Archive for the ‘Respirometers’ Category


A respirometer is an instrument for measuring the movement of air in and out of a patient, they may be mechanical or they may be electronic.
Most work by the air passing in and turning a light vane which, in the mechanical type, will then go on and turn a series of cog wheels that move the hands around to indicate the amount of gas that has passed, in litres. One breath blown through it will give you the patients tidal volume, The minute volume is the tidal volume multiplied by the number of breaths the patient takes in a minute.
They only measure in one direction thus the gas is not measured twice as the patient breathes in and then out.
The electronic ones will often have the same type of vane, this will break a beam of light or a magnet will cause an electrical impulse and the result will be shown on a meter or a liquid crystal display (LCD).
The mechanical one seen most, in England at least, is called the Wrights respirometer and is made by a firm called Fred Ferraris in London, though there are other makes of a similar design.
Repairs to these may not look very easy but with care and the proper tools, very small screwdrivers, quite a lot can be done.
What will probably make repairs difficult for you is that you do need the proper spares to do any major repairs.
These are very specialised and have to be purchased from the manufacturers, though some of them could be obtained from a watch-maker, or even made by a clever watch-maker, in fact come to think of it you can make some of the parts yourself.
The only other way of getting spare parts is to keep old and very badly damaged ones and take parts from them.

Look at the photographs in the photo article to help you take it apart.

Some faults and the cures.
A simple test for smooth running is that they should be able to register a flow as low as 2.5 lpm.
This is how you should test them:
With the respirometer set to zero and connected to your supply of gas, slowly increase the gas flow from zero until the fine scale needle is just seen to have begun a smooth rotation.
The fine scale needle should then be able to make one complete revolution without hesitation.
Next test it at 16 lpm. it should read within 2% .
Now test at 60 lpm. it should read within 5 to 10 % .

For a quick check of smooth running, hold the respirometer resting in your fingers with the inlet pointing to the palm of your hand, blow gently towards your palm so that the air bounces off your palm and into the inlet, the hands should slowly go around. If you have to blow hard to get it to run it needs maintenance.

The calibration can be checked against an anaesthetic machine rotameter as above.
If it found to be incorrect there are two ways of looking at it.
1. If it is reading low, check that it is running smoothly as explained above, if it is not capable of running at around 2.5 litres per minute then you must clean it first, see the section on general cleaning.
If you consider that it is running as smoothly as it should then you will have to look at the calibration jet inside.
To find this look down the connector into which the gas passes, you will see the slots of the stator and next to that a hole or a hole with a threaded jet in it, this is the calibration jet. This jet lets some of the gas bypass the vane, the amount depending on the size of the hole, so if your respirometer is reading low you will need to decrease the size of the hole, thus letting more gas go through the slots to the vane, that is the easy bit, for now you have to get the old jet out, and it is where you find that the jet has corroded in or at the least is very tight.
If you are lucky enough to have got the old jet out then the smaller one can now be fitted, if you have been able to buy the official one, if not you are going to have to make one.
See below for instructions on how to make a new jet.
If it is reading high, you will have to do the opposite and put a larger jet in, with the same problems as before.

It is unlikely that the jet is the wrong one, there is no reason why anything would have changed from when it was made.

But if you are sure it is the cause of the problem proceeds as follows.

First you will need a piece of round aluminium rod (or other non corroding material) just slightly larger than the completed jet (1/4 of an inch or slightly larger) if you can use and have access to a lathe then you will have to turn the rod to the correct diameter to enable you to cut the thread on to it, the thread is 1/4 BSF.
Having turned the bar, cut the thread with the die, and cut the bar to the correct length, put a screwdriver slot in the top, you must now drill the jet.
The size of the jet will depend upon whether you want to allow more or less gas through than you are doing at the moment.
In any case start with a very small hole and by trial and error work your way upwards.
Go very carefully, because if you make your jet too big you will have to begin again, however in that case you could block the hole with an epoxy glue and try to drill the jet smaller, if you are using brass or stainless steel you could silver solder the jet over and re-drill.
What ever you have to do is a nuisance, so taking it slowly in the first place is the best plan.

To do this you have to take the works out of the case, it is fairly easy to see how this is done. Firstly find something to keep the parts you take off safely.
a. Remove the bezel and glass as one unit..

b. Remove the face and hands, take care with the hands as they sometimes fly off unexpectedly. Use your finger nails to grip the hands to pull them off.

c. Unscrew the stop control and take out the stop control components from inside. Note how they are assembled before you dismantle them.

d. unscrew the six screws holding the works into the case.

e. Remove the works from the case.

You may have a little difficulty here, the problem is that between the works and the body is a thin red rubber washer with time this often gets perished and sticks the two parts together. Do NOT try to prize it out with a screwdriver under the top plate that holds the cogs in place, this is sure to pull out the pillars that support it and that will give you a real problem.
Push it out from the back through the connector where the vane is, press on the main plate and NOT on the vane bridge.
If you have an ultra sonic cleaner then it is worth putting it through that before stripping it down, this may cure the problem.
If you don’t have one try any local watch repair shops they may have one.
If you can’t get it ultra sonically cleaned then you will have to strip it down.
Having got the works out it is not as difficult as it might look to take it completely apart, cog wheels and all but do make a sketch or take photographs before you do it to assist you in re-assembly.

Work slowly and keep the parts removed under a cover to keep the dust out.
If you don’t have any spares then there is not much that you can do apart from cleaning all the pivots and holes with alcohol, as you re-assemble it test each cog for free running, and unless any pivots are broken it should run much smoother.

