13.5 Troubleshooting Diagnostic & Therapeutic Equipment
Key Takeaways
- The ACI outline requires the technician to identify fault conditions and apply appropriate corrective action separately for diagnostic equipment and for therapeutic equipment; both are distinct examinable sub-topics from monitoring, infusion, perioperative and laboratory troubleshooting.
- Diagnostic-set faults resolve in a fixed order — handle charge, lamp voltage rating, lamp condition, fibre bundle, then contacts — and fitting a 2.5 V lamp in a 3.5 V handle destroys it instantly while a 3.5 V lamp in a 2.5 V handle gives a dim yellow beam that is often misdiagnosed as a failing transformer.
- Spirometer under-reading is almost always the sensing element rather than the electronics: condensate or saliva in a pneumotachograph, a bent or sticking turbine vane, or protein and aerosol film insulating a hot-wire element; a 3.00 L syringe check at several flow rates separates a leak from a calibration error.
- Position-dependent scale error indicates a load cell, cable or debris fault at one corner; a uniform offset across the platform indicates zero drift or a span calibration error — the distinction determines whether the fix is a calibration or a component replacement.
- Therapeutic-device faults concentrate on the safety layer: verify the independent over-temperature cutoff on warmers and incubators, verify phototherapy irradiance with a radiometer rather than by eye, and verify SCD cuff pressures and leak-down rather than assuming the display, because in every case the protective element fails silently.
Troubleshooting Diagnostic & Therapeutic Equipment
The Problem Solving domain of the ACI outline names diagnostic equipment and therapeutic equipment as two separate corrective-action sub-topics:
- "Identify the fault conditions and apply appropriate corrective action for diagnostic equipment (for example, otoscope, ophthalmoscope, scales, stress test, audiometer, ECG, spirometer)."
- "Identify the fault conditions and apply appropriate corrective action for therapeutic equipment (for example, neonatal/pediatric equipment, patient temperature management equipment, aspiration equipment, SCD, physical therapy equipment)."
These devices are numerous, geographically scattered across clinics and therapy departments, and often serviced under time pressure with no service manual to hand. A disciplined fault tree matters more here than anywhere else.
1. Diagnostic Sets: Otoscopes & Ophthalmoscopes
Work the chain from the wall outward.
| Symptom | Ordered checks | Most common resolution |
|---|---|---|
| No light at all | Transformer base output → handle charge → lamp continuity → handle-to-head contact ring | Discharged or end-of-life rechargeable handle |
| Dim, yellow light | Lamp voltage rating vs handle rating (2.5 V vs 3.5 V) → battery under load → contact oxidation | Wrong lamp fitted, or a tired NiCd holding voltage at rest but sagging under load |
| Lamp fails within seconds of replacement | Lamp rating vs handle rating; measure handle open-circuit voltage | 2.5 V lamp fitted into a 3.5 V handle |
| Dark speckles in the projected spot | Fibre-optic bundle | Broken fibres from a dropped head; head replacement |
| Intermittent, flickers when handled | Contact ring, lamp base, handle threads | Clean and retension contacts |
| Pneumatic otoscope will not insufflate | Sealing ring on the head, bulb, tubing, speculum fit | Missing or perished rubber sealing ring |
| Ophthalmoscope dioptre wheel skips | Detent mechanism, Rekoss disc | Clean and lubricate per manual, or replace head |
Two traps worth naming. First, battery voltage measured at rest lies. A NiCd handle that reads full open-circuit can collapse under the half-amp a halogen lamp draws; measure under load. Second, an LED conversion is not always a drop-in. Some LED lamps require a different drive current or polarity and will not work — or will not last — in a legacy handle.
