Discrete Process Measurement classification
- 2. a “Discrete” variable or measurement refers to a true-or-false condition. Discrete sensors typically take the form of switches, built to “trip” when the measured quantity either exceeds or falls below a specified value. Thus, a discrete sensor is one that is only able to indicate whether the measured variable is above or below a specified setpoint
- 3. “Normal” status of a switch The “normal” status for a switch = its electrical contacts are in during a condition of no physical stimulation. For a momentary-contact pushbutton switch = the status of the switch contact when it is not being pressed Electrical switches are always drawn in schematic diagrams in their “normal” statuses, regardless of their application.
- 5. Example: LED will turn on if the liquid level rises above 14 inches and the pressure falls below 22 PSI and either the flow is less than 3 gallons per minute or the temperature is greater than 125 degrees Fahrenheit
- 6. Multiple switches wired to the discrete input channels of a programmable logic controller (PLC) “red LED indicators denoting the real-time status of each input on the PLC”
- 9. A limit switch detects the physical motion of an object by direct contact with that object. A typical limit switch design uses a roller-tipped lever to make contact with the moving part. Limit Switches
- 11. Detecting the proximity (closeness) of some object “non-contact limit switches” A proximity switch will be in its “normal” status when it is distant from any actuating object. Proximity Switches
- 12. Inductive proximity switches: Sense the presence of metallic objects through the use of a high-frequency magnetic field Capacitive proximity switches: Sense the presence of non-metallic objects through the use of a high-frequency electric field Optical proximity switches: Detect the interruption of a light beam by an object Ultrasonic proximity switches: Sense the presence of dense matter by the reflection of sound waves. Proximity Switches
- 15. Often use diaphragms or bellows as the pressure-sensing element, the motion of which actuates one or more switch contacts. Pressure Switches
- 16. Two pressure switches sensing the same fluid pressure as an electronic pressure transmitter
- 17. The bourdon tube is Grey. The mercury tilt switch bottles have yellow plastic caps covering up their external electrical contacts
- 18. Advantages: • Immunity to switch contact degradation from harmful atmospheres (oil mist, dirt, dust, corrosion) • Safety in explosive atmospheres (No spark) Disadvantages: • Intermittent of electrical contact resulting from mechanical vibration • Sensitivity to mounting angle Open Contact Closed Contact
- 19. Deadband “differential pressure setting”: Determines the amount of pressure change required to re-set the switch to its normal state after it has tripped.
- 20. Pressure switch actuated by differential pressure
- 21. Often use bimetallic strips or filled bulb as the temperature-sensing element causing the switch mechanism to actuate Temperature Switches
- 22. Temperature switch equipped with a separate adjustment for Deadband avoid repeated “nuisance” alarms
- 23. Electronic temperature switch circuits using T/Cs, RTDs, or Thermistors also input 4-20 mA loop current signals
- 24. A level switch is one detecting the level of liquid or solid (granules or powder) in a vessel, often use floats as the level-sensing element Level Switches
- 25. Float Type Level Switches
- 26. Float Type Level Switches
- 27. An electronic circuit continuously excites the tuning fork, causing it to mechanically vibrate. The circuit detects this frequency change and indicates the presence of mass contacting the fork. The fork vibrating motion tends to shake off any accumulated material, such that this style of level switch tends to be Resistant to Fouling. Tuning Fork Level Switches
- 28. Electric motor to slowly rotate a metal paddle inside the process vessel. A torque-sensitive switch mechanically linked to the motor actuates when enough torsional effort is detected on the part of the motor Paddle-Wheel Level Switches
- 29. Sound waves pass back and forth within the gap of the probe. The presence of any substance other than gas within that gap affects the received audio power. The lack of moving parts makes this probe quite reliable, although it may become “fooled” by heavy fouling Ultrasonic Level Switches
- 30. Couple of capacitive switches sensing the presence of water in a plastic storage vessel Capacitive Level Switches
- 33. Often use “paddles” as the flow-sensing element, the motion of which actuates one or more switch contacts. Flow Switches
- 35. Like all other process switches, flow switches exhibit Deadband (also called differential ) in their switching action