Originally Published MPMNOctober 2003

PRODUCT UPDATE

Matching Sensors with Applications

Melody Lee

Sensitivity, response time, and size are among the factors to consider 

Sourcing a sensor or transducer that perfectly fits your needs can be a tedious task. Once the application is identified, flexibility, sensitivity and response time, and size are additional factors to consider. In this article, several suppliers to the device industry describe how they are able to meet the exacting specifications of device OEMs. For a complete listing of sensor and transducer product and service suppliers, please turn to the accompanying buyers guide on page 52.

Sourcing a sensor or transducer that perfectly fits your needs can be a tedious task. Once the application is identified, flexibility, sensitivity and response time, and size are additional factors to consider. In this article, several suppliers to the device industry describe how they are able to meet the exacting specifications of device OEMs. For a complete listing of sensor and transducer product and service suppliers, please turn to the accompanying buyers guide on page 52.

Custom Parts Expand User Flexibility

Not all manufacturers will be fortunate enough to find an off-the-shelf sensor or transducer for their needs. For more complicated applications, a custom product may be required. Loop-powered temperature transducers from Phoenix Contact Inc. allow custom configuration of measuring ranges, inputs, linearization, and diagnostics. 

"Our older modules used dip switches, which means there were fixed points to be set to," says product engineer Paul King. "The new multipurpose modules let youprogram the transducer to a variety of styles of thermocouples. You can also custom configure them to a temperature range. [The old modules] had a set range of 0°-400°F. You can now shrink that range down to the temperature that you will be using to get better results." 

The programmable transducers include an on-line measuring option when they are plugged into a computer. The thermocouples' behavior is monitored and charted using the software. Previously, additional modules and meters had to be installed to record point-to-point measurements. 

NTC Thermistor Offers High Sensitivity

Sensitivity and response time are also key factors in choosing a product. Kevin Moran, group marketing manager for BetaTherm Corp., says that certain surgical procedures would benefit from sensors that are quick to communicate temperature changes. 

BetaTherm offers a microbeta-chip probe to measure or control body temperature in medical applications. With a response time of 30 milliseconds, the product is also helpful in sensing liquid levels, monitoring air and gas flow, and analyzing chemicals. 

"Sensitivity refers to the ability of the sensor to detect small changes in temperature. Similar to the speed of response, the sensitivity of negative temperature coefficient (NTC) thermistors is related to their size," Moran says. "This is one of the main advantages of NTC thermistors over other sensor types. NTCs typically demonstrate a -3 to -6% change in resistance value per 1°C increase in temperature, making them the preferred choice for many medical procedures."

Microsensor Enables Small Samples

In addition to its functionality, a sensor's size can be a critical factor. Some invasive medical procedures or measurement applications may call for minute package sizes. 

Integrated Sensing Systems Inc. had a small idea when it created its specific gravity meter. Small, that is, in terms of developing silicon microtube technology to measure the density of fluid volumes from 600 nl to more than 1 µl. The sensor uses a vibrating microtube that is about as wide as a human hair.

"There are [other] density meters on the market, but by using microfluidics, we can reduce sample sizes," says vice president Doug Sparks. This particular product has been used in drug manufacturing to test the fermentation of antibiotics and for specific gravity testing in urology. More recently, researchers of blood transfusion products have been looking into using the density meter for testing blood and cell density.

According to the company, the microelectromechanical systems (MEMS)-based product is a fraction of the size and weight of conventional desktop meters. MEMS technology also enables integration of a temperature sensor into the micromachine density meter chip, improving sample results. The sensor's small size allows it to achieve test times of 3 seconds to an accuracy of four digits, according to 
the firm.

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