Immersible thermal sensors arc finding growing acceptance in the gas flow measurement field. By using two Platinum Resistance Temperature Detectors (PRTDs) immersed in a stream of flowing gas, modern thermal flow meters can directly measure mass flow, without the need for temperature or pressure compensation. Thermal meters offer very high turndown, accu- racy and repeatability at an economical price. However, all ther- mal sensors are not created equal. Recent optimization of sensor design has led to a marked increase in the performance of ther- mal mass flowmeters in the field.
To understand just what role sensor design plays in thermal flow meter performance, it is necessary to understand the basics of how immersible thermal sensors work.
An immersible sensor typically consists of a heated element (called the velocity sensor) and a passive element (called the tem- perature sensor). Both are precision PRTDs. In operation, the velocity sensor is heated to a temperature Tv. As the gas veloci- ty (the flow) increases, heat (Q) is removed via forccd convection and the velocity sensor begins to cool. The gas temperature (Tg) is simultaneously being measured by the temperature sensor (see Figure 1). The sensor electronics compares the measured tem- perature value of the velocity sensor (the resistance of the PRTD changes linearly with temperature) to that of the temper- ature sensor and is designed to maintain the velocity sensor at a constant temperature differential above that of the gas. In other words, the difference, Tv-Tg , is maintained at a constant 'setpoint' value. This is accomplished by adding more power to the heated PRTD that heats up the velocity sensor until Tv has been increased to its set-point value. The wattage (power) added to accomplish this is proportional to the heat, Q, removed, and this is directly proportional to the mass velocity.