2. Audiometers
| Symptom | Likely cause | Corrective action |
|---|---|---|
| Intermittent tone or crackling on one channel | Headphone cord conductor fatigue near the strain relief | Replace the cord/transducer; recalibrate that pairing |
| Output level wrong at some frequencies only | Attenuator or frequency-generator fault; transducer aging | Full electroacoustic calibration to ANSI/ASA S3.6 |
| Crosstalk between channels | Attenuator leakage, cable routing | Service; verify crosstalk in calibration |
| Distorted tone at high levels | Amplifier clipping or transducer damage | Verify harmonic distortion during calibration |
| Thresholds inconsistent across sessions | Ambient room noise | Verify the booth against ANSI/ASA S3.1 before blaming the instrument |
| Bone oscillator reads out of tolerance | Oscillator damage or artificial-mastoid coupling error | Recalibrate with a proper artificial mastoid |
The critical rule: a transducer and an audiometer are calibrated as a pair. Replacing a headset, an insert phone or a bone oscillator invalidates the calibration until the new pairing is calibrated. Handing back an audiometer with a new headset and no recalibration produces clinically wrong thresholds that nobody detects.
3. Spirometers
The single most common complaint is under-reading volumes, and it is nearly always the sensing element.
Fault tree for "spirometer reads low."
- Verify with a 3.00 L syringe at several flow rates. If error grows with flow, suspect resistance or turbulence in the element. If error is constant, suspect calibration or a leak.
- Leak test the whole path — mouthpiece adapter, filter, tubing, sensor housing seals. Leaks under-read at every flow.
- Inspect the sensing element by type. Pneumotachograph: condensate or saliva in the capillary bundle or mesh, or a failed heater allowing condensation. Turbine: bent vane, sticky bearing, dirty optical interrupter. Hot-wire: protein or nebulized medication film insulating the wire — the wire looks fine to the naked eye but is thermally blanketed. Ultrasonic: contaminated transducer windows.
- Check ambient inputs. BTPS correction uses temperature, barometric pressure and humidity. A failed ambient sensor or an un-entered altitude produces a systematic error that looks like a calibration fault.
- Recalibrate only after 1 to 4 are clean. Calibrating around a leak buries the fault.
Remember the acceptance criterion for daily calibration verification under the ATS/ERS 2019 update: ±3% of the 3.00 L syringe at every tested flow, so roughly 2.91 to 3.09 L. The tighter ±2.5% figure is the ISO 26782 maximum permissible error for the device itself, not the daily bench criterion.
4. Scales
| Symptom | Interpretation | Action |
|---|---|---|
| Reading changes with weight position on platform | Corner load error — one load cell not carrying its share | Inspect that cell and its cable; clear debris bridging platform to frame |
| Uniform offset at all positions | Zero drift or span error | Re-zero, then span-calibrate with certified weights |
| Reading drifts steadily while loaded | Creep in a load cell, or temperature effect | Replace cell; allow thermal stabilization before calibrating |
| Erratic, jumping display | Damaged load-cell cable, poor shield ground, EMI | Inspect and replace cable; verify shielding and ground |
| Reads zero with weight applied | Open load cell or failed excitation supply | Measure bridge excitation and output |
Always calibrate with certified traceable test weights at multiple points across the range, and record as-found and as-left values. Because paediatric and oncology dosing is weight-based, an uncorrected scale error is a medication-error pathway, not a nuisance.
5. Stress-Test Systems
| Symptom | Cause | Action |
|---|---|---|
| Treadmill speed does not match protocol stage | Speed sensor, drive belt slip, controller calibration | Verify with tachometer; adjust belt tension; recalibrate |
| Elevation does not track commanded grade | Elevation motor, limit switch, position feedback pot | Verify feedback; recalibrate elevation |
| Belt slips or drifts to one side | Belt tension and tracking adjustment; worn deck | Adjust tracking; replace deck/belt if worn |
| Emergency stop does not halt the belt immediately | E-stop switch, safety-clip lanyard magnet, controller relay | Remove from service until repaired |
| Severe ECG artifact during exercise | Skin prep, electrode adhesion, cable strain relief, harness | Improve prep and strain relief; replace worn cables |
| ST measurements look wrong or flattened | Acquisition left in monitoring bandwidth | Restore diagnostic bandwidth 0.05–150 Hz |
The emergency-stop and safety-clip verification is the single most important test on the device, and it is the one most often skipped because the treadmill "runs fine."
