Input Devices: Test Your Knowledge

QUESTIONS

1. Illustrate the difference between an on-off (digital) signal and an analog signal. Provide an example of where to use each type of signal.

2. Explain why it's important to understand the theory of operation for each of the types of sensors.

3. Provide an application where one would use a Bourdon tube pressure sensor, a strain gauge pressure sensor, and a piezoelectric pressure sensor.

4. Explain the operation of a strain gauge and indicate the type of signal one would expect from it.

5. Explain the operation of a piezoelectric pressure sensor and indicate the type of signal one would get from it.

6. Identify the two broad categories of load cells.

7. Explain how one would troubleshoot a load cell.

8. Compare the terms laminar flow and turbulent flow.

9. Explain the operation of a sonic type flow meter and explain why one would use this type of flow meter instead of another.

10. Explain the operation of a Coriolis mass flow meter.

11. What is a Reynolds number and what is it used for?

12. Explain the operation of a thermal mass flow meter.

13 Explain why density measurements would be used in industrial applications.

14. Describe the operation of an RF admittance (capacitance) level sensor.

15. Explain how a sounding-level sensor operates.

16. Explain how a sonic-level sensor operates.

17. Provide an application where a linear potentiometer and a rotary potentiometer can be used.

18. Explain the operation of a magnetostrictive position sensor.

19. Describe the basic operation of an accelerometer. Be sure to include the function of the piezoelectric element.

20. Explain the pH scale and give an example of a substance that is an acid, a base, and neutral. Be sure to give the pH strength of the examples that one provide.

TRUE OR FALSE

1. One advantage of a 4-20 mA signal over a 0-10 volt dc signal is that the 4-20 mA signal uses a live zero, which makes it easier to detect a broken wire in a sensor or transducer.

2. A strain gauge is a larger version of a load cell.

3. The positive-displacement flow meter provides a more accurate measure of flow than a ΔP flow meter.

4. A turbine flow meter uses a turbine wheel that is turned when fluid flows past it. The number of revolutions of the turbine shaft is then converted to amount of flow.

5. A vortex-type flow meter uses LEDs to measure fluid flow.

6. Explain the operation of a pressure differential ( ΔP) type flow sensor that uses an orifice plate to create a pressure drop that is used to calculate flow.

7. Pressure can be used to measure temperature, flow, and level because there is a relationship between each that can be used in calculations.

8. A positioning system that uses an absolute encoder does not need a home switch.

9. An incremental encoder produces two waveforms (A pulse and B pulse) that are out-of-phase from each other to determine the direction of rotation.

10. An acid has a higher pH number than a base.

MULTIPLE CHOICE

1. Zero and span are provided on most sensors_______.

a. to make the sensor easier to troubleshoot.

b. to allow the sensor to be adjusted in the field to match the actual minimum and maximum conditions that are being sensed.

c. to make the sensor easier to be removed if it's broken.

2. An eddy-current-killed oscillator (ECKO) proximity switch_______.

a. can detect ferrous metals.

b. can detect any metal or nonmetal part since it bounces an echo off the part and measures how long it takes for the signal to return to the sensor.

c. can detect anything that breaks the beam of light it sends out.

3. Vibrating-tine sensor _____.

a. detects the level of granular material in a bin when the material covers the vibrating tines.

b. detects the flow of liquids as they pass the vibrating tines.

c. detects the level of gases and vapors when they make the tines vibrate more slowly.

4. Light can be used to indicate the level of liquid and solids ____.

a. when the beam of light changes its frequency as it reflects off solids and liquids.

b. when the beam of light is broken by the presence of material indicating the material has reached the level at which the sensor is mounted.

c. when the impedance of the return light beam is 20% higher than the impedance of the light that is sent to the target.

5. One can detect changes in temperature with a(n) _____

a. thermocouple.

b. RTD.

c. thermistor.

d. IC temperature sensor.

e. all the above.

f. only a and b.

6. The difference between an absolute and an incremental encoder is ____.

a. the absolute encoder produces a set of pulses that requires a home switch to determine its exact starting location.

b. the incremental encoder automatically knows its position as soon as power is turned on.

c. the absolute encoder automatically knows its position when power is turned on.

d. all the above.

7. A venturi flow meter____.

a. uses a pressure differential across a venturi that is converted through a calculation to the flow value.

b. measures flow directly from the pressure differential across a venturi.

c. counts the number of revolutions the venturi wheel makes as fluid flows past it.

8. A paddlewheel can be used to measure level ____.

a. by causing the paddlewheel to turn slowly when material is below its location and determining when it stops as material covers it.

b. by counting the number of rotations the paddlewheel turns as fluid moves past it and converting this number through a calculation.

c. by determining the pressure differential across the paddles on the paddlewheel as fluid moves past it.

PROBLEMS

1. Convert the following Fahrenheit temperatures to Celsius: 0°F, 32°F, 72°F, 700°F, 150°F, 200°F, 212°F, 400°F.

2. Convert the following Celsius temperatures to Fahrenheit: 0°C, 50°C, 100°C, 150°C, 200°C, 300°C, 400°C.

3. Compare the following pressures and list them in order of the most pressure to the least pressure: 12 psi, 15 psig, 10 psia.

4. Calculate the amount of resistance a strain gauge would need to produce to balance the bridge circuit shown in this schematic if R1 and R3 are equal to 250 Ω, and R2 is set for 400 Ω.

5. Calculate the flow through a pipe that has a 3 inch inside diameter when the velocity of the flow is 4 ft per second.

6. Calculate the Reynolds number for a liquid flowing through a pipe with a 1.5m. inside diameter, whose specific gravity is 0.60. flow rate is 10 gpm, and whose viscosity is 1.15 ft²/sec.

7. Calculate the flow through an orifice plate when the P1 pressure is 4.3 psi, and P2 pressure is 3.8 psi, and the k value for the orifice plate is 10.2.

8. If a load cell is rated for 2 mV/volt and its power supply is 9 volts, how much voltage would one expects to see if the load cell was loaded to its maximum weight?

9. Calculate the depth of a tank of water if a pressure differential sensor shows the pressure at the bottom of the tank is 27.5 psi.

10. Calculate the resolution of a ball screw mechanism that has a pitch of 20 threads per inch.

DRAW and EXPLAIN

1. Draw a diagram of a potentiometer connected to a power supply to provide a change of voltage as the potentiometer wiper moves. Explain how this circuit operates.

2. Draw the electrical diagram for a linear variable differential transformer (LVDT) and explain its operation.

3. Draw a linear encoder and explain how it creates positional data.

4. Draw the electrical diagram for a resolver and explain its operation.

5. Draw a diagram that shows the operation of a Hall-effect sensor and explain how the Hall effect works.

6. Draw the output signals from an incremental encoder and explain what each waveform is used for.

7. Draw a diagram of a circuit that could use the signal from a strain gauge to provide a reading to a meter to indicate pressure.

8. Draw a rack and pinion mechanism and explain how linear motion can be converted to rotary motion and how rotary motion can be turned into linear motion.

9. Draw a diagram of a circuit that a load cell would use to produce a useful signal. Be sure to identify the exciter voltage and the signal voltage.

10. Draw a pitot tube flow meter and explain its operation.

11. Draw a float-level sensor and explain its operation.

12. Draw a displacer-level sensor and explain how it differs from a float-level sensor.

13. Draw a diagram that indicates how a system can use a single float to indicate several different levels of liquid.

14. Draw a Bourdon tube pressure sensor and explain its operation.

EXAMPLE OF IN-THE-FIELD JOB ASSIGNMENT