LIQUID COMPASS OVERHAUL

Overhaul of P Type, E2B/C and “Landing type” compasses is carried out by:

SIRS Navigation, Compass House, Bowes Estate, Wrotham Road, Meopham, Kent DA13 0QB, build

these compasses, from new, and stock all known spares, since they took over the line from Smiths.

Pandect Instruments, of High Wycombe

Airtron Instruments (Bruce Irvine) 40b St Pauls Street, Stamford, Lincolnshire, PE9 2BH

DIY Refilling:

The recommended liquid is denatured alcohol, or Industrial Methylated Spirit I.M.S. 99, which must be

totally de-aerated for at least 1 hour. The seal, under the glass, is (now) 3mm rubber (used to be 1/8",

or, sometimes, a gooey 2-part mixture,) and the seal, for the filler point, is a fibre washer, which must be

placed in some of the alcohol, during the de-aeration process, so that no air pockets remain in the fibre.

During the filling process, also, the flexible diaphragm/bellows, in the base of the inner bowl, must be

pulled out, about 1/4", to allow for any expansion/contraction of the fluid, due to temperature changes.

The paint is a special alcohol-proof type, which is not proof against many other liquids, especially

cellulose, or white spirit (used in many smaller compasses.)

DIY ‘P’ TYPE COMPASS LIQUID REFILL (based on a P8)

1 - Dismantle compass and remove the bowl. To find leak fill with tap water. Empty contents into measured vessel to

record volume. Add a bit more to make up a volume total, just in case.

2 - Remove grid ring and clean and dry out bowl. If the grid ring was leaking, dry it and repair with clear epoxy resin.

Glasspaper ring underside till extra smooth. Seal the join between bowl and ring with black rubber sealant, thinly

applied over both surfaces (black rubber car window seal repair, in a tube from a car accessories shop). This looks like a

good match to the original.

3 - It you don't have a spirit hydrometer with gauge 0-.99, (a beer gauge is no good), to make up the required mix of 0.81

specific gravity, (as alcohol reduces the specific gravity of water to below 1), try using the following recipe which is good

enough for display purposes only –

Note • pure alcohol is very difficult to acquire in small quantities, so buy a bottle of Surgical Spirit (98% proof) from the

chemist, the odour will not affect anything, together with a small quantity of distilled water (as used to top up car

batteries). A ratio by volume of 89% surgical spirit to 11% distilled water is about right. Pour both very slowly into a

clean measured container. Mix thoroughly, and notice the liquid will appear to be cloudy, leave undisturbed with a cover

over it for a week. It will clear. Try not to move the liquid at all, to avoid bubbles.

Note: Denatured alcohol, Industrial Methylated Spirits “IMS 99”, Isopropyl Alcohol etc. can be purchased from Reagent

Chemical Supplies www.reagent.co.uk (email; info@reagent.co.uk).

4 - Find a new syringe (farmers supplies or car accessories supply shop -fibreglass repairs section) and fix a clean long

thin narrow piece of clear tubing firmly onto the end (old squeezy spray bottle?).

5- Set up the compass bowl so that the filler hole (seen on it’s side) is now securely placed vertically, and begin to very

slowly fill up the syringe and then with great care release the liquid into the bowl. If necessary gently tap the glass if a

bubble appears. It will take time to do. When you reach the top, continue on to fill the outside screw space as well.

Again carefully tap the bowl all round and look out for any sign of bubbles that may rise. If the level falls repeat until you

are satisfied. Replace nut. Wipe dry all over, shake and rotate bowl to check for bubbles and or leaks. Suggest you leave

it out overnight, before refitting the lot, to make sure the job is well done.

Terry’s View:

Bear in mind the materials used in the vicinity of compasses; they must not be iron or steel.

When it comes to the Liquid Filling consider the purpose(s) of the liquid:

1.To partially “float” the magnet system and so reduce pivot friction.

2. To damp the magnet system movement, allowing alignment without overswing or oscillation.

3. To lubricate the pivot and so further reduce pivot friction and reduce pivot wear.

You should determine the manufacturer’s original fluid type if possible and do not mix the original fluid

with any substitute, in case of any interaction. Drain the original fluid and compare it with any

proposed substitute. The requirements of the fluid are that it should be colourless and transparent,

have a small coefficient of expansion, low viscosity, low freezing point, be non corrosive, have no

interaction with dial, markings or internal paint and act as a lubricant.

Various spirit and oil based fluids have been, and are, used: “IMS 99” 90% Alcohol with 10% water*,

Industrial Methylated Spirits*, Ethyl Alcohol de-natured with 5% methanol, Isopropyl Alcohol*, Exxon

Mobil “Isopar L”, odourless mineral spirits. A mixture of 60% glycerole and 40% Isopropyl alcohol, or

supermarket glycerine and methylated spirits has been suggested, mostly these are petroleum

distillates. The most recommended is purified paraffin, best obtained as “lamp oil”.

Available from; Reagent Chemical supplies, 18 Aston Fields Rd, Whitehouse Ind Est, Runcorn,

Cheshire , WA7 3DL

T; 01928 716903

E: info@reagent.co.uk web; www.reagent.co.uk

See also file “Compass Types & Liquids.doc”.

An idea from a contributor

Dissolved Air

The manufacturer and compass repair shops will tell you that you must put the mineral spirits in a

chamber and apply a 28"Hg vacuum to it for 15 minutes to remove dissolved air. If you don't do so, you

will get a bubble on cold mornings. However few of us have access to a vacuum/pressure chamber. After you

fill your compass with Odorless Mineral Spirits (being careful to leave no bubble), put it in the freezer

overnight. In the morning you will find a bubble. That bubble is the dissolved air that the vacuum pump

would have removed. Either apply light pressure to the bellows or capsule to force the bubble out or add more

mineral spirits.

Some Shrinkage Will Occur

If you remove the plug to add more mineral spirits, you will find that a lot more air is sucked into the

compass as the volume of mineral spirits has shrunk due to the change in temperature. No problem, just top

it up. Now this is important - as the compass begins to warm up the mineral spirits will expand and you

will need to allow the excess spirits to bleed out through the plug (remember, you added a lot more mineral

spirits than the volume of the bubble to compensate for the shrinkage). Once the compass is close to room

temperature you can close the plug.

Another Method

I'm sure you're thinking: why not just put the mineral spirits in the freezer overnight. Good question. I used

an epoxy adhesive syringe to fill the compass and it was all that the syringe could do to suck up the spirits

at room temperature. The rubber plunger seal kept coming off the plunger. I don't think it would handle the

colder temperatures and greater viscosity very well. If you decide to cool the spirits in the freezer, consider

that the container will deform badly as the spirits shrink. Leaving the cap loose would solve that problem

but consider that the vapours are flammable and a freezer fire might result. I think it's safer to add the

spirits to the compass at room temperature.