Designing to Achieve Medical Temperature Measurement Accuracy

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Temperature is probably the most common physical measurement made during medical diagnosis or evaluation. Body temperature is an accurate predictor of many types of conditions and illnesses. The typical human temperature is within the range of 97.7°F to 99.5°F (36.5°C to 37.5°C) with 98.6°F (37°C) as being normal.

Such temperature readings are usually measured with a sensor under the tongue by a nurse with a portable thermometer. It takes less than 10 seconds. The precision is only one digit after the decimal point, but that’s generally enough to get an initial assessment for diagnostic purposes.

This approach isn’t good enough for clinical testing or for keeping track of human temperature over time. Therefore, some new method was required. Specifically, he need was for a temperature sensor with the 0.1°C precision of a resistance temperature detector (RTD) but without a 100-Ω platinum wire coil taped into your armpit.

A new approach offered by Texas Instruments gives the desired precision with a maximum error of less than ±0.3°C. This breakthrough development owes its success to a new wearable temperature sensor. TI and the University of California San Diego and San Francisco collaborated to improve wireless patient monitoring within the medical setting via TI's new digital temperature sensor, which offers ±0.1°C high accuracy.

 Sponsored Resources: 

• TI Sensor Innovation: Enabling Research in Wireless Patient Monitoring with the University of California
• Design Challenges of Wireless Patient Temperature Monitors
• Design with medical temperature accuracy

The Need

Demand for precise temperature measurement is critical in clinical studies and in wearable patient-monitoring products. R&D revealed that thermistors weren’t up to the task because of the difficult calibration. Though the RTD is ideal because it provides the necessary precision, it’s out of the question due to the devices’ bulk and complexity.

Semiconductor IC temperature sensors have gradually improved over the years, and with good design and packaging can meet the need. The gold standard for patient-monitoring thermometer accuracy is the American Society for Testing and Materials (ASTM) E1112. Another similar standard is ISO-80601-2-56.

Designing for Minimum Error

It was determined that for any temperature sensor to meet the required accuracy and precision, it must meet these requirements:

• The sensor must always remain in good thermal contact with the patient’s skin.
• The sensor must be thoroughly thermally isolated from all other heat sources.
• The sensor must use minimal power so that self-heating effects are negligible.

The sensor emerging from this development work is Texas Instruments’ TMP117 temperature sensor. It’s contained within a very thin small outline, no-lead, 2- × 2-mm WSON package (Fig. 1). This type of sensor measures the temperature of the silicon die. It’s designed to be used as a skin patch taped into place on the inside of wearer’s forearm.