If you have had to strip it right down re-assemble and check as you go as follows:

a. after refitting all the cogs, give them a spin, they should run smoothly and glide to a halt.
If not check each is clean and not damaged, and that when they are running that they do not touch each other. Other than where they should

b. Check the vane and the vane shaft, do check the inner end of the vane shaft with the small cog wheel, they quite often go rusty, if it has, and you have not got a new one clean the cog on the end with a brass brush, metal polish or something similar. If you have used metal polish clean every last bit off afterwards.

c. When you are going to re-fit the vane pinion, remember to put the mercury seal assembly on the pinion first, this is the little disc that is held in place by two small screws and should have a seal in it that stops moisture getting in to the works.

d. re-fit the vane shaft and the bridge and then check that it runs as smoothly as it should. Now you may find that despite the fact that the cogs are all free running and the vane pinion is in good order, that it will still not run well. Check the mercury seal it is possible that when you refitted it and tightened up the screws, that it was not quite positioned correctly and that the vane pinion is rubbing on it.
Undo the two screws a little, centralise the seal so that it is not rubbing the shaft and re tighten the screws.
This should do the trick.

Re-fitting in to the case.
Put the workings back in to the case and replace the screws, do up these so that they are not quite tight, blow in to the respirometer, the vane should run smoothly and with no real noise.
If it is noisy grip the movement and move it slightly this should reduce the noise and make it smoother running.
When it is as it should be tighten the retaining screws fully.
Refit the on/off mechanism, then the hands and face.
The two screws that hold the face should only be just done up, do not over tighten them, if you do you may well pull the threaded bushes out of the top plate.

When you have finished do the low flow test.

If you have pulled the bushes that take the face screws or they have come loose refitting them is a difficult task but proceed as follows:
First you will need to make a tool to help you do it, what you need is a piece of brass rod about 6 inches long, a small screw of the same thread as the one that holds the face on.
Drill a shallow hole about the same depth and diameter as the head of the screw.

a. Remove the face and hands.

b. Remove the on/off mechanism.

c. Remove the top plate taking care not to lose the cogs.

d. Screw the screw in to the bush, put the bush back into the original hole for it in the top

e. Here you will need some assistance, put the brass rod into a vice with the hole upwards.

f. Turn the top plate upside down and put the screw head into the hole in the top of your brass rod.

g. Get your assistant to hold the plate horizontal and with a very small punch and a light
hammer re-stake the bush into place.
The idea of the screw in the bush is to protect the thread from damage.

Broken `glass’
If you have a broken one of these, your best plan is to go along to a watch repair shop, they should be able to fit a new one for you.
Otherwise make one out of a thin piece of plastic sheet.
I wouldn’t bother to try to buy the official ones they cost far too much and as I said you can get the same thing from a watch shop for a fraction of the price. I have made them out of a plastic petri dish.

Bent tapers.
Find a taper of the opposite type or turn one out of a piece of BRASS rod on a lathe, if you can’t do it yourself then it is worth going to a local engineering firm and getting them to make you one, it’s always handy to keep.
With this taper tap the connector to the correct shape.

On/Off control not working.
a. Remove the bezel that holds the glass

b. Remove the face and hands

c. Inspect the on/off assembly, you will probably find that the piece of wire that stops the cog is bent out-of-place.
Bend it so that it touches the cog when the button is in the off position.

d. re-fit face, hands and bezel.

e. Test

Hands will not zero.
This is normally because the hand is not seated on the return cam correctly. Repair as follows.
a. Remove bezel

b. Remove the faulty hand by pulling it off, it can normally be done with your fingers if you
have reasonably long nails.

c. Press the reset button and hold it down.

d. re-fit the hand  loosely with it at zero, when you are happy with its position press it on properly.

e. check it is now correct.

Bezel will not stay in place.
For some reason you will find that now and again the bezel will not fit properly, to cure this try the following;
Take a pair of wire cutters, grip the edge of the bezel where it presses into the respirometer body and twist the pliers whilst holding the bezel still, do this three times at even distances around.
This has the effect of putting three small ridges in it and there by helping it to grip better.

Damaged vane.
Sometimes you will find that they have autoclaved the respirometer, this will do some damage and one of the things that you will notice is that the vane will have curled up and have become quite useless.
To replace it proceed as follows;
a. Remove the bezel, hands and face.

b. Remove the works from the case.

c. Remove the vane bridge.

d. Undo the two very small screws that hold on the mercury seal plate, and lift out the vane assembly.

e. Pull the vane off the shaft, clean the shaft.

f. If you have a new vane then fit it, but do not glue it in place at this point.

g. Refit it in to the respirometer and fit the bridge.

h. Now position the vane correctly so that it neither touches the bridge at the top nor the mercury seal plate screws at the bottom.

i. When you are happy with its position glue it in place, for this I always use nail varnish, however epoxy glue would do providing that you only put the smallest amount on, too much and the vane will not run as well as it should.
When using nail varnish and no doubt any other glue, make sure that you put glue on to both sides of the vane in the same position. If you don’t the glue dries, shrinks and the vane may have a curve in it, not good.
If you do not have the official vane then you will have to make one.
The real ones seem to be made of a silvered plastic.
I have never had to make one so to some extent I am guessing when I tell you what to do.
Try a very thin tin or aluminium foil, the sort that is sometimes used for cooking with.
Really all you can do is to look around your hospital for something that looks right and then give it a try.
When using a respirometer it should not be left in the patient circuit when not required, and when as a fitting on, for example a Radcliffe ventilator, should also not be left switched in to the gas stream, this is because they get very damp and there are parts that will corrode under these conditions.
Respirometers are not very robust so treat them with respect and try to teach the doctors and nurses to do the same.

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