Heraeus Platinum RTD Pt1000 Temperature Sensors (Qty.5)

On offer is one lot of 5 new Platinum RTDs (Resistance Temperature Detectors) by German specialist manufacturer Heraeus, in original bubble packaging. These are the thin-film type, with a layer of platinum on a ceramic substrate, with platinum plated nickel connecting leads.

Platinum RTDs are the royalty of temperature sensors... which is reflected in their price (see below). Simply put, their resistance increases predictably with increasing temperature. Being made of platinum, a metal costing over $2000/oz (2..3x more than gold), these sensors are usually only used in demanding industrial, aviation and automotive applications, where their accuracy, repeatability and long-term stability give them a clear edge.

In many applications below 500° Celsius they have replaced thermocouples because although individually more expensive they don't require specialised cabling or cold junction compensation.

The front of each is sealed with a mauve glaze, the back is white alumina ceramic.

To the best of my knowledge, platinum RTDs are the only temperature sensors which are standardised, their characteristics set out in IEC-751.
Specifically these are Pt 1000, Class B devices, with characteristics:

• 1.000K ohm @ 0° Celsius
• 0.12% tolerance
• TCR = 3850 ppm/K
• Temperature range = -70°C to +500°C

This means they are interchangeable, and one can be swapped for another without costly recalibration ... try that with the average thermistor!

Most Pt RTDs have a 100 ohm resistance, whereas the ones here are 1000 ohms. This has the advantage that resistance changes are 10 times larger.

A good overview of RTDs is given in the Wikipedia at http://en.wikipedia.org/wiki/Resistance_temperature_detector and similar ground is covered by http://canteach.candu.org/library/20030701.pdf.

Technical references on Pt RTD characteristics are found online at http://www.cyntec.com/pdf/sensor/Technical%20References.pdf and http://www.lakeshore.com/pdf_files/sensors/Manuals/F038-00-00.pdf.

For comparison with other technologies, the Nat Semi Temperature Sensor Handbook is worthwhile.

Over a limited temperature range (say 0°C..100°C) Pt RTDs are essentially linear, so for simpler applications no compensation is necessary. Over a broader range they can still be used for high accuracy measurements because their non-linearity is predictable (according to the Callendar-Van Dusen equation).

The Wikipedia article gives a table of values (between -50°C and 200°C) for the resistance of these sensors, calculated from the equation. At 0°C an ohmmeter reads 1000 ohms, at 100°C it reads 1385 ohms and at 200°C, 1758.4 ohms. Such a table would be programmed into a thermometer/controller based on a microcontroller, with interpolation used for intermediate points.

Interfacing RTDs is covered in most books on temperature measurement / industrial instrumentation, such as Microcontroller-based temperature monitoring and control - by Dogan Ibrahim and A Baker's Dozen: Real Analog Solutions for Digital Designers By Bonnie Baker (which are online in preview form).

And as usual, plenty more references are just a Google away ...

Temperature is probably the most common non electrical quantity to be measured in electronics. These sensors have a wide range of potential applications because of their accuracy and wide operating temperature range. A few that spring to mind for the RF constructor are:

• Bolometer - mount one of these on an RF dummy load resistor, and carefully constructed you have the basis of a fine RF power meter.
• Oscillator Oven Control - as part of a temperature controller loop, these would be great for precise regulation.
• Soldering Thermometer - measure iron tip temperature, or control an SMD reflow oven.

Notes:

• These flat-film sensors are constructed in a similar way to strain gauges, so take care to mount them where the substrate will not be flexed. Best not to use epoxy, but instead use silicon grease or similar.
• Low current should be used, as any resistor will generate its own heat heat when current is passed through, and this will cause errors in temperature readings.

Price Reference:

Very few stockists sell platinum RTDs compared with those who stock regular thermistors. Digikey and Farnell are exceptions, and they list the bare sensors like those on offer here at $15..$50 US each in small quantity. When mounted in steel probes they cost around $50-$100 each.

Starting here at a very fine price for five!

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