DIFFERENTIAL PRESSURE AND ENERGY
built with the low-cost, thin-film batch processing characteristic of integrated circuit fabrication. In addition, there is the option of incorporating active silicon circuit elements on the chip with the microtransducer, thus achieving a larger scale device integration with lower cost, lower device size, and easier applicability. These expectations are now close to being realized in practical devices at Honeywell with the development of an air flow and differential pressure sensor. This paper explores how these techniques may be used for natural gas flow, differential pressure and energy flow measurement.
DISCUSSION
Microtransducer Design for Flow Sensing of Gaseous Fluids
The choice of materials for the transducer was strongly influenced by the objective of making the finished device easy to manufacture. For example, a high temperature ceramic was chosen as the dielectric because it can be deposited to tightly controlled specifications and is an excellent insulator and passlvatlng film. A metal alloy was chosen for the resistor metallization because deposition technology was well established. In addition, the alloy’s higher resistivity than pure metals allows the delineation of resistors in the 500 to 1000 ohm range on a 150×350 micron area using 5 micron line widths while maintaining a substantial temperature coefficient of resistance of about .003 per degree centigrade. Standard (100) silicon was chosen for the substrate because of its desirable anisotroplc etching properties and its suitability for later circuit integration with the transducer.
The basic flow transducer consists of a pair of ceramic film bridge microstructures, as shown in Figures 1 and 2 containing a central heater resistor divided equally between the two bridges, and two identical detector resistances placed adjacent to and symmetrically with respect to the heater resistance. In operation, air flows across the chip perpendicular to the long axes of the bridge structures, cools the upstream detector and heats the downstream detector. The resulting temperature and corresponding resistance differential yields a circuit output measurement of the flow rate. The use of identical detector resistors eliminates zero point offsets. The bridge structures are less than one micron thick, and their small heat capacity provides a short thermal time constant, typically .005 second or less, and a high sensitivity to rapid flow changes.
The air space beneath the bridges thermally decouples the heater and detectors from the silicon to a large extent. Consequently, large heater temperature differences in the 100 to 200°C range relative to the silicon can be sustained by small power inputs. The thermal efficiency is typically 15°C per milliwatt of input power under no-flow conditions. The thin ceramic film permits the detector resistors to be placed closely adjacent to the heater so that they can operate at about 60 percent of the heater temperature elevation, and can develop large temperature differentials under small gaseous flow conditions.
The etched cavity below the bridge pair is precisely limited at the sides by the etch-resistant (111) planes of the silicon, and at the bottom of the cavity and the ends of the bridges by accurate timing of the etch duration. The symmetry and effectiveness of the etched undercut of the bridges is maximized by orienting the axes of the bridges at 45 degrees to the <110> directions in the silicon.
Drive Circuit Operation
Figure 3 shows heater and sensor circuits used to operate the flow sensor. The sensor circuit is a conventional Wheatstone bridge circuit. However, the heater circuit, as shown in Figure 3, is uniquely adapted to the flow sensor to provide an output proportional to mass flow, and to minimize errors due to ambient temperature changes. The circuit is designed to keep the heater temperature at a constant
differential above the ambient air temperature under conditions of ambient-temperature variation and air flow variation. The ambient temperature is sensed by a similar heat sunk resistor on the chip, and the chip temperature (which remains within about one degree of the air) is a satisfactory approximation to the ambient.
This mode of heater operation also provides means to sense changes in air or gas composition (with or without flow) which alter the thermal conductance and thus change the operating temperature of the heater and detector resistances. For example, extreme changes of thermal conductance, such as that encountered between air and helium, cause large differences in power needed to hold the heater at its constant temperature differential. This is one input for the determination of natural gas heating value, which in its first approximation is proportional to its density or average molecular mass.
The error integrator is the active component in the heater circuit. It integrates the voltage differences seen on the Wheatstone bridge, and changes the voltage to the heater to maintain the bridge circuit balance. The error in heater temperature can be kept less than 1.0 percent of its differential above ambient over an ambient temperature range of −40°C to +80°C.
Chip Packaging and Application Considerations
The chip is fabricated with passivation techniques to permit its exposure to gaseous environments during a long operating life. To prevent corrosion, gold pads and gold wire bonds are used, and all other surfaces are passlvated with silicon nitride. The chip housing protects the microstructure from mechanical damage and FIGURE 1. PLAN VIEW OF AIR FLOW SENSOR MICROSTRUCTURE
provides the smooth flow channel that is desired for precise flow measurements. Mass flow measurements require calibrations that depend on channel size and the onset of turbulence, or velocity-dependent flow profiles across the channel, which limits the accuracy of mass flow measurement. Therefore, it is important to keep Reynolds numbers below the turbulent values, and it is desirable to have fully-developed laminar flow at the chip for all channel sizes. The small channel sizes, which can be comparable to or less than chip dimensions, and the resulting high flow impedance, facilitate the sensing of dynamic differential pressures between two rooms, across a duct flow orifice, or any application in which a small flow through the sensor is permissible. The detector microtransducer provides an output response proportional to the mass flow rate.
This response can be calibrated to yield a differential pressure measurement in which the range and shape of the response curve can be determined by the dimensions that determine the flow impedance of the housing.
In addition, the small channel dimensions that can be obtained provide a high impedance relative to the impedance of common pneumatic connecting lines. Consequently, it is possible to remotely locate the differential pressure sensor by using convenient connecting lines, thus reducing installation costs and, in some applications, reducing environmental stresses on the sensor.
