1. Control Loops: Primary Sensors, Transmitters & Transducers
Process Technology (PTAC-1332)
Control Loops: Primary Sensors, Transmitters
& Transducers
This chapter describes the individual control loop components. Most transmitters house both the sensing and measuring function and produces a signal that can be transmitted to the next control loop element (controller, recorder, PLC, DCS) . Each function of sensing, measuring, and transmitting is discussed in more detail to include the purpose and operation of the component.
Objective
Describe the relationship between measuring instruments (pressure, temperature, level and flow) and their role in control loop.
Describe the purpose and operation of transmitters in the control loop
Discuss differential pressure in relation to process input to the transmitter
Compare and contrast the transmitter input and output signals
Describe the function of current to pneumatic transducer (I/P-signal converter)
Describe the relationship between 3 Psig to 15 Psig signal and a 4mA to 20 mA electrical signal.
Given a process control scheme, how a control loop function

2. Process Technology (PTAC-1332)
Control Loop Purpose and Operation
Sensing and measuring of a process variable are two separate function in the control loop. Some transmitter are capable of doing both functions. Transmitters always produces an output signal that carries information to the next loop.
There are actually several elements that perform sensing and measuring in tandem with transmitter (or without). These are sensors and transducers. Sensor detect PV and transducer converts one form of energy to another.
Sensor Drive Loop : Sensor serves as transducer and provide an input directly to controller
Transmitter Driven Loop: Transducer is a part of transmitter and effectively converts PV into standard Instruments signal

3. Control Loops: Primary Sensors, Transmitters & Transducers
Class Activity: Describe this loop both as sensor driven and Transmitter driven

4. Control Loops: Primary Sensors, Transmitters & Transducers
Sensors can be mechanical or electronic. A float/ displacer is mechanical sensor while thermocouple is electronic sensor. As shown in previous diagram, thermocouple can be directly connected to controller (sensor driven loop). The output signal to this sensor driven loop is nonstandard input (i.e. mV rather than standard 4-20 mA).
Sensor Types (Based on connectivity)
Discrete sensing elements (stand alone or individually distinct) are devices that are connected to transmitter by sensor wires. Thermocouple and RTD’s are example of discrete temperature sensors that are usually installed in thermo-well extending to process. Thermocouple wires or lead wires extending out of sheathing of the RTD’s are used to connect to transmitter. The transmitter may be physically connected to the open end of thermo-well or located short distance away.
A d/P transmitter is an example of Integrally mounted sensor transmitter. Since d/P cell is part of transmitter, the P/V has to be brought to it. This is accomplished by impulse tubing (impulse means a wave transmitting through something) between process and transmitter. The impulse tubing must meet material specification of the process where it is connected.

5. Control Loops: Primary Sensors, Transmitters & Transducers
Transmitter either have primary sensing element mounted within them or connected to them. After being sensed and measured, the measurement is transduced (converted) by the transmitter circuit into a standard instrument signal. The standard instrument signal are 4-20 mA (electronic), 3-15 Psig (pneumatic), or digital.
Unlike a directly connected sensor, where connection is non-standard, transmitters provides a standardized signal that is accepted by all instruments in the loop. A d/p transmitter contains special type of pressure sensing and measuring component called a d/p cell. The d/p cell is designed to measure the difference between two pressures and then produce an output signal. Inside the transmitter is transducer that transforms output of the d/p cell into signal such 3-15 Psig or 4-20 mA.

6. Control Loops: Primary Sensors, Transmitters & Transducers
Capacitance D/P Cell
Deflection results in capacitance variation between moving and fixed plates (differential capacitance)

7. Control Loops: Primary Sensors, Transmitters & Transducers
Regardless of the type and magnitude of process variable. Output of the transmitter is converted into standard signal (4-20 mA). In an electronic analog control loop, a thermocouple with milli-volt input to the transmitter has a 4-20 mA. Likewise, an RTD with resistance input to the transmitter also has a 4-20 mA output signal. Similar is the case with pressure and analytical measurements. All these process variables are converted into a standard signal, representing 0-100% of their respective measurement.
Standard signal are the language that an instruments speak when communicating with one another. For every input value applied to an analog transmitter there is a unique output value. In 4-20 signal 4 mA represents 0 percent of the measured process variable and 20.0 mA would represents 100 percent . Thus 12 mA represents 50% value.
% 𝑠𝑝𝑎𝑛= 𝑉𝑎𝑙𝑢𝑒 −𝐿𝑅𝑉 𝑆𝑝𝑎𝑛 X 100%
Calculating the input of transmitter by observing its output is a useful concept

8. Control Loops: Primary Sensors, Transmitters & Transducers
Instrument Scaling
An instrument scale is a range of ordered marks at fixed intervals inscribed on an indicator plate. When calibrating a transmitter, an instrument technician observes two scales: the input scale and output scale. The act of calibrating a transmitter involves applying a known set of input values to the measuring component and then adjusting transmitter output to correlate measurement range to a standard signal range.
Scaling converts process signals so that they are compatible with instrument and control system. Thus, scaling is the act of equating the numerical value of one scale to its mathematically proportional value on another scale. A standard analog electronic pressure transmitter is represented, on an appropriate pressure scale, while its output signal is represented on a milli-ampere scale.
Percent of Scale
Input
Output
0 %
500 oF
4 mA
25 %
625 oF
8 mA
50 %
750 oF
12 mA
75 %
875 oF
16 mA
100 %
1000 oF
20 mA

9. Numerical Conversion of one scale to another
Control Loops: Primary Sensors, Transmitters
& Transducers
Numerical Conversion of one scale to another
Process technician may use formulas to convert the numerical value of one scale to another. To know how to plug in the appropriate numbers in the formula.
Important Terminologies
Upper range value (URV) : The number at the top of the scale.
Lower range value (LRV): The number at the bottom of the scale.
Operating range: The set of values that exist between the LRV and the URV of a scale; expressed as two number (e.g. calibration range may be expressed as 50 Psig to 150 Psig)
Span: The algebraic difference between the URV minus the LRV of a scale; expressed as one number (e.g. if URV= 150 Psig and LRV = 50 Psig, then span = 100 Psig)
Solve Example 9-7 in class
Transmitter
input
Transmitter
output
Value B = V alue A − LRV A Span A x Span B + LRV B
URVA = 50 psig
20 mA = URVB
Where:
A = Original scale
B = New Scale
Proportional factor = Decimal representation of the original value on scale “A”
Value A = 25 psig
12 mA = ValueB
LRVA = 0 psig
4 mA = LRVB

10. Control Loops: Primary Sensors, Transmitters & Transducers
Transducers and Signals
Generally speaking, a transducer is a device that converts one energy into another. Transducer converts quantities such as temperature and pressure into electronic form. Specifically, a primary sensor such as diaphragm produces motion in response to pressure and then a transducer converts the motion into a measurable electrical quantity.
A common signal transducer that converts an analog electronic signal (4-20 mA) into a pneumatic signal (3-15 Psig). This device is I/P transducer
Sensor-Transducer (One and the Same- Special Situation)
A piezo-electric transducer is a crystalline wafer that respond to applied pressure by producing an electric response. Another example is a thermocouple that responds to difference in temperature by generating a millivolt signal. These sensor-related transducer cab produce any level of electrical impulse unlike their macro counterparts (I/P)