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

Contents

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 Scale	Input	Output
0%	500ºF	4 mA
25%	625ºF	8 mA
50%	750ºF	12 mA
75%	875ºF	16 mA
100%	1000ºF	20 mA

Scaled Sensor Input – Transmitter Output

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
VALUE_A	8 mA	$VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}$
LRV_A	4 mA
URV_A	20 mA
SPAN_A	16 mA	$Value B = \frac{(8 mA - 4 mA)}{16 mA} \times 12 psig + 3 psig$
LRV_B	3 psig
URV_B	15 psig
SPAN_B	12 psig	Value_B = 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
VALUE_A	200 ºF	$VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}$
LRV_A	100 ºF
URV_A	300 ºF
SPAN_A	200 ºF	$Value B = \frac{(200 ºF - 100 ºF)}{200 ºF} \times 16 mA + 4 mA$
LRV_B	4 mA
URV_B	20 mA
SPAN_B	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
VALUE_A	175 psig	Insert Equation
LRV_A	0 psig
URV_A	300 psig
SPAN_A	300 psig	Insert equation
LRV_B
URV_B
SPAN_B		% 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
VALUE_A	220 ºF	$VALUE_{B} = \frac{(220 ºF - 150 ºF)}{550 ºF} x 16mA + 4mA$
LRV_A	150 ºF
URV_A	700 ºF
SPAN_A	550 ºF	VALUE_B = (70/550) x 16mA + 4mA
VALUE_B	6.04 mA
LRV_B	4 mA
URV_B	20 mA	VALUE – 2.04 mA + 4 mA 6.04 mA output signal
SPAN_B	16 mA	VALUE – 2.04 mA + 4 mA 6.04 mA output signal

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

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?

VALUE_A ?

LRV_A?

URV_A?

SPAN_A?

LRV_B?

URV_B?

SPAN_B?

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]

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