What RTD sensor do you need?

Choosing an RTD sensor like thin film or ceramic wire wound depends on the following things:

  • Temperature range
  • Nominal resistance
  • Dimensions
  • Costs

The most important one is the temperature range. Depending on the temperature range one could determine what the preferred option will be: Thin film, ceramic wire wound or SMD platinum Pt element.

Temperature range RTD sensors

Ceramic Wire Wound

A ceramic resistor generally has the widest temperature range in combination with the narrowest tolerances for higher temperatures. It can measure high temperatures due to the insulating material covering the platinum wires. However, they are not the best option for high frequency. In case of high frequency, the platinum wires can break after being exposed to high frequency for a long period of time.

Read more about ceramic wire wound.

Thin Film sensors temperature range

A thin film resistor generally has a wide temperature range, but is mostly available in class A up to ± 300°C. A thin film resistor can withstand environments with high vibration well.

Element typeOperating
Temperature
Lead Wire MaterialRecommended
Connection Method
C-196 to +150°CAgPdsoft soldering
L/LN-50 to +400°CAgPd — silver coated Nickelsoft soldering
M-70 to +500°CNi/Pthard soldering, welding, crimping
HM-70 to +600°CPtPdhard soldering, welding
HL-70 to +750°CPt/NiCrhard soldering, welding
HD-70 to +850°CPthard soldering, welding
Source: Heraeus Nexensos GmbH

Read more about Thin Film elements

Platinum Surface Mount Devices (SMD)

SMD platinum elements are perfect for being mounted on a printed circuit board. The SMD’s have a linear temperature coefficient of 3850 ppm/K and are operational within a temperature range of -50 °C up to +150 °C.

Read more about Surface Mount Devices

And the other factors?

When comparing the Pt100 versus Pt1000 in terms of resistance, keep in mind that resistance value readings for the Pt1000 will be higher by a factor of ten than resistance value readings for the Pt100 at the same temperature. For most applications, the Pt100 and Pt1000 can be used interchangeably depending on the instrument used. In some cases the Pt1000 will work better and be more accurate.

In case of a 2-wire configuration, one could prefer a Pt1000 over a Pt100 since the measurement error of a Pt1000 will be lower.

Dimensions

Moreover, the dimensions of a Pt element are of great importance.

The alumina tubes of ceramic wire wound come with outside diameter (OD) that vary from 0.8mm to 4.5mm so they are even suitable for the thinnest mineral insulated RTD cables.

Thin film elements come in different sizes. Thin film elements can be as small as 1.0 mm, they could fit in the very thin tubes. The following table summarizes some size dependent properties;

Smaller elementLarger element
Faster response time Higher source currents possible
Higher self-heating constant Lower self-heating constant
Self-heats at lower power Requires higher power to self-heat
Fits in small ID sensor
housings
Larger contact area for surface
mounting
Source: Heraeus Nexensos GmbH

SMD’s continue to become increasingly compact and the small SMD sizes we supply perfectly match this trend. Kamet currently offers the following dimensions:

Product part numberSize (L x W x H in mm)
SMD 06031.7 x 0.9 x 0.45
SMD 08052.3 x 1.4 x 0.6
SMD-FC 08052.1 x 1.35 x 0.4
SMD 12063.2 x 1.6 x 0.6

Heraeus Nexensos also developed the micro RTD with the dimensions 0.6 mm by 0.3 mm. Please contact us for more information about this tiny SMD.

Cost of RTD elements

Lastly, costs are crucial when selecting a certain type of pt element. Ceramic wire-wound elements tend to be more expensive since a great part of the manufacturing process is still done manually. Thin-film elements being the most economical of the three options.

To summarize: Thin Film versus Ceramic Wire wound

The following table summarizes the advantages of each type:

Thin-Film Element AdvantagesCeramic Wire wound Element Advantages
Low-cost Higher source currents possible
Fast response time Lower self-heating constant
Low thermal mass Wider operating temperature range
High vibration resistance Wider tight tolerance temperature range
High thermal shock resistanceCustomizable R0 values
Small size footprint Larger diameter lead wires
Source: Heraeus Nexensos GmbH