Comparing Sensors

There are a number of different industrial applications that require precise measurements of temperature. In applications such as: building control, food processing and petrochemical manufacturing, it is necessary to monitor temperature with different types of sensors. The two most common types of sensors, measure temperature electrically, or register the variation of resistance with temperature.

In many industrial settings, measurement points are further away from the control point.  As a result, there is often a necessity for the additional processing of measurement in the computer. Direct indicating thermometers, are unsuitable for industrial applications, as they are not completely accurate. Most industries choose to use RTDs and thermistors, which convert temperature into electrical signals.

RTDs use the property that electrical resistance of metals, differ with temperature. The RTD uses a positive temperature coefficient to sense when resistance increases with temperature. The RTD is mostly comprised of nickel or platinum and is an incredibly accurate sensor for large manufacturing endeavors. Most platinum RTD thermometers can register temperature from -200 to 800 degrees Celsius, and have a +0.05 accuracy value. Measuring the resistance of platinum has a ton of advantages including: chemical stability, easy fabrication, and agreeable electrical properties. As a result of these beneficial characteristics, platinum has become the most universally employed temperature sensor in industrial applications.

Thermistors are created from a combination of metal oxides. The metal oxide’s resistance, decreases with increasing temperature. Unlike the RTD, thermistor’s resistance characteristics taper off with increasing temperature, and are therefore known as negative temperature coefficient sensors. Thermistors are primarily used in applications that do not exceed 200 degrees Celsius and are mostly used for monitoring and indicating purposes.  For such simple applications, thermistors are preferred because they are less expensive than RTDs and have simple electronic components and circuitry.  Also, thermistors can be designed with an extremely fast response and low thermal mass.

Knowing the right type of sensor to use can help to make any industrial application more efficient.  Platinum based RTDs are the most accurate of secure sensors, which can be used over a long duration of time. Problematically, some automated industries cannot accept RTDs directly, which adds additional costs of customizing machinery.  In comparison, thermistors are not as stable, nor as accurate, but are easier to wire and entirely more cost effective than an RTD. As a result of thermistors being more popular for simple procedures, there are many different designs on the market.  Thermistors can be created with varying resistances and curves, making it important to pick the correct thermistor for any given application.

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