Archive for the ‘OMV vaporiser’ Category

O.M.V. (Oxford Miniature Vaporiser)


Photographs and drawing in the illustrated version.
The description below might not be the same as for your vaporiser due to the age and model, however, it will be substantially the same.

This is a small Vaporiser which can be used without an anaesthetic machine for giving cheap and simple anaesthetics.
It works in a similar way to the large Vaporiser except that it does not have a temperature compensation device in it.
Instead, there is a chamber underneath filled with anti-freeze to help stabilise the internal temperature.
There are few things to go wrong with these units.
The most likely is that in time it will become stiff to operate the pointer.
This comes about because of deposits of thymol left behind by the anaesthetic agents.
So knowing how to clean it when required (or better still, before required) will be of great value.

It is a job which is easy enough with instruction, but one which must be done with care and careful thought and not rushed.
The only tools you need are a small watch makers screwdriver and a medium screwdriver (flat bladed) and the setting tool described below.

There are two cleaning procedures, the simple one and the less simple one.

If the control being stiff is the problem there is no need to dismantle the whole thing and clean the wicks. In this case use the simple procedure below.

The simple one.

This should be done as soon as the unit feels stiff, doing it then might prevent having to do the more difficult procedure.

The only thing you need for this is a rubber bung or similar to block the inlet and some cleaning fluid, ether or methylated spirits.
With ether this is best done outside to avoid the fumes and falling asleep.
Place the bung into the inlet; turn the Vaporiser on to its side with the outlet port pointing upwards.
Pour cleaning fluid into the inlet whilst moving the pointer to and fro.
The Vaporiser should be completely filled and allowed to stand for 5 minutes before emptying.
Once empty the vaporiser should be left open while air is blown through it for 15 – 20 minutes, this will rid it of any smells.
From time to time you may find that this does not free it up properly and you will have to go through the more difficult procedure.

The more difficult procedure.                                                                                                                                    This is done if you have to do a serious clean if the above procedure does not solve your stiff rotor problem if concentrations are falling off or there is an internal problem.
Apart from a few simple tools, you will need a setting gauge shown in fig A1.2.
Actually, the one shown and sold by the firm is a great deal more complicated than it needs to be.
For almost all models all you need is a piece of brass or stainless steel rod, ordinary steel will do except that in time it may go rusty, it needs to be 1/8 inch or 3.15 mm in diameter.
Cut off about 150mm and bend a right angle at one end, make the arm of the bend about 10mm long. Remove any rough edges.

If concentrations are going off and leaks are not a problem, it could be that the wicks are clogged with thymol, that is the main reason for wanting to remove the regulator and clean the wicks.

Clear your bench off, prepare your tools, all three of them.
You are now ready for the major cleaning procedure.
1. Remove the pointer- remove screw (13) and washer (32), lift off the pointer and abutment washer (33), remove scale.
Place in a safe place.

2. Remove lid – take out the three screws (53), and lever the lid off the body.
Note: These screws are rather short and with a fine thread, treat them very carefully otherwise you WILL strip the thread on the screw or the vaporiser.
If this does happen all is not lost, you will have to tap a new thread into the body and fit a larger screw, The tapping is done with a thread tap which you may have in your maintenance department or try a local engineering firm.

3. Remove M6 nut (35) and washer (36) and remove off – line hook (16).
Remove two screws (34), and take off tenon block (39) and clamp (40).

4. Remove 4 screws (27) and washers (28). Lift out regulator assembly (6), ‘O’ seal (21) and clamp ring (22).
Place all the screws in a safe place.

5. Part fill the vaporising chamber with cleaning fluid, (ether or alcohol), agitate gently to wet all parts of the wicks.
Allow to soak for 2 – 3 minutes repeating the agitation several times. Scrub with a toothbrush if necessary.
Throw away the liquid.
Repeat the process until the liquid appears clean.
Turn the chamber over and allow to dry completely.

6. If indicator glass is still dirty after the above process, remove the 4 screws (42) securing it, lift out the glass and wipe clean. Do not do this lightly, if you damage the seals and have no replacements you will be in trouble.

7. If after cleaning the wicks, corrosion is present on the wick e.g. rusty patches, reassemble the Vaporiser and return it to the manufacturers for replacement of the wicks. (their comments not mine)

8. Regulator – to dismantle.
Note: This description mentions an idler pinion, some models have only the rack and the pinion on the end of the pointer, others will have a pinion between that on the pointer and the rack this is the mentioned idler pinion. This is used to reverse the direction of the rack and slide valve relative to the pointer and scale.

