Odisha CHSE +2 VOC Field Technician Air Conditioner common mistakes
Odisha CHSE +2 VOC Field Technician Air Conditioner · Understand the traps and how to avoid them.
Confusing absolute pressure with gauge pressure when reading a manifold gauge or stating a saturation temperature - quoting the gauge reading directly as the pressure in a P-T table lookup, so the saturation temperature comes out several degrees too low.
Manifold gauges read gauge pressure (above atmospheric), while refrigerant property tables and saturation charts are tabulated in absolute pressure. Candidates learn 'read the gauge, find the temperature' without internalising that 1 atm (about 1.013 bar / 14.7 psi) must be added first.
What to do instead
Before any P-T lookup, write the conversion explicitly: P_abs = P_gauge + P_atm. Practise with a known pair (e.g. R-22 at 0 barg reads about 5.6 bar abs, saturating near 2 °C, not the sub-zero value a gauge-only lookup gives). Always state in your answer which pressure you are quoting.
Describing the refrigeration cycle without correctly placing the metering device, compressor, condenser and evaporator, or reversing the high-side and low-side when tracing refrigerant flow.
Candidates memorise component names as a list rather than as a loop with a pressure boundary. The compressor and condenser sit on the high side, the metering device and evaporator on the low side; without that boundary the order becomes arbitrary.
What to do instead
Draw the four-point cycle from memory daily: compressor discharge → condenser → metering device → evaporator → compressor suction. Mark high side (discharge to metering device inlet) and low side (metering device outlet to suction) with a line, and label the state at each point (superheated vapour, subcooled liquid, two-phase mixture).
Treating all refrigerants as interchangeable - mixing up which refrigerant is an HCFC, HFC or natural refrigerant, or stating that R-22 is ozone-friendly or that R-410A is a drop-in replacement for R-22.
R-22 (HCFC) is still common in older Indian equipment and candidates generalise from what they see in the field. R-410A is an HFC blend with different pressures and requires POE oil, so it is not a drop-in; this distinction is frequently tested and frequently missed.
What to do instead
Build a one-page table with columns: refrigerant, family (CFC/HCFC/HFC/HFO/natural), ODP, GWP, lubricant, and whether it is a drop-in for anything. Learn at minimum R-22, R-32, R-134a, R-410A, R-407C, R-290 and R-717 (ammonia) against those columns.
Answering ozone-depletion questions with vague statements such as 'CFCs are bad for the environment', without naming chlorine as the ozone-destroying atom, the stratospheric ozone layer as the site of action, or the Montreal Protocol and its phase-out schedules.
Candidates conflate ozone depletion with global warming, so they give a greenhouse-gas answer to an ozone question. The two are separate mechanisms: ODP depends on halogen content and atmospheric lifetime, GWP on infrared absorption.
What to do instead
For every ozone question, state the mechanism in three steps: UV splits the CFC releasing Cl; Cl catalytically converts O3 to O2 (one Cl can destroy many thousands of O3 molecules); the Cl is regenerated. Then name the Montreal Protocol and the phase-out order (CFCs first, then HCFCs, then HFC phasedown under the Kigali Amendment).
When tracing an electrical fault in a split AC, jumping straight to the compressor or the PCB instead of first checking supply voltage, the remote/handset setting, the thermostat, and the safety devices (HP/LP cut-out, overload, door/float switch).
Compressor and PCB failures are memorable and dramatic, so candidates anchor on them. In practice a large share of 'not cooling' calls are supply, setting, filter, or a tripped safety device, and exam scenarios are written to reward a systematic sequence.
What to do instead
Use a fixed sequence in every troubleshooting answer: supply present and correct voltage → control setting and thermostat calling → safety devices closed → control circuit continuity → load (fan, compressor) windings and current. State the sequence explicitly in the answer, then give the reading that confirms or eliminates each step.
Ignoring superheat and subcooling and diagnosing charge purely by 'the suction line feels cold' or by gauge pressure alone, leading to overcharging or undercharging diagnoses that are wrong.
Feeling pipes is what technicians do in the field, but pressure alone varies with ambient and indoor load. Superheat (evaporator exit) and subcooling (condenser exit) are the charge indicators, and candidates often cannot compute them or do not know target values.
What to do instead
Practise the two calculations until automatic: superheat = suction line temp − saturation temp at suction pressure; subcooling = saturation temp at discharge pressure − liquid line temp. Learn typical targets for a fixed-orifice system (superheat around 8–12 K) and a TXV system (subcooling around 5–8 K), and always state both the measured value and the target.
Describing evacuation and charging steps without the mandatory sequence: leak test, evacuate to a specified vacuum (typically 500 microns / about 0.7 mbar) and hold it, then charge by weight or by superheat - and omitting that refrigerant must never be vented to atmosphere.
Candidates compress installation into 'vacuum and fill', skipping the standing vacuum test that proves no moisture or leak remains, and they do not connect recovery to the legal and environmental requirement.
What to do instead
Memorise the commissioning checklist as an ordered list: pressure/leak test with dry nitrogen → triple evacuation with nitrogen breaks → standing vacuum test (hold below 500 microns for at least 10–15 minutes with the pump isolated) → charge by weight from the nameplate, verified by superheat/subcooling → record all readings. Add 'recover, never vent' as a standing line in every charging answer.
Using a multimeter or clamp meter incorrectly in a live circuit - measuring voltage with the meter set to resistance/continuity, clamping the clamp meter around both supply conductors and reading zero, or testing a capacitor without discharging it.
Candidates learn meter functions in isolation and do not rehearse the setup step (function, range, lead placement) before each measurement. Clamp meters reading zero on a two-core cable is a classic exam trap, and capacitor discharge is a genuine safety point that is often skipped.
What to do instead
Adopt a fixed pre-measurement ritual: identify what you are measuring → select function and range → verify lead/jaw placement → confirm on a known source if unsure. For current, clamp one conductor only. For capacitors, discharge through a resistor and verify 0 V before touching. State the ritual in answers involving measurement.
Writing safety answers as attitude statements ('work carefully', 'wear PPE') instead of naming the specific hazard, the specific control, and the specific procedure - for example not stating that brazing requires nitrogen purging, that refrigerant vapour is heavier than air and displaces oxygen in pits, or that PPE for refrigerant work includes goggles and gloves against frostbite.
Safety is taught as a general induction rather than as hazard-specific content, so candidates default to generic phrases. Marking schemes award points for named hazards and named controls.
What to do instead
For each task in the syllabus (brazing, evacuation, charging, electrical work, working at height), write hazard → control → emergency action as three short lines. Include the numbers and named items: nitrogen purge while brazing, no smoking near refrigerant, ventilation for heavier-than-air vapour, goggles/gloves for liquid refrigerant contact, lockout of the supply before electrical work.
In the practical/project answer, describing the job without stating the instruments used, the readings taken, the acceptance criteria, and the customer handover - so a 'commissioning' answer has no measured values and a 'service call' answer has no final verification.
Candidates narrate what they did physically and omit the documentary half of the trade, which is exactly what the practical and project assessment rewards. It also reflects a real gap: handover and record-keeping are commonly skipped on site.
What to do instead
Structure every practical answer with five headings: tools and instruments used → readings taken (with units) → acceptance criteria (with numbers) → corrective action if out of range → handover (demonstrate to customer, explain filter cleaning and thermostat setting, record readings on the job sheet). Rehearse this template on three standard jobs: new split installation, gas top-up, and no-cooling complaint.