A self-heated termistor flowmeter for small liquid flow in microchannels

Abstract

A self-heated thermistor flowmeter, based on two tiny, glass-encapsulated, NTC thermistor probes, for measuring liquid flow as small as tenths of a millilitre per minute with long-term stability and fast dynamic response has been developed. A new circuit configuration, mainly consisting of a multiplier as well as an active bridge for computing the power dissipated within the measurement probe, which is taken as the output of the thermistor flowmeter, is presented. The circuit configuration ensures that the thermistor flowmeter really operates in constant temperature-difference mode, and combining the temperature characteristic linearization method for NTC thermistors derived here, allows an easy, accurate fluid temperature-dependence compensation to be made. The compensation network needs only one adjustable parameter, the optimal value of which can be found through a simple computation or experimental adjustment. Thermistor probes with matched temperature coefficients are unnecessary. The flowmeter will be used to measure the liquid flow in microchannels in flow injection analysis.