Scientific instruments can help to shape and re-shape epistemic boundaries, especially those between communities of scienti?c researchers. But how do they function at boundaries between scienti?c communities and communities of non-experts? This paper examines the use of air monitoring instruments at the boundary between petrochemical facilities and nearby residential communities, asking whether a new generation of fenceline monitors shared by industry (and regulatory agency) experts and community members alter the epistemic boundary between the two groups. Arguing that epistemic communities organized (...) around instruments are characterized, in part, by a common understanding of the evidential contexts for instrumental data, the paper shows how the evidential contexts in which experts and residents interpret monitoring data diverge. Instead of the new, shared fenceline monitors helping to reconcile differences over evidential contexts, those pre-existing contexts shape the interpretation of data from the new instruments–perpetuating epistemic boundaries between industry experts and community members. (shrink)