Archive for the ‘Blood pressure machine repairs’ Category

This is rather a long word which could be a joke but is the proper name for blood pressure machines. (sphygmos being the Greek word for pulse)

There are three types:
1. Mercury.
2. Aneroid.
3. Electronic.

The last, the electronic, we can forget about.
They are used in theatres and on some wards, but on the whole are a waste of money for those with little to spend. There is more to go wrong and will be expensive to repair if they do so.
The second is the aneroid, these are compact and portable but do suffer in that they are easily damaged and can go out of calibration without you realising it.
Aneroid units screwed to a wall or attached to an anaesthetic machine are fairly safe from damage and should work well, but should be checked from time to time against a mercury type.
The first on the list, the mercury is by far the best. Easy to look after and endlessly accurate.
Take care if you have to maintain them, do not handle mercury, do not breath in the fumes or the black powder that may be found in the tube or reservoir. Over a period mercury and its oxide are toxic.

Maintenance of these: Should be done once a year.
The Aneroid type.
The first thing to check is that the pointer is at zero, on some it will fall to a 0 figure on others it will rest within an ellipse or square at the bottom of the dial.
In general terms if this is so it will probably be giving you accurate readings.
To check this you should connect it to a mercury machine with a ‘Y’ piece and note down the figures from both on a piece of paper.
If the pointer is not at zero, remove the ‘glass’, gently remove that pointer and put it back on in the correct position.
Take care removing the hand they are made of thin metal and bend easily. Find a couple of small screw drivers put one under each side of the central boss and lever gently upwards.
Take care to note where it goes when it comes off, it may fly off unexpectedly and get lost.
If when you let the air out of the cuff the needle moves in a jerky manner you may need to lubricate the mechanism inside.
Again remove the ‘glass’, face and pointer and lubricate the pivot points inside with a little light oil.
Having checked the accuracy against a mercury B.P. machine look at your figures, if they agree within a few mm all the way up the scale it is o.k.
If they do not agree there are 2 possibilities:
1. The linear error, here the error is the same all the way up the scale, say +10.

Mercury Aneroid

50 60
70 80
100 110
150 160

2. The Non Linear error, here the error might be -20 at the lower end, getting less as you go up, correct at say 70, and getting greater as it gets higher till at 150 it might be reading 170.

Mercury Aneroid

30 10
50 40
70 70
100 110
150 170

as you can see the error is changing as you go up the scale.

Correcting these errors is very hard going and can take a great deal of time. To do this you have to take off the ‘glass’, hand and face.

From there it depends upon the make how you adjust the errors.
The one I describe is a common one made by Accoson and may give you a guide as to the principles.

Below is my photograph of the insides.


Inside you will see a disc shaped corrugated bellows, then a lever arrangement which changes the up and down movement of the bellows to a rotary movement of the pointer.
Part of the mechanism is a pin resting on the curved inner side of a triangle with the centreĀ (of the triangle) cut away.
What happens is that when the pin (which is resting on the bellows) is moved by the bellows expanding, it acts against the side of the triangle. This moves around and the teeth on the end of the triangle engage on a cog which moves the pointer around.

There is a fine coiled spring to return the hand to zero when the pressure is released, take care not to damage this spring.

It is not easy to describe and I could understand if you had to read it a few times to get the hang of it.
Try pressurising it with your blood pressure cuff and bulb and watch what happens.
Now for the adjustment, and it involves bending the pin mentioned above –


If you have a linear error you have to bend the pin either towards or away from the side of the triangle depending on which way you are correcting.
If the error is non-linear you must bend the pin parallel to the side of the triangle.
It is very much a case of trial and error, a slight bend with a small pair of pliers, put the face and hand back on and re-test.
If the error is still there repeat it.
This will have to be done many times before you get the direction and the amount of bend correct.
I can only stress again that each bend has to be done very gently, once you get within 2-3 mm Hg I would consider yourself lucky and leave it alone.
The only other precaution I would take is to open the window, I have known people get so frustrated at doing this that they throw it out.
Leaks and so forth in the rubber parts will be dealt with in the section on the mercury B.P. machine.

The Mercury type.
First a quick description of a common configuration in mercury BP machines.
The whole unit is held in a box. Attached to the lid will be the scale, normally 300 mm long sometimes 270. It is held in place in a groove in the reservoir at the bottom and by a spring tensioned metal cap at the top.
In the cap at the top is a leather washer held in place by a large short screw.
The graduated tube is held at the bottom in a hole in the top of the reservoir and at the top by the spring cap. The leather washer is pressed on to the top of the tube.
The reservoir holds the mercury. The reservoir has a tubing connector on it for connecting the cuff to. Under the tubing connector is another leather washer held in place by a screw cap.
These two washers allow the air in and out but prevent the mercury from escaping.
The mercury should be of sufficient quantity to be level with the zero mark on the tube.

The first thing to do when you need to check one of these is to wrap the cuff around a round object about the size of a drinks can, connect it to the B.P. machine and inflate the cuff.
Check that the mercury goes up the tube, take it to the top of the scale so that it reaches the leather washer at the top, make sure it does not leak out.
Slowly deflate the cuff and check that the mercury goes back to the zero point.

