Due to the need in industry for greater accuracy and stability, the resistance thermometer (RTD) is becoming more widely used. The platinum RTD having a nominal resistance of 100 ohms at 0°C and a temperature coefficient of 0.00385 is emerging as a world standard.

Common Resistance Materials for RTDs: Platinum (most popular and accurate), Nickel & Copper

Detector Tolerance and Accuracy
The RTD bulbs used in Techno assemblies conform to IEC 751-1983 and in standard form are supplied to class ‘B’ tolerance. Other tolerances of class ‘A’, 1/3 & 1/10 DIN are available on request. Also available are Nickel 100, 120 ohm and Platinum 46, 50, 100, 200, 500, 1000 ohms Cu 53, 2000.

RTD Terms

RTD (Resistance Temperature Detector)
An acronym for resistance temperature detector or device. A resistance temperature detector operates on the principle of the change in electrical resistance in wire as a function of temperature.

RTD Element
Sensing portion of the RTD which can be made most commonly of platinum, nickel, or copper. TECHNO features two styles of elements: wire wound and thin film.

RTD Probe
An assembly composed of an element, a sheath, a lead wire, and a termination or connection. The standard TECHNO® RTD probe is made with a 100 ohm platinum European curve element (alpha = 0.00385).

Platinum RTD
Also known as PRTD, Platinum RTD's are typically the most linear, stable, repeatable, and accurate of all RTD's. 

Thin Film RTD
Thinfilm RTD's are made up of  a thin layer of a base metal embedded into a ceramic substrate and trimmed to produce the desired resistance value. TECHNO RTD's are made by depositing platinum as a film on a substrate and then encapsulating both. This method allows for the production of small, fast response, accurate sensors. Thin film elements conform to the European curve/DIN 43760 standards.

Class A RTD
For a Class A sensor the following relationship applies where “T” is the absolute temperature: rT=±(0.15+0.002 * t) For t =75 0C, the tolerance is therefore: rT=±(0.15+0.002 * 75)=±0.30C

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1. Temperature range
2. Tolerance,
3. Accuracy,
4. Time response Vibrations

• When accuracy and stability are a requirement of the customer’s Specification.
• When accuracy must extend over a wide temperature range.
• When area, rather than point sensing improves control.

Temperature range
The internal assembly construction falls into two temperature ranges:

Low  –200°C to +250°C.
High –200°C to +600°C.

Low temp- 3mm, 4.8mm, 6mm, 8mm, 9.5mm dia 316 SS tube alumina powder filled. High temp- 3mm, 6mm dia 316 SS metal sheathed mineral insulated/Ceramic beads insulated

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Features and Benefits of TECHNO make RTDs
Accuracy:- A special process combines strain-free construction with partial winding support for accurate readings and long term stability in standard RTD elements.

Interchangeability. Strain-free construction and precision trimming allow elements from different lots to be substituted without recalibration.

Sensitivity:- Self-heating is minimised, allowing precise measurement. The temperature coefficient (alpha) is carefully controlled at the industry standard while insulation resistance values exceed IEC-751 standards.

Standardisation:- Elements are available to meet or exceed the requirements of various standardisation agencies. IEC- 751 standard tolerance classes A and B respectively are very well suited for industrial applications. Tolerance classes up to 1/10th DIN can also be supplied where higher accuracy is demanded

Physical and chemical stability over a wide temperature range is ensured by Techno’s highly controlled manufacturing process. Standard elements are built to resist mechanical vibration and shock, however where there is high exposure to mechanical vibration, specially manufactured film RTD elements can be supplied to suit the application.

Repeatability:- All elements exceed IEC-751 repeatability values even after long exposure to temperatures within the operating range.

Lead Configuration & Color Code
The resistance in the leads to the RTD (resistance temperature device) may significantly affect accuracy. To deal with this problem, connection arrangements are available which allow the lead resistance to be measured and compensated for, these are shown below:

Used where lead length is short. There is no compensation for resistance of lead wires. Most common type of RTD assembly. Typically connected to standard bridge circuit, which allows lead wire resistance to be compensated. Where higher accuracy is demanded. Lead wire resistance errors are eliminated in this configuration by measuring the voltage across the RTD element supplied with a constant current.

Film Technology
Thick film is produced by spreading Platinum paste through a silk screen onto a ceramic substrate. Thin film is produced by evaporation of metal onto a substrate and then by suitably itching with LASER technology. The basic advantages of both types are they are mass produced elements, they are cheaper as well as they can work in environment. Where extreme vibration is presence

Wire Wound Elements
Wire wound elements are a “hand made “product which involves lot of skilled Labor. It is manufactured by placing a coil of thin wire (7 microns to 35 microns) into a ceramic insulator. The resistance of the coil is suitably adjusted to the ohms corresponding to that particular temperature. Ideally, this coil should be freely suspended for natural expansion and contraction. However, such construction is not immune to vibration and hence it is not adopted for industrial application. For industrial application, normally partially supported or fully supported construction is used which are quite immune to vibration. The extended leads are quite rugged as compared to the alpha wire and normally made from Pt base metal (for Platinum resistors). The choice of ceramic insulator is also very important as any impurities in the insulator may detoriate the characteristic of the alpha wire.

Comparison between Film PRTD and Wire Wound PRTD

1. The film is covered rigidly by glass layer and so the Platinum is not free to expand. The result is they can not maintain characteristics specially at higher Temperature.
2. The film contains very little metal and hence there are more chance of contamination from the impurities of the glass covering.
3. The unit have very little size and mass is also small. Hence they are subject to self heating error when sensing current is pass through it.
   
4. Control of homogenous of metal is difficult as compared to wire used in wire wound elements.