The Level 2 method: checkout results, leak and compliance, the subsystem map and your instruments
Treat a failed checkout or a clinical complaint as evidence, place it on a map of the machine's subsystems, separate them with a test lung, analysers and the machine's own tests, and know which adjustments and repairs are yours to make.
Where Level 2 starts
At Level 1 you learned to run the checkout, replace the PM kit, verify the machine with an analyser and write the report. Level 2 starts when that routine no longer ends in a pass: the morning test fails on the same step every day, the anaesthetist says the bellows does not fill, the oxygen reading drifts during long cases, or the machine passes every test and the complaint still comes back. Your job is now to say which part of the machine is at fault, to prove it with a test rather than a guess, and either to repair it within your authorisation or to hand it on with evidence.
Two rules do not change from Level 1. The machine you are fault-finding is out of clinical use, labelled, and not connected to a patient; if it is needed for a case, the team takes another machine and a self-inflating bag stays available. And the gas supplies are under your control: you know which pipeline outlets and cylinders are connected, and you can isolate them.
The checkout result and the log are evidence
A modern workstation tells you much more than pass or fail, if you read it. The checkout screen usually names the step that failed and often gives a number: a leak in millilitres per minute, a compliance value, a pressure that was not reached or not held. Write the number down before anyone repeats the test. A leak that is just over the limit and stable is a different fault from one that has doubled in a week, and both are different from a test that fails because a pressure was never built up at all.
Next, open the history. Most electronic machines keep a log of checkouts and technical alarms with time stamps, and many record when a checkout was bypassed. The GE Carestation, for example, records a bypassed checkout in its system log, and the Dräger Fabius GS service log stores coded entries grouped by function, with a whole group for the compliance test that distinguishes, among other things, a system that cannot be pressurised from one that cannot hold its pressure. Read the log for the first abnormal entry, not the latest. On an anaesthesia machine a supply problem at 07:40 can produce a ventilator alarm at 07:41 and a checkout failure at 07:45; the first entry is closest to the cause. Check that the clock is right before you trust the order of events.
Finally, talk to the user who saw it. Ask what they set, what they saw, when it happened in the case, what circuit and accessories were fitted, and whether anything had been changed: a new absorbent supplier, a different circuit, a vaporiser moved from another machine, work on the hospital's air plant. Many anaesthesia faults are explained in that conversation.
Leak and compliance: what the numbers mean
A leak test closes a part of the gas path, pressurises it and measures either how fast the pressure falls or how much flow is needed to hold it. A leak value therefore belongs to a zone: the zone the test sealed. The breathing system test on most machines seals the circuit at the patient connection; the low-pressure test seals the fresh gas path from the flow controls to the common gas outlet; a high-pressure test looks at a cylinder and its yoke. Before you search for a leak, be sure which zone the failed test was looking at.
Compliance is the volume the breathing system and circuit take up for each unit of pressure, usually given in millilitres per centimetre of water. The machine measures it during the checkout and uses it to correct delivered and displayed volumes. That gives you a diagnostic tool. A compliance much higher than usual means extra volume or a softer part in the circuit: a longer or more expandable patient circuit, an extra filter or humidifier, a reservoir bag in the circuit during a test that expects it isolated. A compliance much lower than usual means part of the system is not taking part: a blocked branch, a closed valve, a circuit not connected the way the test expects. A machine that cannot build pressure at all is usually open somewhere large: a missing seal, an unlatched absorber canister, a disconnected hose.
Every automated test assumes a configuration: which circuit, where the filters are, whether the bag is connected, what the switches are set to. The machine's screen or the operator's manual describes it. If the test fails, the first question is whether the test was run as designed.
The subsystem map
For fault-finding, split the machine into seven blocks and give each a question. Gas supply: does gas of the right kind arrive at the right pressure, also under flow? Flow control: does each gas leave the flow control at the set flow, and do the oxygen ratio protections work? Vaporisers and the manifold: is the vapour concentration right and is the mount gas-tight? Breathing system: is the circle tight, do its valves work, does the absorber remove carbon dioxide? Ventilator: does it deliver the set volume and pressure and release on expiration? Monitoring: are pressure, volume, oxygen and gas readings true? Power and electronics: are supplies, batteries and boards healthy?
Most symptoms point at two or three blocks, not one. Low delivered volume can be a leak in the breathing system, a ventilator that does not deliver, or a flow sensor that reads wrong. Low inspired oxygen can be a supply fault, a flow control fault, a leak that draws in room air in a circuit at negative pressure, or an oxygen cell that reads low. The skill is to choose the test that splits the candidates. The rest of the course goes through the blocks in the order gas flows; lesson 8 puts them back together.
- Measure independently before you believe a display: an analyser at the patient connection settles whether the machine or its sensor is wrong.
- Split by zone: run the test that seals only one zone, or move the seal point, and watch which zone the leak follows.
- Split by gas: a fault on one gas only points at that gas's supply, valve or sensor.
- Split by mode: a fault in mechanical ventilation that is absent in manual mode points at the ventilator side.