The effects of dust coming through the flow line are minimized by the chip surface being parallel to the flow direction. In contemplated differential pressure sensing applications, a long life in typical industrial atmospheric environments can be assured by removing most of the dust by using a suitable filter. This approach is practical because of the low maximum throughput rate, typically 20 to 50cm−3 per minute, made possible by the small channel dimensions and the high sensitivity of the detector. Accelerated life tests with heavy dust concentrations have demonstrated an equivalent life in normal industrial air in excess of 20 years with no dust accumulation on the chip and no deterioration of response.
FIGURE 2. CROSS SECTION, A–A, OF AIR FLOW SENSOR MICROSTRUCTURE
Performance Characteristics
An outstanding property of the sensor is the large temperature differential that develops between the two detector resistances with only small air flows, thus making it practical to use a simple alloy film to sense FIGURE 3. HEATER AND SENSOR CIRCUITS FOR THE AIR FLOW SENSOR FOR MASS FLOW
temperature even if its temperature coefficient is much smaller than that of diodes or thermistors. Figure 4 shows the temperature changes of the detector resistors in a typical differential pressure measurement, and the resulting temperature differential between the upstream and downstream detector resistances. Figure 5 shows pressure differential characteristics at four ambient temperatures and illustrates the temperature compensation that can be achieved over a broad operating temperature range. The sensor measures mass flow quite accurately under density changes caused by variations in ambient temperature and ambient pressure. Variations of gas composition involve changes in many properties of the gas such as specific heat, molecular weight and size, and others. Therefore, because the gas flow sensor responds to changes in the gas other than just molecular mass, changes in composition may not always lead to sensor outputs indicative of changes in molecular mass, (Fig. 6). Figure 7 shows a comparison between helium (He) and air as an example in which the sensor output appears consistent with the mass flow rather than volume flow, and is independent of gas composition. For minor gas composition differences, the mass flow errors are generally quite small.
Figure 8 shows the excellent reproducibility in signal vs. differential pressure across the flow channel obtained for four sensor units from one process batch. Because of the close relationship between heating value of natural gas and its density, we believe that energy flow sensing is directly possible with this device.
A correlation between natural gas heating value and density, shown in Figure 9, is less satisfactory than generally assumed, since errors of over 10% occur, even if propane-air peaking mixtures are excluded. If included, the accuracy deteriorates further. However, means to improve these accuracies are possible and are being studied. Applying corrections to density can lead to results which may be useful for natural gas energy flow measurement, as shown in Figure 10.
A safety test was made at PSC to demonstrate that a microbridge overvoltage burnout does not ignite a stoichiometric gas-air mixture. The microbridge chips were placed in a premlxed (0.8 stoichiometry) gas air FIGURE 4. TEMPERATURE DIFFERENTIALS UNDER FLOW CONDITIONS IN A TYPICAL SMALL
CHANNEL AS A FUNCTION OF PRESSURE DIFFERENCE BETWEEN CHANNEL PORTS
mixture 0.5 inch above a large bunsen burner flowing at 9 l/min. Seven burnouts were done with no ignition in any case. Even at a pulse voltage 10× the voltage required for slow burnout, no ignition occurred. The . 005" spacing of the resistor from the silicon chip and the low burnout energy of about 10−5 joules (vs.
minimum ignition energy of 2×10−4 joules (4)) make the cooling of the heated volume quite rapid and flame propagation cannot develop.
CONCLUSIONS
We have developed a novel, highly-sensitive air or gas flow sensor which performs well as a mass flow sensor and differential pressure sensor. It is especially suited to applications in the low differential pressure range from 0 to 1.0" of water column.
It provides a more direct approach for temperature and pressure compensation than other presently- available mass flow sensors requiring measurement of temperature and pressure. For some gas mixtures of varying composition, mass flow is indicated accurately (e.g. CO2 and He) without calibration corrections.
Because it can be fabricated by conventional thin film deposition and silicon processing techniques. It offers the possibility of lower cost and broader applications than present commercially available gas flow sensors.
Mixtures of other gas constituents require corrections. It appears to be possible to generate these corrections (using auxiliary real time measurements) to provide a mass flow output under varying temperature, pressure and composition conditions. Refinements of this approach may enable a similar measurement of energy flow in a natural gas stream. We are currently studying these approaches.
FIGURE 5. TEMPERATURE COMPENSATION ACHIEVED IN DIFFERENTIAL PRESSURE MEASUREMENT OVER 0 TO 60°C RANGE
Because this sensor can be fabricated by conventional thin film deposition and silicon processing techniques, it offers the possibility of lower cost and broader applications than present commercially available gas flow sensors.
REFERENCES
1. E.Bassous, “Fabrication of Novel Three Dimensional Microstructures by The Anlsotroplc Etching of (100) and (110) Silico’’. IEEE Transactions on Electron Devices, ED-25. No.10, 1178–1185 (1978).
2. K.E.Bean, “Anisotropic Etching of Silicon”, Ibid., p. 1185.
3. K.E.Peterson, “Dynamic Micromechanics On Silicon: Techniques and Devices” , Ibid., p. 1241.
4. B.Lewis and G. v.Elbe, “Combustion Flames and Explosions of Gases”, 2nd Ed., Acad. Press, NY and London, (1961), pp. 323–346.
UB/jh IGT.wp 04–16–86
FIGURE 6. RESPONSE TO MASS FLOW IN A TYPICAL SMALL CHANNEL FOR AIR AND CO2