Select Page

## AutoQuiz: How to Read a Gauge Pressure Transmitter

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.

### A gauge pressure transmitter that measures the pressure in a 150 # high pressure steam header is mounted 6 feet below the center line of the header. The tap for the impulse line connects to the top of the header and rises 2 feet above the header center line, extends horizontally for 3 feet, and then drops down to the transmitter. In order to read the pressure in the steam header correctly, the transmitter output must be:

a) calibrated for suppressed zero, the suppression equal to 8 feet of liquid head pressure
b) calibrated for suppressed zero, the suppression equal to 6 feet of liquid head pressure
c) calibrated for elevated zero, the elevation equal to 8 feet of liquid pressure
d) calibrated for true zero
e) none of the above

At zero gauge pressure in the steam line, you are essentially suppressing (pushing) the transmitter output back down to zero after the system reaches equilibrium with the 8 feet of impulse line full of liquid. The transmitter will read that impulse line liquid head pressure plus any pressure exerted on top of the liquid. 6 feet of liquid head suppression is incorrect as the impulse arrangement will cause 8 feet of liquid head to accumulate.  Elevated zero is incorrect as that adjustment is used to adjust for negative pressure offsets resulting from the transmitter being mounted above the zero reference point (high pressure tap point) and where a liquid in the impulse line or a capillary system would exert a negative pressure.

The correct answer is A, calibrated for suppressed zero with the suppression equal to the 8 feet of liquid height that is in the impulse line.

Reference: Thomas A. Hughes; Measurement and Control Basics, 5th Edition, ISA Press.

## AutoQuiz: Loop Tuning Terminology for Change in Amplitude

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.

### What common loop tuning terminology is used to describe the change in amplitude of the control loop oscillations shown in the post graphic?

a) 4:1 Decay Ratio
b) Quarter Wave Decay
c) ¼ ZN Diffraction
d) 4/1 Gain Response
e) none of the above

The common method of judging tuning is known as the “quarter wave decay” method.  It has been shown if a loop is tuned so the oscillation decays with each wave being one quarter of the previous wave, it produces satisfactory, if not optimum, set point response and disturbance rejection.

The correct answer is B, Quarter Wave Decay.

Reference: Harley M. Jeffery; Loop Checking: A Technician’s Guide, ISA Press.

## AutoQuiz: Standard Wiring Color for Type K Thermocouple

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.

In the diagram in the post image, which (A, B, C or D) is the standard wiring color for a Type K thermocouple in the U.S. and Canada?

A illustrates the very common Type J, Iron/Constantan thermocouple.

B illustrates the less common Type E, Chromel/Constantan thermocouple.

D illustrates the less common Type T, Copper/Constantan thermocouple.

The correct answer is C, which illustrates the very common Type K, Chromel/Alumel thermocouple.

References:

Control Systems Engineering Exam Reference Manual: A Practical Study Guide, by Bryon Lewis, ISA Press.

Measurement and Control Basics, 5th Edition, by Thomas A. Hughes, ISA Press.

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.

RTDs (resistance temperature detectors) are used to measure temperature in a process stream because they have many distinct advantages over thermocouples, including higher resolution of measurement, are more stable, are more repeatable, and give a more linear measurement response. However, RTDs have some distinct disadvantages as compared to thermocouples. All of the following are disadvantages of RTDs as compared to thermocouples, except:

a) require a temperature transmitter to function
b) must have equal length and same type and gauge of lead wires
c) are not self-powered
d) are more expensive than thermocouples
e) none of the above

Answer B is not the correct choice; equal length and same type and gauge of lead wires is a distinct disadvantage of using RTDs, especially if the lead wires must be extended to the temperature measurement point. RTDs are resistance devices whose measurement can be influenced by variations in lead length and thickness (gauge). Thermocouples do not have this limitation (they are voltage devices).

Answer C is not the correct choice. RTDs are not self-powered (thermocouples generate their own mV signal based on the sample temperature as compared to the reference temperature). An RTD requires a power source, usually a 24VDC supply.

Answer D is not the correct choice. RTDs are much more expensive than thermocouples. The materials from which RTDs are manufactured (platinum and rhodium, among others) are more expensive than copper, iron, or tungsten, common thermocouple materials. Also, RTDs are more expensive to manufacture (e.g., RTD leads are often wound around an insulating core). Common RTD assemblies (3-wire) are typically 2-3 times the cost of comparable thermocouple assemblies measuring the same temperature range.

The correct answer is A, require a temperature transmitter to function. This is not a disadvantage of an RTD, since its function does not require a temperature transmitter to be installed. RTDs do require a power source and a measurement junction, but this can be done with any Wheatstone bridge circuit.

Reference: Maintenance of Instruments and Systems, 2nd Edition, Goettsche, L.D. (Editor), ISA Press.

Image Credit: coastalwiki.org

## AutoQuiz: How to Diagnose Industrial Pressure Sensor Errors

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program. This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.

An electronic differential pressure (DP) cell and pressure transmitter device has been suspected of reading incorrectly during some high-pressure plant operations. The instrument supervisor sends the best technician during the next shutdown to diagnose the problem. The as-found condition of the pressure transmitter as recorded by the instrument technician has given the values shown in the post graphic.

What should the instrument technician report to the supervisor?

a) there is no apparent problem with the pressure sensor/transmitter
b) the pressure sensor/transmitter is giving a non-linear response; replacement of the sensor and/or the transmitter will be required
c) the pressure transmitter span has drifted; the pressure transmitter should be recalibrated
d) the pressure transmitter zero has shifted; the pressure transmitter should be recalibrated
e) none of the above

Response A (no apparent problem) would be appropriate only if the % Deviations calculated for the as-found conditions were all very close to zero (e.g., <0.01%).

Response C (span drift) would be indicated by % Deviations all increasing in the positive direction with a very small deviation (e.g., <0.01%) at the 0% value.  In addition, the % Deviation for increasing and decreasing input values would need to be very close in value, indicating hysteresis is not a problem.

Response D (zero shift) would be indicated by % Deviations of roughly all the same magnitude, either (+) or (-), including the 0% values. In addition, the % Deviation for increasing and decreasing input values would need to be very close in value, indicating that hysteresis is not a problem.

From the table of as-found values, the % Deviations cross from positive to negative several times, indicating a non-linear response. In addition, the as-found % Deviation values as the input signal is decreased are quite different from the values found with increasing input signal, indicating hysteresis in the output signal.  This may indicate the DP cell is damaged or the pressure transmitter electronics are not functioning properly.

The correct answer is B, the pressure sensor/transmitter is giving a non-linear response; replacement of the sensor and/or the transmitter will be required.

Reference: Maintenance of Instruments & Systems, 2nd Edition, by L.D. Goettsche, ISA Press.