Remove inlet and outlet cones (4 screws each Nos 25 and 26) gaskets (24) and obdurator assembly (19).
Remove 2 screws (23) securing the rack (20). Lift off rack and spacers. Note that the plastic sleeves inside the spacers must be retained. Note also the relative positions of the ports and the direction of the cone of the slider.
If there are no marks on the rack and pinion, mark across both with a pencil to assist in re-assembly.
Remove the slide valve by pushing from the outlet end of the regulator housing, if it is stiff use a wooden or plastic stick and a light hammer to tap it out, do not use metal as this will damage the machined surfaces.
Soaking the assembly in ether or alcohol will usually dissolve any deposits that may be causing the stiffness.
Wash the slide valve and regulator housing in cleaning fluid and dry with a clean cloth.
Pay particular attention to the sliding surfaces.
NEVER, NEVER clean the surfaces with sand paper, metal polish may be used to remove stubborn dirt but wash it off properly with alcohol or ether to remove any polish that may have been left behind.
Re-fit the slide valve to the housing.
Do not use oil or grease, check for smooth motion from one end to the other. Check the location of ports and the direction of the cone, re-assemble rack with spacers and screws, lining marked tooth on the rack between two marked teeth on the idler pinion, and the same time the marked teeth on the idler and the pointer pinion. (see fig 2.1).

9. Backlash and adjustment.
Backlash can develop between the rack and pinion and idler or between idler and pinion.
A tappet screw (29) is provided to adjust the engagement of the former.
The pinion (37) is mounted in an eccentric bush and engagement with the idler is adjusted by slackening the lock screw (30), rotating the eccentric bush using a 3mm bar in the hole provided until the engagement is correct and then tightening to lock screw.

10. Re-assemble regulator to body.
The special P.T.F.E. coated ‘O’ ring (21) which seals the assembly must be examined for damage and replaced if necessary.
Separate the clamp ring and regulator housing by removing 4 screws (27) and washers (28).
Insert the clamp ring into the body with the 4 screws at 45 degrees to the regulator housing looking from the top.
Immerse the ‘O’ ring in warm water (40oC to 60oC) for a few minutes to soften it.
Place the ‘O’ ring on top of the clamp ring.
Insert the regulator housing assembly using the 3mm bar to line up the screw holes in the clamp ring and the regulator housing, insert the draw screws into 2 diagonally opposite holes to draw the clamp ring into the body (hand tight only).
Insert 2 screws (27) and washers (28).
Remove the 2 draw screws, insert the 2 remaining screws and washers. Tighten all 4 screws evenly and fully, making sure that the regulator is pressed home fully into the body without gaps between the inlet and outlet square sections.

Note: This procedure can be done differently if you do not take apart the clamp ring and seal when removing the regulator from the body. When taking apart do not completely remove the four screws, just slacken them off.
When replacing you must be careful not to damage the ‘o’ seal when pushing the regulator into place in the body. Tease it in with your fingers.

11. Examine the obdurator assembly to ensure that the adjustment screws are sealed.
If the seal is broken, return the unit to the makers. (their comments not mine)
Assemble gaskets (24), obdurator assembly and inlet connector with 4 screws to the regulator housing.
Check that the obdurator is lined up with the slide valve.
Assemble gasket and outlet connector to regulator housing.

12. Leak test main body seal.
Ensure slide valve is open, block outlet connector, pressurise to about 180 mmHg through the inlet connector.
Run a small amount of cleaning fluid (ether or alcohol) into the joints between the regulator assembly and the body.
Check for bubbles.
Tighten screws or replace ‘O’ ring if necessary to obtain a leak free joint.
Tip out the surplus liquid and allow to dry.

13. Re-assemble tenon block, clamp, off line mounting block, lid and pointer. There is no need to replace the three screws that hold the top in place, but make sure the holes are lined up for the pointer checking procedure.

Do not fit the scale at this point.

Check pointer setting.

