NCCER Instrumentation cheat sheet

29 CFR 1910.147 (lockout/tagout) requires isolation of all hazardous energy and verification of a zero-energy state by test before instrument work begins.

NEC Article 500: Class I covers flammable gases and vapors, Class II combustible dusts, Class III ignitable fibers; Division 1 means the hazard exists under normal conditions, Division 2 only under abnormal conditions.

NEC Article 505 Zone system: Zone 0 is continuously hazardous, Zone 1 occasionally, Zone 2 rarely or briefly; Zone 2 is roughly equivalent to Division 2.

Intrinsically safe loop check: barrier Voc must not exceed device Vmax, barrier Isc must not exceed Imax, allowed Ca must cover device Ci plus cable capacitance, and allowed La must cover Li plus cable inductance.

Surface temperature classes: T1 450 degC, T2 300 degC, T3 200 degC, T4 135 degC, T5 100 degC, T6 85 degC; the class must stay below the gas ignition temperature.

Confined space pre-entry limits: oxygen 19.5 to 23.5 percent by volume, flammable gas below 10 percent LEL, and toxic gas below the exposure limit before entry is permitted.

NFPA 496 covers purging and pressurization of enclosures; Type X pressurization reduces the classification inside an enclosure so general-purpose equipment may be installed.

ISA-5.1 defines P&ID instrument letter codes: the first letter is the measured variable (F flow, T temperature, P pressure, L level) and succeeding letters describe function (T transmitter, C controller, I indicator).

Intrinsic safety grounding is single-point, dedicated, and typically 1 ohm or less to earth, and must be kept separate from power and lightning grounds per the barrier manufacturer's instructions.

Liquid-service impulse lines slope at least 1/8 in/ft (about 1 percent) down toward the transmitter or drain; gas lines slope up toward the transmitter; steam lines slope toward a condensing pot.

Pressure tap location: gas service at the top of the pipe, liquid service at the side, steam service at the top with a pigtail or condensing chamber to protect the element from live steam.

Orifice plate: the sharp square edge faces upstream and the bevel faces downstream; typical minimum straight run is 10 pipe diameters upstream and 5 downstream, more when fittings are present.

Thermowell guideline: immersion of at least 10 sheath diameters for the sensing length and at least one-third of the pipe diameter, within the velocity and vibration limits of the well.

DP level measurement: the high side connects to the lower tap, and a filled (wet) reference leg produces an elevation or suppression equal to the static head of the leg.

Manifold operation: open the equalizer before closing the block valves to isolate a DP transmitter; when returning to service, open the block valves first and close the equalizer last.

API 670 proximity probe setup: linear range of about 40 mils, sensitivity near 200 mV/mil on 5 mm and 8 mm probes, and a static gap set near minus 9 VDC.

Mount transmitters on rigid brackets that avoid structural resonance, keep electronic housings out of direct sun and washdown, and orient liquid-filled legs so they stay flooded.

Swagelok-style tube fittings: tighten 1-1/4 turns past finger-tight on 1/4 in tube (about 1 turn on 3/8 and 1/2 in), confirm with a gap gauge, and avoid over-tightening the ferrule.

Minimum bend radius for instrument tubing is about 3 tube OD; use a lever bender and keep bends away from the fitting hex so the connection is not stressed.

Support tubing about every 3 ft horizontally and 4 ft vertically, with extra support within a few inches of valves, manifolds, and fittings.

Route shielded signal cable with the shield grounded at one end only, normally at the control room, so ground-loop currents do not add noise to the loop signal.

Maximum loop resistance = (supply voltage minus 12 V) / 0.020 A; a 24 VDC supply allows up to 600 ohms total, including indicators, barriers, and long cable runs.

NEC Chapter 9 conduit fill: 53 percent for one conductor, 31 percent for two, 40 percent for three or more, based on total insulated conductor area.

Terminate no more than two conductors under a single pressure terminal unless the terminal is listed for more; use ferrules on stranded wire and apply the manufacturer's torque.

Thread sealant such as PTFE tape or paste is applied to NPT pipe threads only, starting one or two threads back from the end, never on flare, compression, or tube-fitting threads.

Five-point calibration: 0, 25, 50, 75, and 100 percent of span ascending, then repeated descending, recording as-found and as-left values plus the direction of any deviation.

Error in percent of span = (indicated minus true value) / span x 100; a common acceptance tolerance for a smart transmitter is plus or minus 0.5 percent of span.

4-20 mA mapping: 4 mA = 0 percent (LRV), 12 mA = 50 percent, 20 mA = 100 percent (URV); each milliamp equals 6.25 percent of span.

Adjust zero before span and then repeat both, because the two interact; verify the result against an independent precision source rather than the same communicator.

DP flow transmitters extract a square root: flow percent = the square root of DP percent, so 4 mA is zero flow and 25 percent DP corresponds to 50 percent flow.

HART communication needs at least 250 ohms of loop resistance; connect the communicator in parallel with the loop or across the sense resistor.

Pt100 RTD: 100.00 ohms at 0 degC and about 0.385 ohm per degC; a 4-wire connection cancels lead resistance and the measured resistance plus tables gives temperature.

A loop check verifies continuity, polarity, grounding, tag numbers, and end-to-end response from the sensor through the DCS or PLC to the final control element, including alarms.

Proportional band in percent = 100 / gain; a 100 percent proportional band equals a gain of 1, and a wider band gives a slower, more stable response.

Integral setting: repeats per minute = 1 / (minutes per repeat); derivative acts on rate of change and is usually expressed in seconds or minutes.

Fail-safe valve selection: an air-to-open actuator with spring return drives the valve closed on air failure (fail closed), while air-to-close drives it open (fail open).

Cascade control requires the secondary loop to be significantly faster than the primary; the primary controller output becomes the secondary controller setpoint.

Split-ranging assigns portions of one controller output to two valves, for example 4-12 mA to valve A (0-100 percent) and 12-20 mA to valve B (100-0 percent).

Standard signals: 4-20 mA current, 1-5 VDC across a 250 ohm resistor, and 3-15 psig pneumatic, where 3 psi is 0 percent, 9 psi is 50 percent, and 15 psi is 100 percent.

Ratio control sets slave flow equal to ratio times wild flow; the ratio station multiplies the measured wild flow to generate the remote setpoint.

Control valve Cv equals the water flow in gpm at 60 degF that passes through the fully open valve with a 1 psi pressure drop.

Indication below 4 mA or pegged at zero points to an open loop, reversed polarity, missing loop power, or a blown fuse; confirm 24 VDC under load and check continuity.

A DP flow reading that is low or frozen often means a plugged impulse line, a closed block valve, or trapped gas in a liquid-filled leg.

Wet-leg level error: loss of fill in the reference leg lowers low-side pressure, raises DP, and drives the indication falsely high; refill the leg and re-zero the transmitter.

RTD diagnosis: a near-zero ohm reading indicates a short and an infinite reading an open; compare measured resistance to the standard table at ambient temperature.

Thermocouple diagnosis: read the millivolt output and apply cold-junction compensation; drifting or noisy readings often come from a degraded junction, moisture, or a ground loop.

Ground loops appear as 50/60 Hz noise, offset, or drift; correct them by grounding the shield at one end only or by inserting a signal isolator.

Never apply an insulation tester (megger) to signal wiring while transmitters, DCS cards, barriers, or surge protectors are connected to it.

A supply that reads 24 V open-circuit may collapse under load; measure voltage at the transmitter terminals with the loop in service to confirm power and voltage drop.