The dominant property of stainless-steel that is relevant to the decision is that it is essentially non-corroding. A second well-known property is that it is non-magnetic; a third (and less obvious) property is that it is a very poor conductor of electricity.

The Pulse Induction technique used by Elcometer in all its metal detectors, including the Elcometer P350 / P351 covermeter and the Elcometer P330 covermeters, responds primarily to the electrical conductivity of a target.
This means that it will respond to both ferrous and non-ferrous metals (but not to the ionic conductivity of water containing dissolved salts); in the case of ferrous (magnetic) metals, the conductivity signal gets 'magnified' by the magnetic permeability of the target metal.
If stainless-steel were merely non-magnetic (but otherwise a good conductor), the Elcometer P350 / P351 would respond as well as it does to e.g. copper pipes -- i.e. yield an adequate signal for location purposes (though not as strong as that from a ferrous bar or pipe).
However, the poor conductivity of stainless-steel means that the basic conductivity signal will be considerably weaker than that from any other metal.

Also, it is a well-established advantage of any covermeter cover meter that, when measuring high-tensile bars, the signal strength does not depend strongly on bar diameter (thus allowing accurate cover determination even when there is some doubt as to the exact bar size); but the signal from stainless-steel rebars varies very rapidly with bar size, so that the signal from smaller sizes is considerably weaker than that from the medium and larger sizes.

As a result:

stainless bars of 6mm or 8mm diameter are undetectable;
Stainless bars of 10mm and 12mm diameter give signals about 3% and 10% (respectively) of that of a high-tensile bar of the same size; this means that it may be possible to locate them (especially if their cover is shallower than normal, which may well be the case), but the indicated covers are quite meaningless (indicating too deep by about 60 and 35mm respectively!).
Stainless bars of diameters of 16mm and upward give about half the signal that would have been expected from a high-tensile bar. This means that there will be no difficulty in locating them (although the directionality distinguishing between vertical and horizontal bars is not as clear-cut as usual). Indicated covers are overestimated by about 5mm to 10mm, which can be rather confusing (unlike the smaller sizes which are so obviously wrong); dialling up about half the actual size on the DIAM knob will often yield an essentially correct cover for these larger stainless bar sizes.

Thus, if you go looking for 8mm stainless reinforcement, and apparently find some at (say) 35mm cover, then it isn't stainless! (or rather, not grades 304 or 316 -- the ones usually specified -- but possibly a ferritic or duplex grade of not-particularly-nonmagnetic steel; or even galvanised ferrous).
Conversely, if a structure does contain some small-diameter stainless bars, and these must be located, the best solution is to use the Elcometer P150 stainless steel rebar locator which is also perfectly suitable for stainless rebars. This instrument does not measure cover, but by comparing signal-strengths with those obtained from bars at a known distance, an approximate guide to cover can be obtained (assuming the size of the reinforcement is known).

Where it is vital to measure cover over stainless reinforcement, please contact Elcometer by clicking on contact us below.
stainless steel covermeters are currently under development, and a prototype model may be available.