- Swap a known-good part of the same type and approved for the model, and see whether the fault follows the part.
Your instruments, and how to trust them
Level 2 work needs a small set of instruments that you trust more than the machine. A gas flow analyser measures flow, volume and pressure independently and, with an oxygen sensor, the oxygen concentration. A multi-gas or agent analyser checks vaporiser output and the machine's gas monitor. A digital manometer and a calibrated flowmeter cover leak and regulator tests. A test lung with a known compliance and a set of known-good test circuits lets you separate the machine from its accessories. Manufacturers also specify test fixtures for some tests, such as low-pressure and breathing system leak test devices; the Fabius GS service manual lists both.
An instrument you have not checked is a second unknown. Before you condemn a machine, confirm that your test lung does not leak, that your analyser is within its calibration date and reads room air and pure oxygen correctly, and that your test circuit gives the leak and compliance values you have seen before on a good machine. Keep a note of those reference values. Engineers have spent hours on a machine whose only fault was a split test lung.
What you may and may not change
Fault-finding tempts you to adjust things until the fault disappears. On an anaesthesia machine that is dangerous, because several adjustments are safety settings. The pressure of the second-stage regulators, the oxygen supply alarm threshold, the oxygen ratio device or hypoxic guard, the ventilator relief valves and the vaporiser calibration all protect the patient. The Penlon Prima 400 manual, for example, states that its mechanical anti-hypoxic device may be adjusted only by engineers trained or certified by the manufacturer, because a wrong adjustment can remove the hypoxia protection.
Use this simple test. You may make an adjustment or calibration only if the manufacturer's service documentation for that model describes it, you are trained and authorised for it, you have the specified tools and test gases, and you will verify the result afterwards with an independent instrument. Everything else - service passwords and option codes you have not been given, bypassing a protection to 'see if it works without it', drilling, jumpering or blanking parts, repairing vaporisers or sealed modules - is outside your scope. When the fault is in such an area, your result is a clear report and a request to the party who is authorised.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Gas flow analyser | Independent flow, volume, pressure and O2 | Out of calibration; dirty or wet flow cell |
| 02 | Agent / multi-gas analyser | Checks vaporiser output and the gas monitor | Wrong agent setting, expired calibration |
| 03 | Test lung | Known compliance load for ventilation tests | Split bag or loose fitting: false leak |
| 04 | Known-good test circuit | Separates machine faults from accessories | Becomes 'bad' unnoticed if never re-checked |
| 05 | Digital manometer | Regulator, relief valve and leak-down tests | Zero drift; leaking hose or adapter |
| 06 | Leak test devices | Seal the low-pressure or breathing system | Bulb or seal leaks; test device must be checked first |
| 07 | Known-good consumables | Flow sensor, O2 cell for swap tests | Not approved for the model: misleading result |
| 08 | Log and camera | Record screens, values and as-found state | Evidence lost when the test is simply repeated |
The filter that failed the test
An anaesthesia machine began to fail its breathing system self-test with a leak and compliance error. Nothing visible was wrong: the absorber was seated, the seals looked good, the bellows was intact. Another engineer asked how the circuit had been assembled for the test. The theatre team had started placing a third breathing filter at the Y-piece in front of the mask, in addition to the filters on the inspiratory and expiratory limbs. The test algorithm assumed filters only on the limbs, so the extra volume and resistance at the Y-piece pushed the measured values outside its limits. With the circuit assembled as the manual describes for the test, the machine passed every time.
An automated test is only valid in the configuration it was designed for. Before you look for a leak, rebuild the test set-up the way the manufacturer describes it.
The leak that depended on the hose
A Dräger Apollo gave unstable and high leak values in its leak test. The engineer tested with a short tube and with his own test circuit, which had given a stable leak of the same order on this machine for two years, and still got scattered results. Colleagues pointed out two things. The machine shows a diagram of how the circuit must be connected at the start of the test, and the test is only valid when that is followed. And this machine family copes poorly with corrugated expandable hoses in the test. Assembled as drawn on the screen, with filters on both limbs and a smooth silicone tube, the results became stable.
Use the test configuration the machine asks for and keep one reference test circuit whose values you know.
A flow sensor proved dead with a multimeter
A Dräger Fabius Tiro would not complete its flow sensor calibration and showed no volume. Before ordering anything, the engineer inspected the sensor and its cable and checked the conductors for continuity with a multimeter. One wire inside the sensor assembly was open. The sensor was replaced, the calibration passed and the volume display returned. Flow sensors on this type of machine are consumables that wear out with use and cleaning, so the result was not surprising, but the five-minute measurement turned a suspicion into a proven fault before the part was ordered.
A cheap measurement before the order saves the wrong part and the second visit.
On-site checklist
- Take the machine out of use; keep a self-inflating bag available.
- Photograph the failed step, values and log before any retest.
- Find the first abnormal log entry; check the clock.
- Ask what was set, what circuit was used and what changed.
- Verify your test lung, circuit and analyser against reference values.
- Split by zone, gas, mode or known-good part; change one thing at a time.
- Adjust only what the model's service documentation allows you to adjust.