The Prima SP platform: gas supply, protections, flowmeters, backbar, breathing system, ventilator and options
Identify the configuration of a Prima SP2 or SP3 in front of you and trace every gas from its inlet to the common gas outlet, naming the protection that acts at each point and the separate units the machine depends on.
A mechanical machine with separate partners
The Prima SP2 and SP3 share one service manual and one gas system design. What you meet on site is a cart with a cast aluminium base, aluminium uprights and panels, four castors with brakes on the front pair, gas blocks behind a front cover, a glass flowmeter bank behind a clear shield and a backbar for up to three vaporizers. Everything that ventilates, absorbs CO2 or measures oxygen is a separate product mounted on the cart: typically the A200SP absorber on a pole mount, an AV-series ventilator such as the AV-S, a Sigma Delta or other Selectatec-compatible vaporizer and an oxygen monitor. Each of those has its own user and service documentation.
That split decides how you work. The Prima SP itself has no processor, no automated leak test and no error log. Its protections are pneumatic and mechanical, and they only prove themselves when someone tests them. In anes-l1 you learned the generic gas path and in anes-l2 the generic faults. Here we add what is specific to this machine: where each protection sits, what its acceptance values are in the manual, and which part on this cart is likely to be the cause when the symptom appears.
The manual we work from is the Prima SP2/3 range service manual of 2005, in a USA specification copy, so it quotes pipeline and regulator values for both USA and UK specifications. Machines in your region may differ in gas colours, pipeline connectors and pressures. Always check the data plate and the specification your machine was built to.
Identify the configuration before you touch it
Write down the model, the serial number and the configuration. The manual's service schedule starts exactly here: the serial number decides which service is due and which parts apply. Then record what is fitted, because two Prima SP machines in the same theatre suite can be very different machines.
- Gases: oxygen and nitrous oxide are the base; air is common; CO2, helium and xenon exist as special options. Up to four cylinder yokes and up to three pipeline inlets.
- Flowtubes: single tubes per gas, or dual cascade tubes for oxygen, air and nitrous oxide (a low range tube up to 1000 ml/min and a high range tube to 10 L/min). This matters for the basal flow and the hypoxic guard tables.
- Auxiliary oxygen flowmeter on the left of the bank, auxiliary gas outlets at the rear, optional auxiliary mains panel and optional flowmeter lighting.
- Backbar with two or three Selectatec-type stations, or a cagemount vaporizer on older or special builds.
- Rear or optional front common gas outlet with the Safelock taper.
- Absorber, ventilator, vaporizers, oxygen monitor and any electronic flowmeter display, each with its own model and serial number.
- SP2 or SP3 badge. Our manual covers both but does not set out the differences in detail; record what the label says and check parts against the serial number.
Gas supply: yokes, pipelines, gas blocks and regulators
Each cylinder yoke and each pipeline inlet feeds its own gas block, and every block has a gauge tapping and a non-return valve, so a cylinder cannot back-feed a pipeline and an empty yoke cannot vent the other supply. Yokes are pin-indexed and the latch should not close unless the pins are fully engaged; a Bodok seal sits between cylinder valve and yoke and filters sit at the yoke and pipeline inlets. Pipeline hoses on the USA build connect with DISS threaded unions.
Cylinder blocks also carry the first stage: a diaphragm regulator that drops cylinder pressure to the working level, with a factory-set relief valve under the diaphragm that vents if the regulator seat starts to leak. The manual gives a reduced pressure of 310 kPa on USA and 380 kPa on UK specification at 5 L/min, against pipeline supplies of 340 kPa and 400 kPa. Downstream, secondary regulators bring oxygen and nitrous oxide to 152 to 241 kPa and air to 207 to 283 kPa before the needle valves. The secondary regulators are not just for stable floats: the hypoxic guard depends on oxygen and nitrous oxide arriving at the needle valves at the right relative pressures.
