HIGH MEASUREMENT ACCURACY PLATINUM THIN FILM RTD TEMPERATURE SENSORS

Dave Boylan

23.07.2020

Common methods for measuring temperature include:

• Thermocouples

• Thermistors

• Resistance temperature detector (RTD)* Best for stability in production, and ability to be calibrated to very high sensitivity levels with very low drift

in the specified temperature range

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• Pyrometer

• Infrared

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https://en.m.wikipedia.org/wiki/Thermocouplehttps://en.m.wikipedia.org/wiki/Thermistorhttps://en.m.wikipedia.org/wiki/Resistance_temperature_detectorhttps://www.ist-ag.com/en/products-services/temperature-sensorshttps://en.m.wikipedia.org/wiki/Pyrometerhttps://en.m.wikipedia.org/wiki/Infrared

RTD sensors can be thin-film, wire-wound, or coiled elements

Advantages of platinum resistance thermometers:

• High accuracy

• Low drift

• Wide operating range

• Suitability for precision applications.

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Fundamentals of Pt thin film RTD temperature sensors

• Platinum RTDs are the most common metal based RTDs used in industrial

applications. This is because platinum offers corrosion resistance, long-

term stability, low drift, low hysteresis and measures across a wide range

of temperature,

(-200…+850°C).

• Thin film RTDs are mass-produced and cost less than the other RTD types.

They are smaller, have a mechanical robust construction and have a faster

response time than other types (wire-wound, coiled), which is desirable in

many applications. They are made by depositing a thin pathway

(meander) of platinum on a ceramic base.

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IST AG Pt Thin Film RTD Construction

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Al2O3 for chemical and temperature robustness

Pt Thin film RTD Parameters

• Sensor is basically described by the following parameters:• – TCR – Temperature coefficient of resistance usually 3850 ppm/K

• – R0 – base resistance at 0°C

• - Tolerance

• The real characteristics of the resistance in regards to the temperature is defined by

the• – Polynomial or characteristic curve

• – With values A,B,C

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R0 is the nominal ohmic value at 0°CCommon is 100 Ohm, 500 Ohm, 1000 OhmSometimes:

– 50 Ohm (mostly for heaters)– 200 Ohm (mostly automotive)– 2000 Ohm– 10000 Ohm

Pt100 or Pt1000?

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Previously, wire-wound RTDs with R0 = 1000Ohm were expensive and fragile (long Pt wires with small diameter)• Pt100 was mostly used for wire-wound RTDs

However using thin-film technology, Pt1000 R0 could be a better choice:• Higher resistance change• Less influence of connecting wires• Less influence of contact resistances• Same price as Pt100

However using Pt100 could be a good choice for• Small Die sizes and/or high temperature die, as lower ohmic resistances are better

IEC 60751 standard for Pt thin film RTDs

• Platinum RTDs generally conform to the IEC 60751 standard. In a typical industrial

application, the RTD is protected by inserting it into a stainless steel sheath.

• The standard describes the behaviour of the sensor in regards to the temperature

• All sensors according to the standard can be interchanged without recalibration

• The standard defines the measurement accuracy tolerance bands and tolerance ranges

• It also regulates the differences between temperature sensing elements and probes

Limitation: Standards can be limiting and forbid product differentiation based on performance

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- 1/5 DIN (F 0.3) : ±(60mK +1 mK/K)- 1/10 DIN (F 0.3): ±(30mK +0.5 mK/K)*

Unofficial norms:

* Best in class today

1/10 DIN Class B => Limited Range of Validity ~

30degC

Accuracy deviations increase with increasing

Temperature

Highly priced due to tight selection criteria

IEC

Question: How Can I get the very best Accuracy calibrated Sensor?Solution: Individually Calibrated Sensors from IST AG for high measurement accuracies

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High accuracy RTDs generally do not exactly

match the IEC 60751 R:T curve. To further

improve measurement accuracy, IST AG can

very precisely characterize an RTD based on

more than 25 years experience. This is done

by carefully measuring the RTD resistance at

a different temperatures and then using that

data to derive the polynomial A, B, and C

coefficients.

Method: Each Thin-film RTD sensor is individually measured in the normal IST AG manufacturing process

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• Temperature Characteristic Curve (Polynomial) is calculated from multiple measurement points

• +/- 50mK measurement accuracy possible across the full range -40degC to +100degC T Range extension possible) => No increase of measurement uncertainty with temperature!

• +/- 30mK measurement accuracy also possible (More measurement points necessary )

• Ro A, B & C Polynomial values for each serialised sensor provided in Spreadsheet (Look-up values), Data-Matrix or QR code on packaging

Individual calibrated Sensor Benefits:

• High Volumes possible

• 4 – Wire options available

• Reasonably priced

• Pre-compensated (shifted) tolerance band is also possible

[mostly customers add offset and drift while assembling

IST AG sensors.]

Contact us to discuss ➔ https://www.ist-ag.com/en/contact

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Example: Pt1000 Thin Film RTD

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P1K0.1206.1K.K.SResistance: 1000 Ohm at 0°CTolerance: individual polynomial values for accuracy: +/- 50mK between -10°C and +60°CIEC 60751 F0.3 in rangeT.-Range: -20°C - +100°CPackage: single packaged and numbered, delivered with electronic and paper versions of polynomial values R0, A & B

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Example: New Glass Sealed RTD with Class A (IEC 60751 F0.15) Accuracy to 400degC

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Resistance: 100 Ohm at 0°CTolerance: IEC 60751 F 0.15 from -200°C to +400°CTCR: 3850 ppm/KT.-Range: -200°C - +400°CDimension: 2.4 x 1.4 x 0.8 mm (LxWxH, element)Contacts: Pt-wire, Ø 0.2 mm, 10 mm longSpecial: in round glass housing 8 x 2.25 mm (LxD)

PW0K1.216.4K.A.010.S (Order # 151495)

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Application Example: High Accuracy RTD that can be used as both heater and temperature sensor in a Heated Razor

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Option 1:Panel / area heater (uniformly distributed oversurface) • Heater size about 35 x 2.5 mm

Solution:1. Ceramic heater (Thick- or Thinfilm)

Directly integrate the heater on the ceramic?

2. Heater direcly processed on metalNo assembly needed to heat spreaderNo thermal bridge to the spreaderVery fast response time (

Application Example: High Accuracy RTD that can be used as both heater and temperature sensor in a Heated Razor

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Option 2:• Local heating spots (in orange)• Heater size about 2.1 x 1.2 mm • Sensor backside in contact with the heat

spreader• Very low cost solution• Fast response time ~

Customised assemblies also available from IST AG

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Contact

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Innovative Sensor Technology IST AG

Stegrütistrasse 14

9642 Ebnat-Kappel

Switzerland

Phone: +41 71 992 01 00Fax: +41 71 992 01 99info@ist-ag.comwww.ist-ag.com

mailto:info@ist-ag.comhttp://www.ist-ag.com/

THANK YOU!

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