Back in November 2011, Seamus posed the question to me of whether compasses balanced for UK/Ireland would be satisfactory for use in PNG. My reply, which I have reasonable confidence of being valid, was as follows –
I don’t have experience with using out-of-zone compasses, but here are my thoughts.
1. Your comment "We were initially thinking that we could simply calculate any variations" is not applicable since the problem is difficulty-in-reading, not a systematic error.
2. I imagine UK-balanced compasses (or Zone-5 Aus-balanced ones) will be radically different from PNG-balanced. The balance issue is related mainly to latitude: the angle with which magnetic lines of force intersect the earth’s surface varies with latitude, being zero on the (magnetic) equator. Since PNG is an equatorial location, its own compasses would require very little weighting, while those in the UK, being used quite far from the equator, would be significantly weighted. Not a good match!
However, in your favour –
3. Usage of compasses in caves is often quite different to usage on the earth’s surface (eg nautical, bushwalking). We cavers frequently have situations where we are not sighting horizontally: we are looking up or down to varying degrees, and these situations produce the problem of ‘sticking-card’. We get around it by being smart – tilt the compass up-and-down to get the card to move, sight onto something above or below the real target, etc. This problem is much the same as using an out-of-zone compass in a flat environment, where the issue is also ‘sticking-card’. In certain in-cave situations of looking up or down, an out-of-zone balance might actually be better! If it’s not, then back-sight that shot to make the out-of-balance issue go the other way.
For this reason, I think you will probably get away with using UK compasses in PNG. But don’t count on it, or blame me!