NCCER Instrumentation flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Class I Division 1 location

An area where flammable gases or vapors are present continuously, intermittently, or periodically under normal operating conditions, or where equipment failure could release ignitable concentrations.

Intrinsic safety entity parameters

For an IS loop the barrier open-circuit voltage and short-circuit current must not exceed the device Vmax and Imax, and allowed capacitance and inductance must cover the device plus cable.

Temperature class T4

T4 limits equipment maximum surface temperature to 135 degrees Celsius, so it suits gases with ignition temperatures above that value but not those requiring T5 or T6.

Lockout/tagout verification

OSHA 1910.147 requires all hazardous energy to be isolated, locked, and tagged, and a qualified person must verify a zero-energy state by test before work begins.

Confined space oxygen limits

Entry testing must show oxygen between 19.5 and 23.5 percent by volume; a lower value is oxygen deficient, and a higher value is enriched and increases fire risk.

Liquid impulse line slope

Liquid-service impulse lines slope at least one-eighth inch per foot downward toward the transmitter or drain so gas bubbles cannot collect and offset the measured pressure.

Gas service tap location

For gas or vapor service the pressure tap is placed at the top of the pipe and impulse tubing slopes upward toward the transmitter, keeping condensate out of the lines.

Three-valve manifold sequence

To isolate a DP transmitter, open the equalizer valve first, then close the high and low block valves; to return to service, open the blocks first and close the equalizer last.

Proximity probe gap voltage

Eddy-current probes are normally set to about minus 9 volts DC, within roughly minus 8 to minus 10 volts, which places them near the center of their linear range.

Orifice plate orientation

An orifice plate square, sharp edge faces upstream flow and the bevel faces downstream; the plate must be installed with the correct face and any drain hole positioned per the drawing.

Swagelok pull-up turns

A 1/4-inch tube fitting is tightened one and one-quarter turns past finger-tight; a gap gauge confirms full pull-up, and over-tightening can damage the ferrule and cause leaks.

Cable shield grounding

Instrument signal cable shields are normally grounded at one end only, usually in the control room, to prevent circulating ground currents that add noise and offset to the signal.

Maximum loop resistance

Loop resistance must not exceed supply volts minus 12, divided by 0.020 amps, so a 24 VDC supply permits no more than 600 ohms total loop resistance.

Conduit fill limit

NEC Chapter 9 limits raceway fill to 40 percent of cross-sectional area for three or more conductors, 31 percent for two, and 53 percent for one.

Minimum tubing bend radius

A practical minimum is a bend radius of three times the tube outside diameter, formed with a lever bender; tighter bends oval the tube and restrict flow or crack it.

Tubing support spacing

Copper and stainless instrument tubing is supported about every 3 feet horizontally and 4 feet vertically, with additional support near valves, fittings, and bends.

Five-point calibration

The instrument is driven to 0, 25, 50, 75, and 100 percent of span ascending and again descending, with the output recorded at each point to check linearity and hysteresis.

Error percent of span

Error equals indicated value minus true value, divided by span, times 100; a 0.4 mA deviation on a 16 mA span is 2.5 percent of span.

12 mA meaning

A 12 milliampere signal represents exactly 50 percent of the 4-20 mA span and 50 percent of the calibrated range on a linear transmitter with 4 mA at the lower range value.

DP flow square root

Because differential pressure varies with flow squared, flow percent equals the square root of DP percent, so 25 percent DP corresponds to 50 percent flow.

HART minimum loop resistance

HART digital communication needs at least 250 ohms of resistance between the supply and the communicator connection point, or the frequency signal is shorted by a stiff supply.

Pt100 reference resistance

A platinum 100-ohm RTD reads 100.00 ohms at zero degrees Celsius and changes about 0.385 ohm per degree; a 4-wire connection cancels lead resistance errors.

Proportional band formula

Proportional band in percent equals 100 divided by controller gain, so a gain of 2 gives a 50 percent band; a wider band means a less aggressive response.

Air-to-close valve failure

An air-to-close actuator needs air pressure to open the valve, so the spring returns it open on air failure; it is specified where fail-open action is required.

Cascade control

In cascade control the primary controller output becomes the setpoint of a faster secondary controller, so the secondary loop absorbs disturbances in the utility or valve before they reach the primary variable.

Split-range control

One controller output drives two valves over different portions of the signal, for example 4 to 12 milliamps for one valve and 12 to 20 milliamps for the other.

Pneumatic signal standard

The standard pneumatic transmission signal is 3 to 15 psig, where 3 psi is 0 percent and 15 psi is 100 percent, so 9 psi corresponds to 50 percent of range.

Reading below 4 mA

An indication below the lower range value usually means an open loop, reversed polarity, lost loop power, or a transmitter fault; check supply voltage and wiring continuity first.

Wet leg loss symptom

If a wet reference leg loses its liquid fill, low-side pressure falls, differential pressure rises, and level indication reads falsely high; the leg must be refilled and the transmitter re-zeroed.

Thermocouple open circuit

An open thermocouple shows extremely high or infinite resistance and may drive the indicator upscale depending on burnout detection; check output with a millivolt source and cold-junction compensation.

Ground loop symptoms

Ground loops produce noisy, drifting, or offset readings in low-level signal circuits; the remedy is single-point grounding of shields or adding a signal isolator.

Megger on instrument wiring

Insulation resistance testing applies high voltage, so signal wiring must be disconnected from transmitters, DCS cards, intrinsic safety barriers, and surge protectors before meggering.