The FridgeRock's fan comes with a thermistor attached by the outside leads which monitors case temperature, and adjusts RPM to suit. It's not the best way of scaling a fans speed to meet the demands of a warm computer, but it is an easy method to implement. The little black thermistor is tucked in there so well we just about missed it altogether in fact.

It should be noted that as you look at the results of this heatsinks performance on the test platform, the fan was rotating at about 3500RPM because we test in ambient temperature conditions. The downside to this technique which is used to keep fan noise down, is that the sensor doesn't actually measure the temperature of the heatsink. The actually metal body of the heatsink will always be much hotter than the surrounding air temperature.

	Heatsink Specsheet: Model: 5U213C1H3G HS Material: extruded aluminum, copper insert Fan: 3500-4200, 12V, 0.3A Fan Dim: 25x70x70mm FHS Dimensions: 68x73x78mm Made by: Spire Sold By: www.spirecoolers.com

The basic design of the Spire 5U213C1H3G is comparable to what we have seen in the past from them. The extruded aluminum heatsink comes with a small copper insert which is held to the rest of the heatsink with a pair of screws. The thermal interface between the copper and aluminum is standard white silicon-based thermal compound. The effect this will have on the overall performance of the heatsink should be pretty obvious compared to a copper plate which has been soldered on via nickel plating the aluminum surface. In the image below you can see some of the white thermal compound showing at the joint.

The extruded heatsink has an interesting assortment of fins - with one cross cut for the clip, and a few extras on the outside for good measure.