Pt-100 sensor technology.pdf

Pt-100 Sensor Technology

A White Paper

TURCK Inc.

3000 Campus Drive

Minneapolis, MN 55441

Phone: (763) 553-7300

Fax: (763) 553-0708

Application Support:

1-800-544-7769

www.turck.com

Not Your Average Thermostat

There are quite a few devices commonly used to measure temperature in industrial applications.

Thermocouples, thermistors and resistance temperature detectors (RTDs) are just a few commonly used

devices for temperature measurement applications, like those found in the pharmaceutical or food and

beverage industries. Each device has inherent design advantages, but some are better suited for certain

applications. Where there may be no right or wrong device for your application, newly designed temperature

sensors incorporate an array of features to aid in usability and functionality.

Let’s compare a few different technologies. Take thermocouples, for example. These sensors contain two

dissimilar metals that are soldered at a junction. The bimetallic junction develops a small voltage, only about

50µV per degree Celsius, that varies with temperature. The manufacturer supplies tables to convert the

voltage to temperature.

Thermocouples are simple, inexpensive devices. The main limitation of a thermocouple is accuracy, as system

errors of less than 1°C can be dif cult to achieve. The main source of error in a thermocouple is parasitic

voltages that develop in the system where the thermocouple wire is attached to the measuring instrument.

To accommodate for this, they require cold junction compensation, often done by exposing one junction to a

reference temperature, generally 0°C, and the other to the desired temperature to measure. The thermocouple

wire must run the entire distance between the measurement point and the processor. Thermocouples are

generally less repeatable and stable than other sensing devices, but their no-frills styling provides a product

that gets the job done.

Thermistors are non-metal resistors that measure the change in temperature via resistance. Thermistors are

generally made of ceramics and metal oxides and can therefore measure much higher temperatures than

thermocouples and RTDs. Thermistors are generally used in applications where only a small temperature

range is required for measurement since their output can be approximated as linear over a narrow range.

Their output is exponential in nature.

There are two kinds of thermistors: positive temperature coef cient (PTC) thermistors where the

resistance increases with increasing temperature, and negative temperature coef cient (NTC)

thermistors where the resistance decreases with increasing temperature. With PTC thermistors,

there is generally a sharp rise in resistance at a critical temperature, while NTC thermistors are

used as resistance thermometers in low temperature applications.

RTDs also use electrical resistance to measure temperature. They require a small power source

in order to measure resistance. The resistance experienced by the sensor is proportional to the

temperature changes being experienced by the media being sensed. RTDs contain a platinum

wire that is wrapped around a core or patterned as a thin lm on a substrate so that it experiences

minimal differential expansion or other strains. RTDs made using platinum, preferred for its linear

resistance-temperature relationship, are referred to as Pt-100 and Pt-1000.

The International Electrotechnical Commission’s IEC 751:1983 speci es the tolerance and

temperature to electrical resistance for platinum resistance thermometers. The most common

TURCK Inc. 3000 Campus Drive Minneapolis, MN 55441-2656

Phone: 763-553-7300 Application Support: 1-800-544-7769 Fax: 763-553-0708 www.turck.com

devices used in the industry, Pt-100s, have a nominal resistance of 100 ohms at 0°C. Pt-1000s measure

1000 ohms at 0 °C, and have a ner resolution but a smaller range than Pt-100’s. All Pt-100 sensors are

manufactured to the 100 ohm standard to provide an increased level of reliability and accuracy. However, two

speci cations exist: the European standard a = 0.00385, and the American standard a = 0.003916. Both are

commonly used, and it is only important to note which one a processor accepts.

Manufacturers often incorporate basic Pt-100 technology into their advanced temperature sensing

technologies to make the whole package more desirable for temperature measurement. Though they generally

come with a higher price tag, these sensors are typically designed with added features that make them easy

to use, like push button programming and control, with highly visible digital readouts. Several manufacturers

also style their Pt-100 sensors in a compact housing that permits its usage in harder to reach positions.

When applying a temperature sensor it is important to consider all the components that make the

system work. Often when applying an RTD, there are several accessories that make installation easier.

Thermowells and compression ttings allow RTDs to be installed in pipes, tanks and other applications

that require special process connections, like sanitary ttings. Thermowells allow the user to insert the

thermowell into the process and to remove/insert the RTD as needed. Thermowells provide a barrier

between the RTD and the media being sensed, and therefore reduce the response time of the sensor while

protecting the probe. A compression tting threads into the pipe or tank and then compresses on the

probe xing it in place.

RTDs come in many different styles for different applications. Probes with dairy style sanitary ttings

are available, as well as cable versions. Furthermore, remote mounting allows higher temperatures to

be measured. When the electronic processor is moved out of hot ambient conditions and connected

to a probe via a cable, temperature measurements up to 900°F can be reached.

Many different temperature sensors exist for applications that require temperature measurement.

Choosing the temperature technology for your application depends on several factor. While there may

be no right or wrong solution for your application, different technologies have been, and continue to be,

developed to meet the needs of new temperature sensing applications.

TURCK Inc. 3000 Campus Drive Minneapolis, MN 55441-2656

Phone: 763-553-7300 Application Support: 1-800-544-7769 Fax: 763-553-0708 www.turck.com

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