressure sensor consisting of a diaphragm with strain gauges bonded to its surface has the following information in its specification: Range: 0 to 1000 kPa Non-linearity error: +/-0.15% of full range Hysteresis error: +/-0.05% of full range The total error due to non-linearity and hysteresis for a reading of 200 kPa is: A. +/-0.2 kPa B. +/-0.4 kPa C. +/-2 kPa D. +/- 4 kPa

C

A Wheatstone bridge when used as a signal processing element can have an input of a change in resistance and an output of: A. A bigger resistance change. B. A digital signal. C. A voltage. D. A current.

C

A sensor gives a maximum analogue output of 5 V. The word length is required for an analogue-to-digital converter if there is to be a resolution of 10 mV is: A. 500 bits B. 250 bits C. 9 bits D. 6 bits

C

Decide whether each of these statements is True (T) or False (F). A cold junction compensator circuit is used with a thermocouple if it has: i. No cold junction. ii. A cold junction at the ambient temperature. Which option BEST describes the two statements? A. (i) T (ii) T B. (i) T (ii) F C. (i) F (ii) T D. (i) F (ii) F

C

The water level in an open vessel is to be monitored by a diaphragm pressure cell responding to the difference in pressure between that at the base of the vessel and the atmosphere. The range of pressure differences across the diaphragm that the cell will have to respond to if the water level can vary between zero height above the cell measurement point and 1 m above it is (take the acceleration due to gravity to be 9.8 m/s^2 and the density of the water as 1000 kg/m3 ): A. 102 Pa B. 102 kPa C. 9800 Pa D. 9800 kPa

C

The change in resistance of an electrical resistance strain gauge with a gauge factor of 2.0 and resistance 50 Ω when subject to a strain of 0.001 is: A. 0.0001 Ω B. 0.001 Ω C. 0.01 Ω D. 0.1 Ω

D

Suggest the type of signal processing element that might be used to: a. Transform an input of a resistance change into a voltage. b. Transform an input of an analogue voltage into a digital signal.

For example: (a) Wheatstone bridge, (b) ADC

Suggest sensors which could be used in the following situations: a. To monitor the rate at which water flows along a pipe and given an electrical signal related to the flow rate. b. To monitor the pressure in a pressurised air pipe, giving a visual display of the pressure. c. To monitor the displacement of a rod and give a voltage output. d. To monitor a rapidly changing temperature.

For example: (a) turbine meter, (b) bourdon gauge, (c) LVDT, (d) thermocouple.

. The measurement instruments used in the tool room of a company are found to have a failure rate of 0.01 per year. What does this mean?

In 1 year, 1 in 100 will be found to fail.

The reliability of a measurement system is said to be 0.6. What does this mean?

Only give required accuracy 6 times in 10

Explain what is meant by 'fitness for purpose' when applied to a measurement system.

See Section 1.6.

Explain what is meant by calibration standards having to be traceable to national standards.

See Section 1.6.3.

Explain how a Wheatstone bridge incorporating a metal resistance element can be used to compensate for when the cold junction is at the ambient temperature rather than 0C and determine the parameters for the bridge if a nickel resistance element is used with a resistance of 10 Ω at 0C and a temperature coefficient of resistance of 0.0067 K^-1 and the bridge voltage supply is 2 V.

See Section 2.9.3, R2 5 19.9 Ω

Explain the terms (a) reliability and (b) repeatability when applied to a measurement system.

See sections 1.4 and 1.3.3. Reliability: the probability that a system will operate to an agreed level of performance for a specified period, subject to specified environmental conditions. Repeatability: the ability of a system to give the same value for repeated measurements of the same variable.

List and explain the functional elements of a measurement system.

Sensor, signal processor, data presentation, see Section 1.2.