High Frequency, High Pressure and High Temperature Semiconductor Strain Gage Sensors

Herbert Chelner, CEO and Chief Scientist

Dr. Robert A. Mueller, President and General Manager

Abstract

High frequency pressure at high temperatures means close coupling, since even a small amount of tubing will reduce the frequency response. It also means that the most applicable design for high frequency is a fixed edge diaphragm. By design, diaphragm stress is converted into an electrical signal by the semiconductor strain gage bridge when properly conditioned and excited; the signal is linear and proportional to pressure.

Semiconductor Strain Gages

Semiconductor strain gages were discovered during the transistor era and became commercially available early in the 1950's. These gages may be homogeneous or diffused. Diffused gages have a variety of problems that limits useful life and affects performance. There are two main elements from which semiconductor gages are made. These elements are Geranium and Silicon and they can be P or N doped. At Micron Instruments, the P doped (Boron) Silicon gage is selected for the basic strain sensor, and the N doped Silicon is used for the temperature sensor. Silicon gages have been proven to be more stable and more corrosion resistant than Geranium gages.

The Micron Instruments Miniature P-doped Silicon Semiconductor Strain Gage

Micron’s strain gage is manufactured from a Boron Doped Silicon ingot grown as a single crystal. The strain gage crystalline axis used is where the longitudinal over the transverse ratio is maximized. The reverse is true for the Silicon temperature sensor. This means that the finished gages will be unidirectional and transverse strains will have no significant effect on performance when properly manufactured.

Gage shape is application sensitive. Semiconductor strain gages are normally bar shaped; the length and resistivity varies but the width is nominally 0.005 inches and the thickness 0.0005 inches for a bar gage. Normally, a gold lead is bonded to ends of the gage for electrical connection. For miniature sensors, it can be important that the gold electrical leads come out the same end, requiring U shaped gages. The U shaped gage also has twice the resistance over the same length, making it desirable for small areas of high strain or for wireless applications where higher resistances are important. There are also M shaped gages, which provide four times the resistance of the same length bar gage when even higher resistance is required.

High Frequency, High Pressure, High Temperature Sensor Applications

There are many applications where high frequency pressure at high temperatures is required.

One application would be testing and tuning jet aircraft engines to optimize performance and quickly detect any deleterious harmonics that could cause rapid engine failure.

Another application would be down-hole oil measurement detecting sudden high pressure or excessive pressure oscillation. Such sensors would provide unattended, early failure warning, and could be designed into a system to automatically detect anomalies and gracefully shut down the sub-system at risk before it destroys itself or blows out the well. When integrated as a wireless sensor, it could also autonomically provide alert and alarm notifications.

High frequency, high pressure, high temperature strain gages could also be used as wind tunnel skin friction sensors. Exploiting the high gage factor in semiconductor strain gages makes it easier to design for higher frequencies, and the high temperature gages makes the sensor especially useful for wind tunnel ultrasonic and hypersonic wind skin friction measurements. Data from such a sensor leads to designs that minimize friction and optimize performance.

There are many other applications requiring flush mounting and high temperature sensors, but some would not require high frequency. One would be high temperature injection molding.

Using Micron Instruments’ High Temperature Semiconductor Strain Gages to Build a 500°F High Frequency Pressure Sensor

The sensor design starts with the semiconductor strain gage. Sensor performance can be no better than the gage performance, so there is no better choice than a semiconductor gage. In addressing technical considerations such as electrical conductivity and oxidation, it is preferable to use gold pads at the ends of the silicon strain gages. Gold begins to slowly amalgamate with silicon at about 500°F. At higher temperatures, the amalgamation accelerates.

Micron Instruments established an initial goal to produce a gage that will sustain 1000°F for short periods and operate reliably for many years at 500°F. This goal was achieved with sophisticated barriers. The gage itself has a minimum of molecular slippages or dislocations, and as such, has a 99-year life and a gage factor of over 150. This gage factor is 50 to 75 times more sensitive than the normally available foil gage. These are some of the characteristics that make the semiconductor strain gage desirable when high performance for a long operational life is required.

Sensor housing material should be selected to optimize the gage performance, but this is also application sensitive. For instance, if in a very corrosion high temperature environment, Titanium 6AL4V should be considered.

Additional Technical Challenges

Bonding Agents

Bonding strain gages with standard carbon based adhesives such as Epoxylite 6203 or M-Bond 610 is limited to under 250°F. These adhesives will lose bonding power (modulus of elasticity decreases) as temperature increases. Full-scale creep starts to become problematic, with the best of procedures, at about 300°F.

Therefore, bonding the gages at high temperatures with glass fritz or ceramic-based adhesives is preferred. Low temperature melting glass fritz has been used successfully for high temperature work, but it contains lead. Lead-free glass fritz is available and typically requires over 600°F but below 1000°F, which the new Micron Instruments’ high temperature gage can accommodate.

Soldering

The lead free solders contain a large amount of tin. Although they will not open at 500°F, they can grow tin whiskers, oxidize, and begin to amalgamate with the gold leads. Micron Instruments has a gold base solder which avoids these problems.


Wires and cables

Few exit cables rated at 500°F are commercially available. For those cables that must operate at 500°F, Teflon and other high-temperature overcoats are available. If there is a potential that the temperature may have short excursions to 550°F or more, Teflon can produce Phosgene and will deteriorate, so cable selection is highly application sensitive.

Sensor metal

Matching the metal to the gage optimizes performance. Also, the metal should be one that will not oxidize or corrode at 500°F to the extent it affects performance. This is also application sensitive; e.g., if operated in a highly corrosive environment.

Wireless Sensing Considerations

A major benefit for monitoring in high frequency, high pressure, and high-temperature environments is early failure mode warning and response. An additional benefit offered by wireless sensing in these operational conditions is real-time awareness and decision-making. Also, wireless sensors are non-intrusive and avoid the many problems with tethered wired sensors. And, it’s important to avoid power sources that have limited life when operating in environments where extended unattended operation is required. The good news is there are several wireless technologies, both active and passive and covered by international (ISO) standards, which meet these requirements. The transceivers are commercially available and relatively inexpensive.

Wireless sensor data can be acquired from both stationary and mobile readers. By using industry-standard wireless technologies, data can be read with commercially available smart phones, tablets, laptop computers, and mobile devices. Micron Instruments’ forthcoming wireless sensor options will all be based on industry standard technologies.

Contact Us.

Micron Instruments has already worked with innovative companies on optimal selection, placement, and processing of semiconductor strain gages for high frequency, high pressure, and high-temperature applications. If you’d like to discuss your application or design, please contact us for a free, confidential consultation either by email using our Contact Us Form or phone (805-522-4676).
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