Representable posets

A poset is representable if it can be embedded in a field of sets in such a way that existing finite meets and joins become intersections and unions respectively (we say finite meets and joins are preserved). More generally, for cardinals $\alpha$ and $\beta$ a poset is said to be $(\alpha ,\beta )$-representable if an embedding into a field of sets exists that preserves meets of sets smaller than $\alpha$ and joins of sets smaller than $\beta$. We show using an ultraproduct/ultraroot argument that when $2\le \alpha ,\beta \le \omega$ the class of $(\alpha ,\beta )$-representable posets is elementary, but does not have a finite axiomatization in the case where either $\alpha$ or $\beta =\omega$. We also show that the classes of posets with representations preserving either countable or all meets and joins are pseudoelementary.