Archive for the ‘Anaesthetic machine testing in pictures’ Category


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TESTING ANAESTHETIC MACHINES in more detail and in words.

At its easiest testing these machines is quite simple, and the main thing that you will be called upon to look for are leaks in the gas system.

a. To do this first turn off or unplug any items such as suckers or ventilators that run off the oxygen supply, make sure that the rotameters are turned off.
Then turn on the cylinder of gas till the full pressure has been reached.
Then turn off the cylinder and watch the pressure gauge for that circuit.
You are now in a position where the pipe work from the reducing valve to the rotameter fine adjustment valve is pressurised to the output pressure of the regulator, perhaps 55 or 60 p.s.i. and between the reducing valve and the pillar valve to a pressure of the cylinder, 2000 p.s.i. or so (for oxygen).
The volume of gas in the system at this time is very little, and because the pillar valve is closed is a sealed system at that moment.
Now should any gas leak out of that pipe work, even a very small amount, then it will show as a drop on the pressure gauge.
If a drop is seen then this is the moment to get out the soapy water and paint brush, brush the soapy water around all the joints in the system that you are testing and look carefully, any leak of gas will show as a collection of bubbles around the sight of the leak.
Large leaks will be noticed by the hissing of the gas and are easy to find and often a tweak with a spanner on the joining nut will cure it, do be careful how tight you do up the nut, they are only made of brass as a rule and the thread is easily stripped.
If that doesn’t then the joint must be taken apart and re-tightened using new P.T.F.E. tape, (polytetrafluroethelene, to give it its full name).
Don’t forget the inside of the gauge, unscrew the back to get at it, you may find that the bourdon tube is leaking, this is the curved brass tube which the gas goes in to and which with the pressure of gas will try to unbend, it is this unbending that eventually moves the pointer via the linkages.
The same test is used for the other gases.

b. Back-Bar testing.
First the low pressure back-bar test, this is testing the back- bar from the fine adjustment valves to the outlet.
For this you will need a blood pressure machine, a length of tube and a rubber bung with a hole drilled through it, into this hole you have to push a piece of brass tube, the rubber tube will connect this rubber bung to the blood pressure machine.
Look at your flowmeter block and you will see a plug in the body at the top somewhere, unscrew this, it is from here that you test for leaks, so your rubber bung has to be able to push in this hole to make a GOOD seal.                                                                  Put a bung without a hole in it in to the outlet of the back bar, and make sure that it is tightly in and there are no leaks around it. Now hold your rubber bung with the attached pressure gauge into the hole, hold it in so that there are no leaks around it.
Slowly turn on the oxygen fine adjustment valve and pressurise the back-bar to 30 mm Hg. When you have reached this pressure turn the flow off and see if the needle on the BP machine stays at 30, the chances are that it won’t, and it will slowly drop as the gas leaks out of the back- bar.
Now turn on the fine adjustment valve again, but do it very slowly and keep an eye on the flow tube and the BP machine, as you increase the flow the BP machine needle will start to rise again, when it gets to 30 turn the flow down a little, adjust with this flow till you are able to keep the pressure at 30 mm Hg.
When the pressure is steady at 30 note the flow on the flow tube, this should not be above 100 cc.
What you are doing, if you stop to think about it, is adding gas to a closed system to replace the gas that is being lost in any leaks that there may be, thus the flow showing on the flow tube, when the BP machine is steady at 30, will be equal to the leak.
Leaks of less than 100 cc. are acceptable at that pressure.
The low pressure test is really testing that the seals at the end of the flow tubes are in good condition, at low pressures they can leak, but at higher pressures they will tend to seal better.

c. Now the high pressure back-bar test.
This is done in the same way as the low pressure test, but instead of 30 mm Hg, you pressurise it to 150 mm Hg, the allowable leak is still 100 cc.
The high pressure test is to check that the metal to metal joints on the back bar are tight.
If you find a leak greater than 100 cc. then you will have to get out your soapy water and paint brush and look for it.
Should you find that one of the taper connections is not gas tight, it may be because the connection has been dented.
In this case the back bar will have to be dismantled, the item with the damaged connection removed and repaired.
To do this you will need to find a piece of round wood or brass, something soft which will go inside the bore of the connection, then with a soft hammer, that is one with a nylon face, or even a piece of wood, carefully tap the connection round testing it every so often on the opposite kind of taper, when it seems to be sealing correctly, replace it on the back-bar and re-test.
It may be that a number of the joints are simply loose, in this case loosen off all the nuts that hold the items on to the back bar, and again with something soft like a piece of wood to protect the metal, tap all the items tightly together.

