The SPK3 is a great tool to quickly take in-duct, ambient, and register temperatures.  It's a handy tool to assess a system and it fits conveniently in your pocket.  The ROD has a sharp, narrow tip to easiy insert into duct work, and the hanging hooks allows you to take hands-free measurements.  The bright back lite display flips between the IR and ROD measurements so readings are always easy to view.  Plus this handy tool quickly calculates T1 - T2 across both the IR and ROD modes, or a combination of the two.