Image Number 4 for United States Patent #7971480.
One embodiment of the present invention involves a thermal sensor and a method of using the same. One thermal sensor is adapted to output a signal which is unaffected by external longitudinal and orthogonal thermal gradients. In one embodiment, the mass flow controller thermal sensor comprises a capillary tube having an upstream tube portion, a tube bend portion, and a downstream tube portion, the downstream portion being substantially parallel to the upstream portion. A distance between the upstream tube portion and the downstream tube portion in one embodiment is no greater than half the upstream portion and downstream portion lengths, a first pair of thermal sensing elements are coupled to the upstream tube portion and a second pair of thermal sensing elements are coupled to the downstream tube portion.