The front panel gauges, 50 mm and colour-coded, are marked CYLINDER or PIPELINE and scaled in kPa x 100. Remember from anes-l1 that a nitrous oxide cylinder gauge says nothing about contents until the liquid is gone. Internally, the low pressure tubing is diameter-indexed: 8 mm oxygen, 6 mm nitrous oxide, 5 mm air, 4 mm for CO2 and switch feeds, 10 mm mixed gas. If you find a different size on a gas, someone has been inside before you.
The oxygen protections: delivery switch, whistle, indicator and cut-off
The gas delivery switch on the front is a pneumatic switch on the oxygen supply. With it off, no gas reaches the flowmeters; with it on, oxygen flows at a basal rate that the needle valve cannot close off, and the other gases become available. The same switch also switches the electrical supply to the ventilator and the flowmeter lighting, so a switch fault can look like a ventilator fault. The whistle sounds briefly each time the switch is operated, which is a free daily test of the whistle.
Three devices watch oxygen pressure. A whistle, fed only by the oxygen left in the machine and prolonged by a small reservoir, starts as pressure falls and must sound for at least 7 seconds. A visual indicator on the front turns from green to red. A cut-off valve, the safety capsule valve in the fault tables, shuts nitrous oxide and CO2 when oxygen pressure falls and reopens them only once it recovers to a higher pressure; air is not cut and keeps flowing. The whistle starts a little before the cut-off acts, and the cut-off resets only at a clearly higher pressure, so the protection does not chatter. For acceptance, test to the values in the manufacturer's service schedule for the machine version you are servicing, and record the document and revision you used.
Downstream of the flowmeters a fresh gas pressure relief valve under the gauge cover limits pressure in the backbar and CGO to about 41 kPa. It protects flowtubes, vaporizers and seals if the outlet is blocked; it does not protect the patient's lungs. Pressure limitation in the breathing system is the job of the APL valve and the ventilator.
Flowmeters and the mechanical hypoxic guard
Each gas has a needle valve with a polished stainless needle and a silver seat in a common manifold, directly below its flowtube. Tubes and floats are matched pairs and length-indexed by gas, with an antistatic coating and an earthing contact; the manual quotes an accuracy of 2.5 percent of full scale. In cascade tubes gas always passes the low range tube first, and the high range tube shows nothing until the flow passes 1 L/min.
The hypoxic guard, the mechanical anti-hypoxic device or MAHD in the manual, is a gear linkage between the oxygen control and a primary needle valve in the nitrous oxide line. Turning oxygen up opens the primary nitrous valve in proportion. The nitrous oxide knob you see works a second needle valve downstream that can only restrict what the primary already allows. The result is a minimum of 30 percent oxygen, plus or minus 3, in the oxygen and nitrous oxide mixture at any knob position. The guard works on ratios of two flows, so it depends on three things at once: the gear mesh and grub screws, the secondary regulator pressures of both gases, and needle valves that are not worn. It does not look at air, and it does not know what oxygen concentration is actually delivered if the oxygen supply contains something other than oxygen.
The oxygen knob has a mechanical stop that keeps the basal flow: 50 to 75 ml/min with cascade tubes and 100 to 200 ml/min with single tubes. Users and engineers who try to close it to zero damage the seat and the linkage.
Backbar, outlet, breathing system, ventilator and power
The backbar carries two or three stations, each with two spring-loaded valve capsules that open when a vaporizer is locked on and close when it is removed. With interlocking vaporizers only one can be on at a time, and with two vaporizers on a three-station bar they must sit on adjacent stations for the interlock to work. Fresh gas leaves through a short hose to the CGO block with its 22 mm male and 15 mm female taper and the Safelock collar. The oxygen flush delivers 35 to 75 L/min to the CGO, bypassing flowmeters and vaporizers, which is why nothing that vaporizes may be fitted between the CGO and the breathing system.
