IFPS CFPS common mistakes
IFPS CFPS · Understand the traps and how to avoid them.
Treating gauge pressure and absolute pressure as interchangeable when applying the ideal gas law or sizing a pneumatic actuator, so the force or volume result is off by roughly one atmosphere.
Most field gauges read gauge pressure, and candidates carry that habit into calculations where Boyle's law and F = p × A require absolute pressure; the ~100 kPa offset looks small as a number but changes the answer materially at low pressures.
What to do instead
Before every gas-law or force calculation, write down which pressure the formula expects and convert explicitly: p_abs = p_gauge + p_atm. When a problem states a gauge reading, immediately annotate it with the absolute equivalent so the conversion is visible in your working.
Computing actuator force from bore area alone and ignoring the rod-side annulus on retraction, or forgetting to subtract the rod area when the question asks for return force.
The extension case (full bore area) is the one practised most, so candidates default to it even when the rod is on the pressure side; the rod area is small relative to the bore, making the error easy to overlook.
What to do instead
Always sketch the cylinder with the rod on the correct side and label the effective area before substituting numbers: A_extend = πD²/4, A_retract = π(D² − d²)/4. Check the direction of motion named in the stem and confirm which area is pressurised.
Mixing up flow rate, actuator speed, and displacement, for example using pump displacement in litres per minute directly as flow without multiplying by drive speed.
Displacement (cm³/rev) and flow (L/min) share the same family of units and are often quoted side by side in the same problem, and candidates substitute one for the other under time pressure.
What to do instead
Keep a unit ledger: write displacement with per-revolution units, flow with per-minute units, and show the rpm conversion explicitly. Sanity-check the magnitude - a 50 cm³/rev pump at 1500 rpm must give about 75 L/min, not 50.
Confusing pressure drop across a flow-control valve with system pressure, or assuming a pressure-compensated flow control holds flow constant regardless of load.
Candidates memorise 'flow control sets speed' without distinguishing meter-in, meter-out, and bleed-off circuits, and without recognising that only the pressure-compensated version decouples flow from load pressure.
What to do instead
Learn the three circuit types by what they control and where they sit, and state the load dependence for each. For any valve question, first ask whether it is pressure-compensated; if not, flow varies with load and the answer must reflect that.
Ignoring pressure drop and parasitic losses when calculating pump input power, using output hydraulic power as if it equalled shaft power.
The formula p × Q gives hydraulic power cleanly, and overall efficiency is a single multiplier that is easy to drop when the stem does not mention it explicitly.
What to do instead
Write the chain explicitly: hydraulic power = p × Q, shaft power = hydraulic power / overall efficiency, and check whether the question asks for input or output. If efficiency is unstated, note the assumption in your reasoning rather than silently omitting it.
Applying hydraulic formulas to compressed air without accounting for the compressibility of the medium, for example treating air flow as incompressible when sizing lines or estimating actuator speed.
The hydraulics and pneumatics domains share symbols and formulas, and candidates transfer hydraulic intuition directly; air's compressibility and its pressure-dependent volume flow are not intuitive.
What to do instead
For any pneumatic calculation, convert to free air delivery (normal or standard litres) before comparing with compressor ratings, and remember stored energy in receivers. Treat actuator speed as load- and pressure-dependent, not a simple Q/A relationship.
Misreading ISO 1219 pneumatic symbols, particularly 5/2 versus 5/3 directional valves, spring return versus detent, and the difference between pilot and solenoid actuation.
Symbols are compact and similar at a glance; candidates learn them visually from diagrams rather than by the port and position conventions, so small differences in end caps or centre conditions are missed.
What to do instead
Learn to read symbols position by position: count the boxes for positions, count ports on one box, and identify actuation from the end markings. Practise describing a symbol aloud in words before answering, and confirm the centre condition for 3-position valves.
Sizing pneumatic cylinders or choosing compressors using only the force at full stall pressure, ignoring air consumption per cycle and duty cycle when the question asks for average demand.
Force calculations are the familiar part, and average consumption requires multiplying by cycles per minute and summing extend and retract volumes - steps that are easy to skip under exam timing.
What to do instead
For consumption questions, compute the swept volume for both strokes, convert to free air using the compression ratio, then multiply by cycle rate. Show each step; the mark scheme rewards the method even if the final arithmetic slips.
Treating a proportional or servo valve as a simple on/off directional valve, so answers assume full flow or no flow rather than flow proportional to the command signal.
Candidates from a purely pneumatic or basic hydraulic background have not internalised current-to-flow or voltage-to-flow relationships, and the exam stem may describe a valve without naming it as proportional.
What to do instead
Look for words like 'ramp', 'command signal', 'spool position feedback', or '0–10 V' and treat these as proportional. Learn that flow scales with command and that dead band and hysteresis exist; answer accordingly rather than in binary on/off terms.
Designing or interpreting relay and ladder logic without tracing the holding (latch) circuit, so a momentary start button is assumed to keep the circuit energised after release.
Ladder diagrams are read left to right as static pictures, and candidates forget that a momentary pushbutton opens again; the seal-in contact is a single parallel branch that is easy to skim past.
What to do instead
Trace the circuit with the button released, not just pressed: ask what holds the coil in after the input drops. Practise drawing the seal-in contact yourself, and for any 'what happens when the button is released' question, re-trace from the supply rail.