- Terms to Know
- Sensor Signals
- Sensor output to Transmitter
- SPAN, Operating Range
- Transmitter Scaling
- Span, %Span
- Scaled Sensor Input – Transmitter Output
- Transmitters: Input to Output
- Sample Scaling Problem: In a standard I/P transducer, an 8-mA input corresponds to what output signal?
- Scaling Problem : A temperature transmitter uses a thermocouple sensor and is calibrated to 100 deg F — 300 deg F as a 4-20 mA output signal. If the fluid temperature is 200 deg F, what is the output signal in mA?
- Scaling Problem: A pressure transmitter is calibrated at 0-300 psig, with an operating setpoint of 175 psig. What is the percent span of the setpoint?
- Scaling Problem: A thermocouple has an operating range of 150 deg F – 700 deg F. Current reading is 220 deg F. What is the scaled output from a standard electronic transmitter at this reading?
- Describe the relationship between sensors, transducers, and transmitters in process control loops
- Compare and contrast the transmitter/transducer input and output signals
- % span
- Scaling: Input to Output (linear)
- Review control loop function based on a process control scheme diagram
Terms to Know
- Discrete Sensing Element
- Integrally Mounted Sensing Element
- Linear Scaling
- LRV, URV
- Operating Range
- Standard Signals
- Pressure, Temperature, Level, Flow
- Discrete Sensors or Elements— wired or connected to the transmitter
- Thermocouples, RTDs
- Should be shown on PID as TE and TT (and TW)
- Flow orifices — The orifice is the Flow Element, often discrete from the transmitter, even though the ‘pressure sensor’ is integral to the sensor
- Integrally Mounted Sensors — physically part of the transmitter
- d/p cell, TT, PT
- Note the need to connect to the Process — external to the sensor in a d/p
- PID: The process connections are not normally shown for the d/P connection points
- Can be shown on PID as PE/PT or PT or PE
What are the standard signals?
- Electronic ???
- Pneumatic ???
- Digital ???
Sensor outputs are most likely non-standard
- Ex. Thermocouple in mV
- RTD — resistance – ohms
- Pressure — actual process pressure
Controllers need standard input signals
- Convert non-standard input signals to standard output signals
- I/P Current to Pneumatic — very common
- P/I Pneumatic to Current
- I/E Current to Voltage
- E/I Voltage to Current
- E/P Voltage to Pneumatic
Sensor output to Transmitter
SPAN, Operating Range
- SPAN = URV — LRV
- Operating Range is ‘LRV to URV’
- Temperature transmitter calibrated for operating range 100 deg F to 400 deg F
- Span = 300 deg F
- Temperature transmitter calibrated for operating range 1500 deg F to 1800 deg F
- Span = ?????
- Transmitter output signal calibrated for operating range 4mA to 20 mA
- Output of Transmitter represents 0-100% of measured process variable
- 4 mA = 0%
- 20 mA = 100%
|Percent of Scale||Input||Output|
Scaled Sensor Input – Transmitter Output
Transmitters: Input to Output
Transmitter Input vs. Output
A = Original Scale (input)
B = New Scale (output)
LRV = Lower Range Value
URV = Upper Range Value
SPAN = URV – LRV
Input = electrical signal
Output = pneumatic signal
|SPANB||12 psig||ValueB = 6 psig|
|SPANB||16 mA||Value B = 12 mA|
|VALUEA||175 psig||Insert Equation|
|SPANA||300 psig||Insert equation|
|SPANB||% Span = 58.3%|
|SPANA||550 ºF||VALUEB = (70/550) x 16mA + 4mA|
VALUE – 2.04 mA + 4 mA
6.04 mA output signal
Example: Pressure transmitter is calibrated to measure from 0-80 psig, and it is measuring 20 psig. What is the output of its standard 4-20 mA transmitter?
Why is I/P one of the most common transducers?
Is this control loop open or closed?
|Component||Element Type||PV being controlled or manipulated||Component Function|
|TW-002||Thermowell||n/a||Housing the sensor|
|TE-002||Temperature element||Temperature||Sensing the temperature|
|TI-002||Temperature indicator||Temperature||Indicating and transmitting the temperature|
|FE-001||Flow element||Flow||Sensing the flow|
|FT-001||Flow transmitter||Flow||Transmitting the flow of data|
|FY-001||Flow transducer or flow computer||Flow/Temperature||Calculation - temperature and flow to calculate net of mass flow|
|FI-001||Flow indicator (net)||Flow||Indicates the final flow rate|