6. Therapeutic Equipment: The Safety Layer Fails Silently
Across every therapeutic device in this list, the pattern is the same: the primary control loop keeps working and the protective element fails without announcing itself. Verification must therefore target the protection, not the function.
Neonatal phototherapy. Complaint: "the baby's bilirubin isn't coming down." The lamp still looks bright. Measure spectral irradiance with a radiometer at the manufacturer's specified distance in the 430–490 nm band; LED and fluorescent emitters degrade gradually and invisibly. Check the emitter hour meter, the distance actually used at the bedside, and any yellowed or scratched diffuser.
Infant incubators and radiant warmers. Verify all three temperature layers independently: the servo skin probe (substitute a calibrated probe or resistance standard), the primary air-temperature control, the independent secondary air cutoff at 38 °C, and the hardwired thermal cutout at 40 °C. A warmer that heats correctly but whose independent cutoff has failed is more dangerous than one that does not heat at all, because nothing reveals the defect until it matters.
Patient temperature management (hypo/hyperthermia blankets, forced-air warmers). Measure delivered temperature with an independent thermometer at the patient end, verify the secondary safety thermostat, and verify hose-disconnect and over-temperature alarms. For circulating-fluid systems, check for leaks, correct fluid, pump flow and blanket integrity.
Aspiration and suction. Loss of suction resolves in a fixed order: canister lid seal → overflow float valve stuck closed → hydrophobic filter wetted and occluded → tubing kink or crack → regulator drift → pipeline terminal. Verify regulator setting against a calibrated vacuum gauge, not the dial, and perform a leak-down test. A stuck-open float is the serious one: it allows aspirate into the pipeline.
Sequential compression devices. Complaints are usually "the sleeve doesn't inflate" or "it alarms constantly." Verify each chamber's peak pressure against specification (Kendall-style gradient: 45 mmHg ankle, 40 mmHg calf, 30 mmHg thigh), verify cycle timing (approximately an 11-second compression with a decompression interval), and verify leak-down. Faults concentrate in the sleeve and tubing — pinholes, cracked quick-connects, kinks under bedding — followed by the compressor diaphragm and the solenoid manifold. Vascular-refill-detection models add a sensing function that must also be verified.
Physical therapy modalities. Electrotherapy: verify the waveform on an oscilloscope across a 500 ohm load — it must be charge-balanced biphasic, and any DC offset is a burn hazard requiring immediate removal from service. Verify that intensity truly reaches zero at minimum. Therapeutic ultrasound: verify acoustic output on a radiation force balance; a delaminated crystal still warms the head while delivering a fraction of the indicated power. Traction units: verify applied force against a calibrated dynamometer and verify the patient-held emergency release. Hydrocollators and paraffin baths: verify thermostat and over-temperature cutoff, because contact burns are the failure mode.
A clinic reports that a new otoscope lamp burned out within seconds of installation in a rechargeable handle. What should the technician check first?
A spirometer under-reads a 3.00 L calibration syringe by an increasing margin as the technician discharges the syringe faster. What does this flow-dependent error pattern indicate?
An infant radiant warmer maintains its skin-servo set point accurately, but bench testing shows that the independent secondary air-temperature cutoff no longer interrupts heater power when the cutoff threshold is simulated. Why is this a higher-priority defect than a warmer that fails to heat at all?
A physical therapy department returns a TENS unit after a patient reported a small skin burn under one electrode. Bench testing across a 500 ohm resistive load shows the output pulses are biphasic but have a measurable net DC offset. What is the correct action?