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01 March 2005

Wheatstone the flow

Many times in industrial application, measurement of very low flow rates is required with fair accuracy. Fluids like natural gas, petroleum, and oxygen flow to customers on a continuous basis through dedicated pipelines. Precise online measurement of quantum of supply is essential for payment purposes. The present work has aimed to design a sensor to measure low flow rate accurately. This sensor uses the important properties of the thermistor and is cost effective.

Anemometer Source: Institut für Meteorologie und Klimaforschung

Different patented versions of thermal flow meters are available. All of them platform on the concept that fluid carries heat away and that heat varies as the fluid flow rate. One thermal sensor senses the temperature of fluid in motion. The other carries a steady current. The latter is near the former, but it's in the fluid flow. Its temperature changes vis-à-vis the former calibrates to the fluid flow rate.

Another version uses a pair of thermistors kept at a definite distance from each other in the flow stream. A definite amount of heat energy, as a sharp pulse, injects into the flow stream through the thermistor placed upstream. By sensing the change in temperature profile of the thermistor placed down stream, the time delay for the thermal pulse to travel registers. This represents the transit time of the fluid from the first thermistor to the second.

A thermal anemometer measures fluid velocity by sensing changes in heat transfer from a small, electrically heated sensor exposed to the fluid.

The heated sensor holds at a constant temperature using an electronic control circuit. Increasing the current flow to the sensor compensates for the cooling effect resulting from the fluid flowing past the sensor. The magnitude of the current increase needed to keep the temperature constant relates directly to heat transfer and the flow velocity. This parameter is available as an output voltage.

At the heart of a thermal anemometer are two sensors: an air velocity sensor and a temperature compensation sensor. The velocity sensor heats to an elevated temperature (relative to the surrounding air) by means of control electronics. The temperature compensation sensor senses the ambient, or surrounding, air temperature and forces the velocity sensor to stay at a constant "overheat" above the ambient.

The design here has a pair of sensors connecting to an inverting amplifier such that the current through both sensors is the same. One exposes to airflow and mounts on a copper plate around which winds thin copper wire. The other is in the vicinity in a closed vessel. In the absence of airflow both the sensors will be at same temperature and have equal resistance values. When air flows through one of the sensors, its temperature comes down and its resistance increases. The increment in resistance converts in terms of voltage since current is same. This voltage is conditioned and fed back to the coil wound on copper plate. This coil heats up the same thermistor so as to maintain it at a constant temperature. The output voltage links to the data acquisition system and displays there.

This system achieves a threshold of 0.02 liters per minute (LPM). The range of the instrument is 0.1 to 10 LPM.

Measuring flow with Wheatstone bridge

Nicholas Sheble edits the Sensors department. This piece comes from Amit Kumar Pandey's paper that he presented at ISA's SIcon/05 in January in Houston.