Thermal mass flowmeters use heat to measure mass flow. These meters are bypass or shunt flow meters. The meter uses a probe to introduce heat to the fluid, at midpoint of the sensor tube. RTDs are located upstream and downstream of the meter to measure the fluid temperature. The temperature variation (or ΔT) between the two measurements is proportional to mass flow. As the fluid velocity increases, the ΔT goes to zero. These types of meters are most accurate in low flow settings, and available as pressure or flow controllers.