Thermopile Flow Sensors and Differential Pressure Flow Meters

Xiang Zheng Tu

 

Air conditioning can refer to any form of technology that modifies the condition of air including heating, cooling, (de-)humidification, cleaning, and ventilation. In order to do so air movement needs to be created and air flow needs to be measured. This is hwy differential pressure flow meters are popular for a long time. But in recent years things have changed. Differential pressure flow meters have been gradually and irreversibility replaced by thermopile flow sensors.

It is not surprising in view on the working mechanism of the differential pressure meters and related limitations to the flow measurements. The working mechanism is based on Bermoulli’s Equation. Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate, as shown in the following figure.

 

It can be seen from the above figure that using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flow meter accuracy is dependent upon the meters being accurate over a 100:1 range of differential pressure. The meters accuracy is typically degraded at low differential pressures in its range, so flow meter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flow meters. Remember that our interest is the accuracy of the flow measurements instead of the differential pressure measurements.

In addition, the flow rate measured by the differential pressure flow meters is not mass flow rate that is required by many applications. According to ideal gas law, gas pressure changes with its temperature and volume. To obtain a mass flow rate, it is necessary to measure additional parameters: differential pressure; absolute pressure; and absolute temperature. These measurements with the differential pressure measurement then sent to a computer for calculating the mass flow rate.

All these limitations with the differential pressure flow meters can be eliminated by the thermopile mass flow sensors. The thermopile flow sensors use the thermal properties of the fluid to measure the flow rate. A measured amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing fluid. As flow rate increases, more heat is lost. The amount of heat lost is measured using the thermopile(s) in the sensor. The output of the thermopile(s) represents the fluid flow velocity or flow rate.

The thermopile flow sensors are fabricated using micromachining techniques in a CMOS production line. They offer many advantages over the differential pressure meters, including but not limited to:

• Large dynamic range
• High accuracy
• Excellent low flow sensitivity
• Direct mass flow sensing
• Low pressure drop
• Very low power consumption
• Miniaturization and small device footprint
• Manufactured in CMOS production line and low cost

The thermopile flow sensors are not only used for air flow measurement in air conditioning, but also for monitoring flow in clean room, in fan/filter units, controlling flow in production facilities in the pharmaceutical, food processing and semiconductor industries; and monitoring flow in glove boxes, insulators, medical equipment such as anaesthetic machines and respirators in order to maximize energy efficiency, and also increase the accuracy of gas flow control.