Thermopile Natural Gas Mass Flow Meters

Xiang Zheng Tu

  

Natural gas flow meters are used at residential, commercial, and industrial buildings that consume gas supplied by a gas utility. Several different types of gas flow meters are in common use, depending on the flow rate of gas to be measured, the range of flows anticipated, the type of gas being measured and other factors.

Diaphragm gas flow meters are one type of the most common and oldest gas meters. The advantage of these gas meters is simplicity of construction and, therefore, low cost but their limits are as follows:

(i)             presence of moving parts subject to wear;

(ii)           high pressure losses;

(iii)         mechanical output and

(iv)          inability to indicate an instantaneous flow rate value.

Nowadays new safety-related and consumption-control-related functions have brought about the development of better performing flow meters, with features such as:

(i)             distribution network and user-connection/disconnection blockage valves remote control;

(ii)           remote consumption reading; (iii) overflow and minimum level flow rate alarms;

(iii)         self-control and diagnostics;

(iv)          metrological performance improvement (accuracy, rangeability, stability and thermodynamic condition compensation); (vi) size reduction; and

(v)           advanced computing functions (prepayment and time bands).

Ultrasonic flow meters could represent a good solution in such better performing flow meters. There are two leading types, the transit-time and Doppler style meter. In the transit-time ultrasonic flow meter, the transducers are upstream and downstream of each other and each act as a transmitter and receiver. One transducer, of course, emits the ultrasound signal with the flow, while the other emits it against the flow. The meter measures the difference in transit time between the two transducers, and the velocity difference is used to calculate volume flow.

Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. They vary greatly in purchase price but are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.

Actuary, gases are more difficult to measure than liquids, as measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quantity or quality of the gas flowing through the meter. Temperature, pressure and heating value compensation must be made to measure actual amount and value of gas moving through a meter.

To solve these problems thermal mass flow sensors could be the best choice. As shown in the above figure, thermal flow meters measure mass flow, not volumetric flow, and use heat disperse to compute the measurement. The primary reason thermal mass flow meters are popular in many applications is their particular features including no moving parts, nearly unobstructed straight through flow path, require no temperature or pressure corrections and retain accuracy over a wide range of flow rates.

The thermal natural gas mass sensors provided by POSIFA Microsystems are manufactured using an US patented technology. The sensor comprises a porous silicon wall with numerous vacuum-pores which is created in a silicon substrate, a porous silicon membrane with numerous vacuum-pores which is surrounded and supported by the porous silicon wall, and a cavity with a vacuum-space which is disposed beneath the porous silicon membrane and surrounded by the porous silicon wall.

Compared to the other thermal natural gas flow sensors, the vacuum-cavity-insulation flow sensor presents superior properties in many aspects.  Among them are easiness of fabrication, perfection of thermal isolation, strength of membrane structure, and lower cost of manufacturing. They have additional performance such as auto-diagnostic, data-recording, block and other functions that can be integrated in an electric output sensor. In the near future they could be an excellent replacement for the ultrasonic flow meters.