The WATO EX platform: models, assemblies and the gas path of this machine

Identify which WATO EX model and configuration is in front of you, name its main pneumatic assemblies and boards, and follow its gas from the wall to the scavenging outlet, knowing where this machine differs from the generic workstation of anes-l1.

One platform, four badges

The WATO EX-20, EX-25, EX-30 and EX-35 are one family. They share a cart, a gas delivery system with mechanical flow control, an electronically controlled bellows ventilator, the same boards and the same service manual. The manufacturer's service manual is written for the maximum configuration and covers the whole family, with separate notes where a model differs. The breathing system design is taken from the larger WATO EX-55/65 machines, which is useful to know when you read field reports from those models: a good part of what happens in their breathing circuit applies here too.

Before you touch anything, establish exactly what you have. Read the data plate for model and serial number, open the system information screen for the software versions, and walk round the machine noting the gas configuration, the number of cylinder yokes, the vaporiser manifold, ACGO, O2 sensor, gas module, AGSS and suction. Write it down. Every later decision depends on it: which seals are in the annual kit, which tests apply, which software package is allowed and which leak results mean what.

In anes-l1 you learned the generic gas path and the generic tests. This course assumes all of that. Here we only add what is specific to this machine: where its regulators and switches sit, which pressures the manual gives for them, how its ventilator is built, which tests and calibrations its software offers, and which faults engineers actually meet on it.

How the models differ

The differences inside the family are mostly configuration, not architecture. Treat the list below as a checklist for identifying the unit; the exact feature set of a given machine is defined by its order, its options and its software, so confirm it on the machine and in its documentation rather than from the badge alone.

Two differences matter most in daily service. First, the EX-20 uses a different CO2 absorbent canister arrangement, so its canister seals have their own part numbers and the large BYPASS seal of the bigger models does not apply to it. Order the annual kit and the canister seals for the actual model. Second, on machines with cylinders, count the yokes: the four-yoke version has its own pneumatic diagram in the manual and its own cylinder inlet layout, and if suction is fitted it brings a Venturi vacuum generator and a liquid collection bottle into the gas path.

• Gas configuration: O2 only, O2 + N2O, O2 + Air, or all three gases, each from pipeline and, where fitted, cylinder yokes.
• Cylinder yokes: standard layout or a four-yoke configuration with its own inlet assembly.
• Vaporiser manifold: single or double, Selectatec-type mounting with interlock on the double manifold.
• ACGO: absent, combined with the breathing system outlet, or a separate outlet (with or without pressure sampling); the variant is set in the configuration and must match the hardware.
• Display and keypad: 7-inch or 8-inch display, each with its own conversion or inverter board.
• Ventilation modes: volume control is the base mode; pressure control and other modes depend on model and options.
• Options: O2 sensor (can be enabled later), CO2 module, breathing system heating module, AGSS (low-flow or high-flow), negative pressure suction, isolation transformer for the auxiliary outlets.
• CO2 absorbent canister: EX-20 has its own canister and seal set; EX-25/30/35 share another.

Supplies and the regulator chain

Each pipeline gas enters through its own non-interchangeable inlet with a filter and a check valve. The manual gives a normal pipeline range of 280 to 600 kPa, and you will use that range in the pipeline test. Cylinders enter through pin-indexed yokes and a cylinder regulator that brings them down to about 400 kPa. If both are open, the machine draws from whichever supply is at the higher pressure, so keep cylinder valves closed while the pipeline is in use; the cylinder is the reserve if the pipeline fails. A relief valve on the supply side opens at roughly 750 kPa to protect the machine if a regulator fails high.

After the inlet, each gas passes a second regulator that reduces it to about 200 kPa. This is the working pressure of the flow control and of the O2 flush. On the O2 inlet sits a pressure switch that raises the O2 Supply Failure alarm when the O2 supply falls below about 200 kPa. Remember these three levels: pipeline or cylinder regulator on the outside, about 200 kPa inside, and the switch that watches O2. Most supply faults on this machine are found by measuring one of them with an external gauge.

The O2 inlet also feeds the ventilator. Drive gas is taken before the flow controls, so the ventilator keeps running from O2 (or from Air on machines configured for air drive) regardless of the fresh gas setting. That also means a failing O2 supply can raise two alarms at once, supply failure and low drive gas pressure, and a contaminated air supply goes straight into the ventilator, as the case at the end of this lesson shows.

From system switch to fresh gas outlet

The system switch is both a gas valve and an electrical switch. Turned on, it lets O2 and Air through to the flowmeters and powers the electronics at the same time; turned off, it stops the gas and shuts the system down. A machine that will not power up although mains is present, or one where gas keeps flowing after switch-off, points first at this assembly: its contact switch or its internal seal.

N2O does not pass the system switch. It goes through the O2-N2O cut-off valve, a pneumatically piloted three-way valve that lets N2O through only while O2 pressure on its control port stays above roughly 0.1 MPa. Lose the O2 and N2O is cut off. The cut-off valve does not affect Air.

