Prepare for the CCST Exam

The correct answer is D, A/D converter. A digital controller requires a digital signal as its input. A 4-20 mA transmitter outputs an analog signal. Therefore, a device to convert an analog (A) signal to a digital (D) is required. This class of device is referred to as an A/D converter.

An I/P transducer is used to convert an analog current (I) signal to a pneumatic (P) signal, as for actuation of final control elements. A P/I transducer is used to convert a pneumatic signal (P) to an analog current (I) signal, as for a pneumatic transmitter in a programmable logic controller loop. A DP transmitter is a differential pressure transmitter, which can output a pneumatic, an analog, or a digital signal, depending on the model of transmitter used.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is C, "sample conditioning system." Answers A and C are items not generally associated with extractive field analyzers. Capillary tubes are used for collecting small samples (water, for instance) from a larger container. There are special capillary tubes that can be used in the analyzer chamber of a gas chromatograph, but they are not constructed from glass. Smooth-walled pipe is important for reducing friction losses in piping systems.

A sample probe calibration system is important to the overall function and maintenance of an extractive field analyzer. However, these systems are not used to prepare the sample for analysis, but rather to provide a mechanism to verify and maintain analyzer performance.

A sample conditioning system can contain devices, such as filters, demisters, flow regulators, and heaters. Sample conditioning systems are used to bring the sample to the ideal process conditions for accurate measurement in the analyzer itself. The sample conditioning system can be a key maintenance item in an analyzer system, since each device needs to be calibrated, cleaned, etc.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is B, "equal to." In order for air to be discharged from the end of a bubbler purge tube, the air pressure in the tube must be equal to (or higher than) the pressure exerted by the liquid head in the tank.

As the tank level is decreased, the liquid head pressure at the tip of the purge tube decreases, and more bubbles per unit of time can escape. The corresponding reduction in pressure in the purge tube is proportional to the level in the tank. Therefore, the point at which the liquid head pressure and the purge tube pressure are equal is the highest level (URV = 100%) that the device will measure.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is A, "51 K ohms ± 5%."

The four-color band coding is:
Color    Value    Multiplier
Black    0    1
Brown    1    10
Red    2    100
Orange    3    1000
Yellow    4    10 K
Green    5    100K
Blue    6    1 M
Violet    7    10 M
Gray    8     
White    9     
Gold    ± 5%    0.1
Silver    ± 10%    0.01

So a resistor with four bands, green-brown-orange-gold, has a value of: 5 1 x 1000 ± 5% or 51 KΩ.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is D, series and energized. To measure current, you must connect the two leads of the ammeter in the circuit so that the current flows through the ammeter. In other words, the ammeter must become a part of the circuit itself. The only way to measure the current flowing through a simple circuit is to insert your ammeter into the circuit (in series) with the circuit energized.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is A; it prevents the formation of a second temperature measurement junction.

A thermocouple measurement junction is formed wherever two dissimilar metals are joined. KX-type thermocouple extension wire is made of the same metals as the K-type thermocouple (chromel and alumel). When extending the thermocouple leads with an extension wire back to the control system input card, KX thermocouple extension wire must be used, and the chromel wire and the alumel wire must be joined to the wire of the same metal in the extension cable. If JX or another type of extension wire is used, another measurement junction is formed. For instance, if JX extension cable is used in the example in this problem, the point where the iron and chromel wires are joined would form another thermocouple. This will negatively affect the intended measurement signal. Proper installation of thermocouple extension wires also requires special terminal blocks to prevent additional junctions from being formed.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is B, "hydraulic actuation." Although many pneumatic actuators can provide a large force, they require either a large diaphragm area (in the case of a diaphragm actuator) or a large cylinder (in the case of a rack and pinion actuator).

Hydraulic actuators are driven by a high-pressure fluid (up to 4,000 psig) that can be delivered to the actuator by a pump that is remote from the actuator itself. Hydraulic cylinders can deliver up to 25 times more force than a pneumatic cylinder of the same size.

Manual actuation is accomplished by turning a valve handle, and is limited to the amount of force that an operator can exert on the lever or hand wheel.

Electric actuation delivers high torques for rotary-style valves, but electric actuators tend to be large and heavy compared to hydraulic actuators.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is D; they measure pressure by sensing the deflection of the diaphragm. For most pressure applications, changes in pressure are detected by the change in deflection of a measuring diaphragm.

The deflection is converted into an electrical signal (voltage) by a piezoelectric or capacitance device. The small electrical current is converted to a standard signal (e.g., 4-20 mA or a digital signal) by a transmitter. Therefore, answer B is not correct.

Answer A is not correct, because pressure sensors can measure very small pressure changes (inches of water) and in some cases, millimeters of water.

Pressure measurement devices are not affected by volume, since they are measuring force over an area only. Many pressure sensors are sensitive to temperature (capillary tubes are filled with fluids that can expand with temperature). Therefore, answer C is not correct.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is C, "Gather information about the problem." Once a problem is identified, data must be gathered and analyzed to determine a viable set of potential actions and solutions.

The logical analysis troubleshooting method consists of (in order):
1. Identify and define the problem.
2. Gather information about the problem.
3. Evaluate the information/data.
4. Propose a solution or develop a test.
5. Implement the solution or conduct the test.
6. Evaluate the results of the solution or test.
7. If the problem is not resolved, reiterate until the problem is found and resolved.
8. If the problem is resolved: document, store/file, and send to the appropriate department for follow up if required.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

The correct answer is B, “location, elevation, and tag number.” Instrument location plans are most often used to support new plant installations and give the installer information about the actual physical location of the installation of an instrument, the elevation of installation (at grade, on a platform, at what height on a process line, etc.), and the tag number of the instrument to be installed.

Specification numbers (part of answers C and D) are usually indicated on instrument lists and instrument installation details. Wiring plans (part of answer A) are typically shown on conduit and wiring schedules or cabling diagrams. Although these details are useful in the installation of a plant, they are not part of the instrument installation plans.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.