# 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

### 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.

### 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%

$\frac{Value&space;-&space;LRV}{Span}&space;x&space;100&space;=&space;Span$

### Span, %Span

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

### Transmitters: Input to Output

Transmitter Input vs. Output

(linear)

$VALUE_{B}&space;=&space;\frac{VALUE_{A}&space;-&space;LRV_{A}}{SPAN_{A}}&space;\times&space;SPAN_{B}&space;+&space;LRV_{B}$

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 $VALUE_{B}&space;=&space;\frac{(VALUE_{A}&space;-&space;LRV_{A})}{SPAN_{A}}&space;\times&space;SPAN_{B}&space;+&space;LRV_{B}$ LRVA 4 mA URVA 20 mA SPANA 16 mA $Value B&space;=&space;\frac{(8 mA&space;-&space;4 mA)}{16 mA}&space;\times&space;12 psig&space;+&space;3 psig$ 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 200 ºF $VALUE_{B}&space;=&space;\frac{(VALUE_{A}&space;-&space;LRV_{A})}{SPAN_{A}}&space;\times&space;SPAN_{B}&space;+&space;LRV_{B}$ LRVA 100 ºF URVA 300 ºF SPANA 200 ºF $Value B&space;=&space;\frac{(200 ºF&space;-&space;100 ºF)}{200 ºF}&space;\times&space;16 mA&space;+&space;4 mA$ 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 220 ºF $VALUE_{B}&space;=&space;\frac{(220 ºF&space;-&space;150 ºF)}{550 ºF}&space;x&space;16mA&space;+&space;4mA$ LRVA 150 ºF URVA 700 ºF SPANA 550 ºF 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
$VALUE_{B}&space;=&space;\frac{(VALUE_{A}&space;-&space;LRV_{A})}{SPAN_{A}}&space;\times&space;SPAN_{B}&space;+&space;LRV_{B}$

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?

REVIEW/DISCUSSION

Is this control loop open or closed?

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