The A200SP absorber takes fresh gas from the CGO, carries the unidirectional valves, APL, bag/vent switch and usually the oxygen cell. The AV-S takes drive gas from the auxiliary oxygen or air outlet, powers up and down with the gas delivery switch through an interface cable, and has a back-up battery that the Prima manual rates at about 30 minutes when fully charged, with a long recharge after deep discharge. The auxiliary mains panel has up to four sockets, each fused, with a circuit breaker in its switch; the manual limits the machine to four operator-accessible outlets because leakage currents add up.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Cylinder yoke, Bodok seal and yoke filter | Pin-indexed cylinder connection with seal and inlet filter | Missing or worn seal leaks; damaged pins or clamp screw; blocked filter |
| 02 | High pressure gas block with primary regulator | Gauge tapping, non-return valve, first-stage regulator and relief valve | Creeping or low outlet pressure, relief valve venting, NRV leak |
| 03 | Pipeline inlet block and hose | Gas-specific inlet with non-return valve and filter | Leaking NRV O-ring, blocked filter, aged hose or loose ferrule |
| 04 | Gas delivery switch, whistle, indicator and cut-off valve | Oxygen-powered on/off, failure warnings and nitrous oxide cut-off | Weak or no whistle, slow indicator, cut-off sticking open or not resetting |
| 05 | Secondary regulators (O2/N2O and air manifolds) | Stable pressures for needle valves and the hypoxic guard | Drift out of range, failure to return to setting after switch cycling |
| 06 | Needle valve capsules and gear linkage (MAHD) | Flow control and the minimum 30 percent oxygen ratio | Worn seat after overtightening, loose grub screw, gear out of mesh |
| 07 | Flowtubes, floats and bobbins | Flow indication, with filters and seals in the bobbins | Sticking float from dirt or static, perished seals, blocked filter |
| 08 | Backbar manifold, valve capsules and relief valve block | Vaporizer stations, fresh gas path and over-pressure relief | Perished top O-seals, sunken pillar insert from a failed spring, leaking hose |
| 09 | CGO block and oxygen flush valve | Fresh gas outlet with Safelock taper; high-flow oxygen bypass | Worn Safelock O-ring, flush that leaks or does not shut off |
Field case: a different vaporizer on the same mount
An engineer looking after a Dameca anaesthetic machine wanted to replace a key-fill sevoflurane vaporizer with a pour-fill one from another maker; the Selectatec-type mounts were the same. He asked colleagues whether a vaporizer from Penlon is calibrated for a particular make of machine. Colleagues answered that it is not: a vaporizer of this type is calibrated as a device in its own right, so a change on a compatible mount was acceptable, provided the vaporizer's own manual allows that mount. What remained was the engineer's part: the right agent and filling system for the department, a correct lock on the station, the interlock with the neighbouring vaporizer and a leak test with the new vaporizer on and off.
On a Prima SP the vaporizer is a partner, not a part of the machine. Compatibility of the mount is only the start; the interlock, the station seals and an output check with an agent analyser decide whether the combination is fit for use.
Oil and water from the air supply
In a hospital using Dräger Primus machines, the original medical air compressor was replaced by an old oil-lubricated compressor without a dryer. Water mixed with oil reached the machines through the air pipeline. On one machine, the engineers reported, oil vapour ignited at a heated sensor element during a test and destroyed the inspiratory and expiratory valves and the breathing circuit. The machines were dismantled, the flowmeter parts, tubing and inlet filter dried and flushed, and the air supply problem was handed to the hospital. The Primus has electronics the Prima SP does not, but the upstream cause is the same for any machine.
The Prima SP manual requires clean and dry gas and puts filters at every inlet. Water or oil found in a filter, flowtube or gas block is a supply fault first: stop, report it to the hospital in writing and do not return the machine to service until the supply and the contaminated parts are dealt with.
On-site checklist
- Model badge, serial number and country specification recorded
- Gases, cylinder yokes and pipeline inlets fitted, and connector system noted
- Single or cascade flowtubes, auxiliary flowmeter and auxiliary outlets noted
- Backbar stations, vaporizers with agent, serial and interlock type recorded
- Absorber, ventilator, oxygen monitor and any electronic flowmeter display recorded with serials
- Auxiliary mains panel: number of outlets and what is plugged in
- Hose dates and last service label read