Do this by inserting the setting gauge (omv1) through the outlet port, (see fig 2.2) with the pointer set between ’50’ and ’60’ on the engraved scale on the lid.
Move the pointer until no further movement is possible. The pointer should indicate ’35’ on the engraved scale.
A positional error of +/- 1mm is acceptable, this allows for slackness due to old age.
If the gear train has been wrongly re-assembled, an error of 7.5o will be introduced per tooth, so errors are easy to detect.
Remove setting gauge, assemble the outlet connector and gasket (24), fit the scale back in place.
After completing the check then put in the three screws that hold the top in place.

Leak testing.
Equipment required for this test:

a. Source of compressed gas at about 200 mm Hg.
The book does say a source of compressed gas, however, I would advise you do not try to use gas from a pressurised cylinder, this is best provided by a blood pressure machine inflation bulb or an oxford inflating bellows.

b. Rubber bungs to fit the inlet and the outlet of the vaporiser.
One of which should have a steel or plastic tube through the middle with a diameter of about 6mm.

c. A reservoir of about 4 litres capacity (or so) capable of withstanding 200 mm Hg pressure.

d. A pressure gauge that reads about 200 mm Hg, a blood pressure machine would do nicely.

e. Liquid soap solution and a small paint brush.

f. Ether or Methylated spirits.

g. A stopwatch.

h. A pair of clamping forceps.

i. Some rubber tube to fit the connector in the rubber bung.
This procedure should be carried out if the unit has been reported to be giving low concentrations or is using excessive quantities of liquids.

The Vaporiser is connected to the reservoir, pressure gauge and compressed air source as shown in Drawing No 1
Air is pumped into the system until the pressure is reading 210 mm Hg, the air supply is them clamped off with the forceps.
The pressure in the system will fall slowly and the time taken for the fall from 200 mm Hg to 190 mm Hg is recorded by the stopwatch.
The check is carried out with:
1. The pointer set to the off position to test the connectors and the top end of the regulator housing.
2. The control pointer in the ‘3.5’ position to test the vapour chamber joints generally.

3. The control pointer to the OFF position and the filler held open to test the vapour seals.

Acceptance values.
1.30 seconds or more.
2.30 seconds or more.
3.15 seconds or more.

If the Vaporiser does not pass this test, the position of the leak is best found by brushing the suspected joints with soap solution whilst maintaining the pressure.
It will show by bubbles forming at the site of the leak.
Do not apply soap solution to the opening around the rack where it could enter the slide valve, it will dry up and cause the slider to stick.
Note: There is always a certain amount of leakage from the slide valve, but this can be ignored if the test figures given earlier are obtained.

Specific repairs.

1. Level indicator and filler unit, to fit a new indicator glass.
Remove 4 screws (42) take off retainer, old glass, seal and centring ring. When fitting a new glass always fit new seals.
Glasses vary a little in thickness, three seals are provided in the spare parts kit and sufficient seals should be used to obtain a good compression on the glass when the retainer is screwed back into place. Check again for leaks.

2. To fit a new drain seal.
Remove drain screw.
Use pin spanner (A1.3) to unscrew the old drain seat assembly (49) from filler block.
Discard old assembly and seal.
Fit new assembly and seal (52), tighten securely.
Leak test again.

3. To fit a new back seal.
If a leakage develops between the filler block and the body, remove level indicator glass.
With a screwdriver, lever out the engraved back plate of the level indicator, this will expose the heads of 2 socket screws (47).
If tightening does not cure the leak, remove the screws, lift off the filler block and fit a new seal (48) between the block and the body.
Re-assemble all parts and leak test.
4. To replace the complete base.
The top is fixed to the base using ‘Araldite 100’, an adhesive.
If this joint is broken due to a fall, the same adhesive should be used to re-affix the base.
Clean off the old adhesive before re-affixing and ensure that the recess in the base is aligned with the water filling screw (12).

The firm advises that the unit should go back to the factory every 2 years, I should leave this to the judgment of the anaesthetist.
For many hospitals this will not be practical, cleaning as required and treating the unit with care should ensure a long life.

Should the sight glass break and you do not have an official replacement and are unlikely to be able to get one, you will have to make your own. I did this once for a different reason. In this case we were doing some experiments on the vapouriser and wanted to measure the temperature inside. I made a glass out of some window glass, shaping it on a grinding wheel, then drilling a hole in it for the temperature probe to go through.
Do wear goggles and a mask when doing this.

