PRT 140: Lesson 10 Control Loops, Sensors, and Transmitters

Objectives

  • Describe the relationship between sensors, transducers, and transmitters in process control loops
  • Compare and contrast the transmitter/transducer input and output signals
  • Calculate:
    • % span
    • Scaling: Input to Output (linear)
  • Review control loop function based on a process control scheme diagram

Reading

Terms to Know

  • Discrete Sensing Element
  • Integrally Mounted Sensing Element
  • Linear Scaling
  • LRV, URV
  • Span
  • Operating Range
  • Standard Signals

Sensors

  • 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

Sensor Signals

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

Transducers

  • 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
  • Etc.

Sensor output to Transmitter

A diagram of sensor output to a transmitter
Sensor output to transmitter
[image 140-9-1]

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

Transmitter Scaling

  • Output of Transmitter represents 0-100% of measured process variable
  • 4 mA = 0%
  • 20 mA = 100%

\frac{Value - LRV}{Span} x 100 = Span

Span, %Span

Percent of ScaleInputOutput
0%500ºF4 mA
25%625ºF8 mA
50%750ºF12 mA
75%875ºF16 mA
100%1000ºF20 mA

Scaled Sensor Input – Transmitter Output

A table showing scaled sensor input - transmitter output
Scaled sensor input – Transmitter output
[image 140-9-2]

Transmitters: Input to Output

Transmitter Input vs. Output

(linear)

VALUE_{B} = \frac{VALUE_{A} - LRV_{A}}{SPAN_{A}} \times SPAN_{B} + LRV_{B}

Where:

A = Original Scale (input)

B = New Scale (output)

LRV = Lower Range Value

URV = Upper Range Value

SPAN = URV – LRV

Sample Scaling Problem: In a standard I/P transducer, an 8-mA input corresponds to what output signal?

Input = electrical signal

Output = pneumatic signal

Data Equations
VALUEA 8 mA VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}
LRVA 4 mA
URVA 20 mA
SPANA 16 mA Value B = \frac{(8 mA - 4 mA)}{16 mA} \times 12 psig + 3 psig
LRVB 3 psig
URVB 15 psig
SPANB 12 psig ValueB = 6 psig

 

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?

 

Data Equations
VALUEA 200ºF VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}
LRVA 100ºF
URVA 300ºF
SPANA 200ºF Value B = \frac{(200ºF - 100ºF)}{200ºF} \times 16 mA + 4 mA
LRVB 4 mA
URVB 20 mA
SPANB 16 mA Value B = 12 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?

 

Data Equations
VALUEA 175 psig Insert Equation
LRVA 0 psig
URVA 300 psig
SPANA 300 psig Insert equation
LRVB
URVB
SPANB % Span = 58.3%

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?

 

Data Equations
VALUEA 220ºF VALUE_{B} = \frac{(220ºF - 150ºF)}{550ºF} x 16mA + 4mA
LRVA 150ºF
URVA 700ºF
SPANA 550ºF VALUEB = (70/550) x 16mA + 4mA
VALUEB  6.04 mA
LRVB  4 mA
URVB  20 mA

VALUE – 2.04 mA + 4 mA

6.04 mA output signal

SPANB 16 mA
VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}

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?

VALUEA ?

LRVA?

URVA?

SPANA?

LRVB?

URVB?

SPANB?

Why is I/P one of the most common transducers?

A diagram of an I/P Transducer [140-10-01]
An I/P Transducer
[140-10-01]

REVIEW/DISCUSSION

Is this control loop open or closed?

A diagram of a control loop
Flow control loop – open or closed?
[140-10-02]
ComponentElement TypePV being controlled or manipulatedComponent Function
TW-002Thermowelln/aHousing the sensor
TE-002Temperature elementTemperatureSensing the temperature
TI-002Temperature indicatorTemperatureIndicating and transmitting the temperature
FE-001Flow elementFlowSensing the flow
FT-001Flow transmitterFlowTransmitting the flow of data
FY-001Flow transducer or flow computerFlow/TemperatureCalculation - temperature and flow to calculate net of mass flow
FI-001Flow indicator (net)FlowIndicates the final flow rate