d. Next test that the pressure gauges work smoothly.
Turn on the pressure and let the pointer rise to full, then turn the cylinder off and open the fine adjustment valve and watch the pointer drop, this drop should be smooth, and it must come to rest on the stop before the gas stops flowing, on no account should the gas stop flowing before the needle has reached the stop.
If it does then take the cover glass off the gauge very carefully prize off the pointer and replace it so that it is touching the stop.
If the pointer does stop or hesitate then take off the back of the gauge and lightly lubricate the linkage, when you have lubricated it work it to and fro with your finger until it is free.
If you find that there is damage inside that is preventing the movement of the pointer, take the gauge off the machine (easier to work on) repair the damage if you can or replace the gauge with one that works.
It is most important that the pointer does move freely up and down, should it stick part of the way, the anaesthetist could be led into believing that there is gas still in the cylinder when in fact it is empty.

e. Next check the rotameters.
Turn on the gas and open the needle valve slowly.
Make sure that the bobbin does not stick and that it revolves at low gas flows (for those bobbins that are meant to rotate), turn up the flow till the bobbin rises to the top of the tube, and check that when the flow is turned off that the bobbin does not stick at the top of the tube on the bobbin stop but sinks to the bottom.

The checks for flow rate are given below, test as follows;
1.Slowly open the fine adjustment valve and set a flow through the tube under test, this will be connected to a test flowmeter that has an accuracy at least as good as the flowmeter under test.
Test that the flow rates are within the following limits:
a. Flow rates below 100 ml/min: measure the deviation from the line indicating flow rate, distance allowed +/- 1 mm.

b. Flow rates above 100 ml/min: +/- 5% of indicated flow rate + 0.5% of the full-scale

Continuation of testing procedure:
f. Check the fine adjustment valves. These should be firm to turn but not to tight, and not so loose that they can be turned accidentally. Tighten or loosen the nut behind the control knob as required.

g. Test that the vaporisers are secure on the back bar ( In this case I am talking of the Cyprane Tec type of vaporiser made by British Oxygen), and that the control knob can be turned on easily, if it is stiff then the control has become clogged up with Thymol and it should be taken apart carefully and cleaned with halothane or trichloroethylene, and then reassembled. (see
section on cleaning Mk 2 vaporisers)                                                                                                                                                                     The vaporiser should be checked for accuracy, to do this you need an anaesthetic gas analyser, these are very expensive, if you do not have one all you can do is a simple test, it goes like this.                                                                                                                    Turn on the oxygen then turn on the vaporiser to a low percentage like then smell the outlet and you should get a slight smell of anaesthetic agent, now turn the vaporiser to maximum, smell the outlet again and you should get a strong smell of anaesthetic agent, this is quite unscientific but will give you a VERY rough guide, and at the very least will tell if the vaporiser is giving
none, a little or a lot of anaesthetic agent. If it is a Cyprane Mk3 there is an accuracy testing procedure in the article on it.

h. Test the oxygen flush valve. Press it and see that a good flow comes out, check that when you do this that the needle on the oxygen pressure gauge does not drop too much, if it does this either indicates that the cylinder is low, that the cylinder is not turned on properly or that the reducing valve is faulty.

i. Test the oxygen failure warning device. By turning on the gases, then turn the oxygen off, and make sure that the whistle sounds just before the needle comes to rest on the stop. Check also that the nitrous oxide stops when the whistle sounds, if you have that kind of alarm fitted to your machine. (more details of this are given earlier).

j. If the machine is fitted with pipeline gas hoses these must be checked very carefully as follows.
Start with the OXYGEN hose, inspect that is fixed tightly to the machine, check that the hose has no damage to it, cuts, splits or bubbles.

k. Check that the probe has OXYGEN stamped on it and that the hose is WHITE in colour (this colour applies to English standard machines, in America the colour donating Oxygen is green). Now do the same for the Nitrous Oxide hose, if you have one fitted, for English standard machines the hose colour is Blue.

l. If the machine is a very old one it may have a chain hanging down from the frame to touch the floor, this is to allow static electricity to discharge away, make sure that it touches the floor and that it has not been painted, thus breaking the electrical path. Newer machines will have anti-static wheels, these will be black with some kind of yellow stripe on.