Calibration procedure.
These instructions are for trained personnel using the equipment listed below:-

1. A supply of dry, clean compressed air, with a pressure regulating system producing
15 – 45 psig.

2. A flowmeter with a needle valve control to produce a measured flow of 4 – 6 lpm with 5% accuracy. (My comments: most flowmeters can manage this accuracy).

3. A fresh gas supply tube to connect the flowmeter outlet to a cagemount female taper connector to fit the vaporiser inlet.

4. A mixing chamber with a cagemount fitting to fit the vaporiser outlet, with an exhaust port for 22 mm tubing and a sampling port for 6 mm tubing (part number MH561)

5. 22mm tubing to the disposal system. (My comments: out of the window / to a scavenging system or suitable filter or sit outside in the sun.)

6. A Riken gas analyser, model 18, calibrated 0 – 6% halothane. (My comments: or other similar).

Caution: The sampling tube between 4 and 6 must be of nylon, PTFE, or similar material which does not absorb anaesthetic vapours. The use of rubber or other such materials may cause large errors in measurement. Short lengths of rubber may be used to join nylon tubing to the analyser and the sampling tee.

Choice of agent:
If the vaporiser is known to be dedicated to a specific agent, it should be calibrated to that agent.
If the vaporiser is used with various agents, it should be calibrated using halothane, as that agent gives the highest numerical scale values and therefore maximum accuracy of measurement.
The design features of the vaporiser which determine the relative output of the various agents at a specific setting of the control are dependent upon agent, temperature and pressure.
These three factors can be considered individually as follows.

1. The halothane in air Riken analyser can be used for other agents by multiplying the readings by the factors given below to give true Vol %. These factors are determined by the relative refractive indices of the vapour.

Agent Factor
Halothane 1.00
Enflurane 1.05
Isoflurane 1.06
Chloroform 1.14
Trichloroethylene 0.88
Methoxyflurane 0.86
Di ethyl ether 0.92

2. Temperature.
Calibration should be carried out at 20oC +/- 2oC. Within this range a temperature correction factor is not required, given the accuracy of measurement available.

3. Pressure.
Normal variations in barometric pressure due to weather changes are not significant given the accuracy of measurement available, but altitude may produce significant effects, as shown below.

Correction factor
Barometric pressure

610 m (2000ft)
x 0.9
910 mbar

1229 m (4000ft)
x 0.84
850 mbar

1830 m (5000ft)
x 0.8
813 mbar

Calibration procedure.

1. Connect the fresh gas directly to the mixing chamber (bypass the vaporiser).

2. Set a flow of 4 lpm and take several readings with the Riken to establish a true and consistent zero.
3. If necessary, flush the reference cell of the Riken with air and ensure that that this cell is equilibrated to atmospheric pressure.

Note; This is most important if the Riken analyser has itself been moved from one altitude to another.

4. Fill the vaporiser approximately three quarters full with the selected agent (see choice of agent) and attach the appropriate scale.

5. Allow the vaporiser to stand with the control at zero for 30 minutes to ensure all temperatures are equilibrated.

6. Connect the vaporiser to the fresh gas inlet and the mixing chamber.

7. Turn on an air flow of 4 lpm and allow the vaporiser to stand for 4 – 5 minutes with the control at zero.

8. Take a series of readings at various control settings with the Riken and record the results on the record sheet (see below). Each time squeeze the sample bulb a minimum of 3 times starting at the given time on the sheet.

Acceptance figures.
The readings, when corrected, should be within 20% of scale. a greater degree of depression than the above indicates that the wick system has been incorrectly assembled or that it is clogged up.

9. After calibration, drain the vaporiser chamber and connect the unit to an air supply.

10. With the control set full on and the exhaust system attached, blow air through until the smell of anaesthetic agent has been removed.

Halothane contains thymol as an additive and it is not possible to get rid of the smell of this substance.

11. Close the vaporiser control to zero before passing the unit for use or storage.

Inspection sheet:
Service at:
Serial number of unit:
Agent used:


Riken reading.
Correction factor (agent).
Vol %

After all this you will realise that you cannot do anything to adjust a vaporiser out of calibration, (unless as they say, the wick has been incorrectly assembled or the wick needs cleaning.).

The E.M.O, the O.M.V and OXFORD BELLOWS, amongst other things, are made by the English firm of PENLON, they are a very helpful firm who are interested in making good quality equipment suitable for use all over the world.
In the unlikely event of you having to send something back to them here is their address.

Penlon ltd.
OX 14 3PM

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