Inflate the cuff again but stop halfway up, watch what the mercury does, it should come to a stop as soon as you stop pumping.
If when you stop pumping the mercury carries on going up there is a problem ( the same will happen when allowing the air out of the cuff), those not very experienced at taking blood pressures may not get this problem because they let the air out slowly and thus the problem is less serious.
You can allow for a little further rise or fall after you have stopped pumping but to much and the machine could be giving you high blood pressure results.
How high depends upon how blocked the leather disc is and how quickly you let down the cuff.
Why is this so? well, in simple terms, it is because the leather disc at the top is blocked or too thick and is not allowing the air in or out quickly enough. (less likely is the leather disc in the cap of the reservoir being blocked or very perished tubing is stuck together restricting the movement of air).
What happens when the mercury is going down is that the air is not able to get into the tube quickly enough, it is held back slightly because the leather washer is blocked or too thick, this means that the pressure in the cuff is slightly less than the column of mercury reads, thus the high reading.

There is a cure for this problem, turn the B.P. machine on it’s side (with the reservoir downwards) remove the calibrated tube, the top part which holds the tube is sprung so lift it up to take the tube out. Where the top of the tube rested there is a washer and then the leather disc, remove these carefully.
Take the leather disc and make sure that it is clean, if having cleaned it, reassembled and re-tested the problem still exists, try it without the disc in place, do not pump the mercury to the top because it will fountain out. You should find that the mercury stops as soon as you stop pumping. If it does stop as it should then the problem is with the top leather disc, if the problem still exists then the fault lies else where, either the leather disc at the top of the reservoir or a tubing problem between the cuff and the machine.
If after cleaning the top leather disc there is still a problem then it may be that the disc is to thick, this is something that will always have been there. Remove the disc again and while holding it between finger and thumb scrape it thinner with a scalpel blade.
Keep it turning around otherwise you will distort the shape of it.
If this still does not improve matters try some other material in its place.
The idea of the seal is to let air in and out but not to let the mercury out.
If after every effort you fail to improve it the user will just have to let the cuff down slowly.

Other problems.
The mercury drops before you open the release valve. Unless it is a very fast leak it is not that much of a problem because you allow the air out anyway when you take the blood pressure.
The obvious cause of this is a leak in the system, first check that the tubing and the cuff are not perished and that the connections are not loose.
The most likely cause is a leak in the one way valve of the pressure release valve.
To get at this, remove the inflation bulb, in the pressure relief valve you will see a piece of rubber with a hole in it (different makes may have different ways of doing the same thing), pull this out with a pair of forceps.
Some times you have to remove a fine mesh filter to get at the one way valve.
Clean this while it is out otherwise it makes it difficult to inflate the cuff.
The older valves were a piece of tube with a small ball bearing in the end and a slit in the side of the tube, the newer ones had the split at the end, this lets the air into the cuff but not out of it. Either way you will probably find fluff and things stuck in it, this will keep it slightly open so giving you the leak. Clean it out so that the lips close together.
Re-assemble the valve and bulb and test, it should have cured the problem.
The release valve itself may be leaking, there is little that you can do about this as most are made so that you cannot take them apart.

Dirty mercury and / or tube.
First a word of caution.
Mercury is considered to be a hazardous chemical, its dangers lie in prolonged exposure to the fumes given off as well as the mercury itself and the black mercuric oxide powder which forms after a while.
Take care to store all waste mercury in plastic bottles with a little water on top, the water stops the fumes.
When cleaning a B.P. machine of the black powder do so in a fume cabinet or outside.
Mercury will, in time, oxidise and start to turn to a black powder, this will stick to the side of the tube and make readings difficult.
From time to time the tube will need to be cleaned, you should empty out the reservoir and clean that at the same time.
Proceed as follows (do this outside if you can):

Remove the tube, if it is dirty you will have to clean it, the official mens of cleaning is with a long pipe cleaner longer than the tube, a piece of twisted wire with soft fibres woven into the twists. You probably won’t have one so you will have to get a small piece of cloth and push it through with a thin stick or metal rod.

To clean the reservoir. Leave the tube in place, remove the top of the reservoir, clean the sealing washer in the top, remove the mercury from the reservoir by pouring it into a clean bottle.
Remove the tube and clean it. Clean the reservoir by blowing out the black powder and wiping the inside with a cloth.
Replace the tube, refill the reservoir to the zero point on the tube, screw the top back on re-connect the tubes and cuff and test.

Rubber cuffs and tubes, repairs to.
Should you find a hole in the cuff it should be replaced, if a new one is not available it can be patched with a patch from a bicycle repair outfit.
New cuffs (and other rubber products) should be stored in a cool, dark and dry place. If you have any silicone oil, rub a light smear of that on before you put them away. Talcum power rubbed on to it will help, but not both together.
Tubing on B.P. machines always seems to crack where it is frequently bent, for example near connections.
Quite often it is possible to cut off a short length to make a good leak free connection again.

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