m. If there is a sucker attached to the machine check its good operation, if there is a blood pressure machine check this also.

n. Check the 2 ltr bag is not perished and that any rubber pipes are sound and connecting well.

o. Check the patient safety valve is working. The patient safety blow off valves on the B.O.C. machines come in two colours silver which opens at 180 to 200 mmHg, the blue one opens at 40 m.m.Hg. for other machines you will have to consult the operators manual for the correct blow off figure, but it will be around 180 to 200 mm Hg.

p. Check that the reducing valves are set at the correct pressure. I’ve explained how to do this in the section on reducing valves, when the valve is on an anaesthetic machine it is done in the same way, by measuring the static and dynamic pressures, it is normally done on the machine.
For this you will need a piece of pipe with a connector that goes in to the auxiliary oxygen outlet (schrader connections).

q. To this is attached the same pressure gauge that was used before (0 to 100 p.s.i.), but with the different tubing connection on it. To set the oxygen regulators, turn on one cylinder at a time and then adjust the reducing valve connected to that cylinder, if it needs it.

r. If you have pipeline supply set it to 50 p.s.i., if no pipeline set it to 60 p.s.i.

s. For the nitrous oxide you will have to look underneath at the nitrous pipe work and look for a `T’ piece or a `X’ connection with a free arm and connect your pressure gauge into that, adjust as with the oxygen.

t. For the carbon dioxide you have to disconnect the pipe work where it leaves the reducing valve and connect your pressure gauge there, adjust as before.
I must stress that these figures apply to the machines made by BRITISH OXYGEN, other makes of machine will quite probably be different.

When lubricating any piece of medical equipment that involves oxygen, only use SILICONE GREASE or whatever your service book recommends, do not use the ordinary sort of grease that you would use in a car for example. (do not use SILICONE GREASE for metal-to-metal lubrication). This is because there is a very slight possibility of an explosion should there be a gas leak where the grease is, the theory is that as the gas leaks out heat is generated, which in the presence of oxygen could cause a fire or explosion.

When you have finished testing or repairing an anaesthetic machine of any kind give it a quick clean and polish, always hand back the machine in a cleaner state than you got it, it will give the user confidence that you have done a good job, which of course you will have done.
Precautions when repairing Oxygen supply equipment.
When using white plumbers tape to wrap around threads on joints, strictly speaking you should only use certified degreased P.T.F.E. tape, for the same reasons as with the grease, however ordinary tape will do until you are able to get the correct type, if fact I would say that reason for not using ordinary tape is so feeble and the risk so theoretical that it may not be worth bothering about, still I mention it so that you can decide for yourself.

On English standard machines cyclopropane comes in an ORANGE cylinder, is a very explosive gas, often it is only put on the machine when it is actually needed, it is not used these days and indeed some machines are not even fitted to take it.

I will at this point say something about NITROUS OXIDE cylinders and pressures.
There is a common misconception that the pressure inside a cylinder containing NITROUS OXIDE is always 750 lb/sq. in.
However the pressure of the vapour above the liquid nitrous oxide, like that above any other liquid varies with temperature, but so long as there is some liquid in the cylinder then the pressure will always be around 750 p.s.i. depending upon room temperature.
This is why some anaesthetic machines do not have a pressure gauge fitted to the nitrous oxide, it can be misleading.
All it tells you is that there is SOME liquid left in the cylinder, but not how much.
During the use of a nitrous oxide cylinder a moment arrives when the last drop of liquid evaporates.
Assuming the temperature to have been kept constant, a state of affairs which never happens during clinical anaesthesia, from then onwards, however, the pressure within the cylinder gradually falls.
The rate of fall in pressure depends on the size of the cylinder and the flow rate used.
The contents can also be found by weighing the cylinders.
This is of no help when the cylinders are fitted to an anaesthetic machine.

Servicing of Vaporisers, additional comments.
In an earlier paragraph I have outlined some servicing procedures that you can undertake on some of the more simple vaporisers.
In general terms it is recommended that vaporisers are returned to the manufacturers or their agents from time to time.
If you have your own anaesthetic gas analyser you will be able to check the output on a regular basis.
If you find that there is an error of more than +/-10 % of the dial setting (@ 2%, 1.8 to 2.2 ), the unit should be returned for re-calibration.
You should tell the anaesthetist of any errors you find and let him decided what to do bearing in mind the above.

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