The flowmeter assembly is mechanical: needle valves and glass flowtubes in low-flow and high-flow pairs, from 0.05 up to 10 L/min. It contains a mechanical O2-N2O link that prevents the user from setting a mixture below a minimum O2 concentration. On this machine fresh gas flow is set by hand and read on the tubes, not by an electronic mixer, so there is no flow sensor check of the fresh gas in the software. The link and the flowtubes are checked by the flow control test in lesson 2.

From the flowmeters the mixed gas goes to the vaporiser manifold. The manifold has a check valve after the vaporisers, which keeps flush and back pressure from pushing gas back into the vaporiser and changing its output; on the double manifold the interlock prevents two vaporisers being on together. After the manifold, fresh gas and the O2 flush meet in the ACGO assembly. It contains a flow restrictor, a relief valve that limits the pressure of fresh gas and flush to about 38 kPa, a selector switch, and on the ACGO outlet a second relief valve that limits the pressure there to about 110 cmH2O. When the user turns ACGO on, fresh gas leaves through the auxiliary outlet and the ventilator stops. The O2 flush works from the regulated O2 supply at about 0.2 MPa, does not depend on the system switch and is self-returning.

The ventilator: a pneumatic module under electronic control

The ventilator is a bellows driven by gas. Drive gas from the O2 (or Air) inlet passes a filter, a regulator set to about 0.2 MPa and a proportional inspiratory valve that the monitor board opens to the flow it needs. A differential-pressure flow sensor measures the drive flow; this is the machine's third flow sensor, the ventilator flow sensor, and it becomes important in calibration and fault-finding. A mechanical overpressure valve dumps drive gas if the circuit pressure exceeds about 11 kPa, roughly 110 cmH2O.

Expiration and PEEP are handled by the expiratory valve, a membrane valve. A small proportional PEEP valve bleeds gas through a pneumatic resistor and builds a control pressure behind the membrane; the higher that pressure, the higher the PEEP. Upstream of the expiratory valve sits the PEEP safety valve, an electrically driven on-off valve that the boards can open to vent the drive circuit if pressure becomes unsafe. A pressure switch raises Drive Gas Pressure Low below about 140 kPa, and a PEEP pressure sensor on the monitor board watches the closing pressure of the expiratory valve. In the manual most of this sits in one block called the integrated pneumatic circuit of the expiratory valve assembly; in the field it is often simply called the expiratory valve assembly, and it is replaced as an assembly.

In the breathing system, inspiration pushes the bellows down and sends gas through the bag/vent switch, the absorber (or the bypass when the canister is removed), the inspiratory check valve, the O2 sensor and the inspiratory flow sensor to the patient. Expiration returns through the expiratory flow sensor and the expiratory check valve to the bellows. Airway pressure is measured electronically on the monitor board through a sampling line, and mechanically on the airway pressure gauge on the breathing system.

Breathing system, AGSS and suction

The breathing system is a compact block that plugs onto the machine through the circuit adapter. Almost all its gas passages are internal; outside you see only the patient tubes and the O2 cell cable. Inside the adapter run the fresh gas, drive gas and the pressure sampling lines for the flow sensors and airway pressure, each sealed by small seals that are in the annual kit. A breathing system that is not fully locked on the adapter leaks in places no external inspection can see. The block is designed to be taken apart for cleaning and, per the manual, can be autoclaved at 134 degC; check the operator's documentation for what is allowed on your unit and its accessories.

The AGSS is a receiving system with a float that shows whether the hospital disposal system pulls enough flow. The manual specifies two versions, low-flow (about 25 to 50 L/min) and high-flow (about 75 to 105 L/min), identical outside and different inside; the float must sit between its marks. Suction, where fitted, is a Venturi generator driven by pipeline Air or O2, with an adjustable regulator, a filter and overfill protection in the bottle and the regulator, so that fluid never reaches the generator.

The electronics are few and worth naming correctly: the main control board runs the system software, display and alarms; the monitor board carries the pressure and flow sensors, valve drivers and the auxiliary monitoring that cross-checks airway and PEEP pressure; an auxiliary control board takes part in supervising the PEEP safety valve; the power board manages mains, the lithium-ion battery and the circuit-in-place detection; and there are smaller display, keypad, network, calibration interface and, where fitted, isolation transformer and heating boards.

On-site checklist

• Model, serial number and software versions recorded from the data plate and system information
• Gas configuration, number of yokes, vaporiser manifold type and ACGO variant noted
• Options noted: O2 sensor, CO2 module, heating module, AGSS type, suction, isolation transformer
• Drive gas source confirmed (O2 or Air) and the quality of that supply questioned
• Annual kit and canister seals ordered for the actual model (EX-20 differs)
• Machine's own operator and service documentation at hand for the exact configuration

Check yourself