Platinum Resistance Elements are temperature sensitive sensors mounted in RTDs. These sensors require an adequate housing as protection during direct exposure to surrounding media. The temperature sensitive part of the sensor is comprised of a platinum wire wound or a platinum thin film material. These sensors are fixed in or on a ceramic former and attached with two connection wires.
Gekon Trading offers a wide selection of RTDs for specific applications. These vary according to our customers requirements for temperature measuring range, physical dimension and connection technology.
The proper construction of RTDs requires expert skill regarding connection methods, housing techniques and the selection of appropriate materials. Based on Gekon Tradings years of experience, we have listed several recommended methods of lead connection. This list is by no means all inclusive since the multitude of applications require the adoption of suitable materials and processing parameters.
All Gekon Trading Resistance Elements comply with the current internationally accepted DIN IEC 751 standard. This standard refers to the relationship between temperature and electrical resistance of a Platinum 100 resistance thermometer expressed as a polynomial with the coefficients A, B, and C.
R = Ro x ( 1 + A T + B T² + C T³ )
A = 3,9083 x 1E-3
B = -5,7750 x 1E-7
C = -4,1830 x 1E-12 x (t-100) ( T < 0°C )
C = 0 ( T > 0°C )
The basic values for T>0°C is a quadratic equation which can be solved for the temperature. After solving for the coefficients A and B listed above, the following formula should be used:
T = -A / (2xB) - SQR ( A² / (4xB²) + 1/B x ( R/R0 - 1 ) )
A third degree polynomial may be used to solve for the basic values for T<0°C. The temperature, as a function of the resistance, can be calculated by an iteration procedure. However, it may be easier to use the standard tables.
The permissible deviations from the basic values are defined as the deviation of the temperature when actually measured from the true temperature. Each individual sensor type may be classified as either 'accuracy/tolerance' Class A or Class B depending on this permissible deviation.
ΔT = 0,30 °C ±C 0,005 T (Class B)
ΔT = 0,15 °C ±C 0,002 T (Class A)
The permissible deviation of the measured temperature from the true temperature is therefore dependent on the Class or accuracy of the sensor. In addition to these internationally accepted tolerance classes, Gekon Trading has available special tolerances upon customer request. For example, 1/3 DIN Class B (± 0,1°C at 0°C). Generally these special tolerances are used for temperatures not exceeding 200°C.
Resistance elements are suitable for all generally used connection technology including crimping, soldering, brazing, spot welding and laser welding.
When extending connecting leads, care should be taken to insure that the leads are only stressed in the axial direction giving consideration to the maximum permissible tensile strength. In cases where the connecting leads must be bent, the leads should be strain released especially at the glass frit. Strain by pressure and extreme bending radii should be avoided. Kinks and twists in the connecting leads represent soft spots which can create a failure during operation at elevated temperatures.
Soldering (Recommended temperature range -30°C to +120°C) Good results can be achieved with soldering materials containing silver (> 2% silver content) along with halogen free water soluble flux materials. After soldering, the remaining flux must be removed in order to prevent corrosion at the joints which in turn could affect the stability of the assembly and the insulation resistance. An adequate strain relief should be made so that the stability of the joint is not affected. For temperature ranges of 180°C there are some additional high melting soldering materials that can be used.
Adhesion (Recommended temperature range -50°C to +200°C) This method only applies to resistance element types without connection wires such as Gekon Trading series SMD or DSC. Conductive adhesion compounds containing silica have performed very well.
Crimping (Recommended temperature range -50°C to +200°C) On crimp connections it is important that the maximum allowable tensile strength of the wires used are not exceeded. The higher temperature level is limited by both the crimping material used as well as by the contact area where corrosion can occur. For this reason, Gekon Trading recommends that the material joint be homogeneous and gas tight.
Brazing ( Recommended temperature -50°C to +400°C) Gekon Trading has found that low melting brazing materials have performed well. During the brazing process, the temperature at the chip itself should not be higher than the processing temperature. Heat must be eliminated (e.g. by holding the wires with a pair of pliers). The used fluxes can corrode making it absolutely essential that they are removed carefully to avoid destroying the joint.
Spot welding (Recommended temperature range -50°C to +600°C)
Laser welding (Recommended temperature range -50°C to +850°C) Both of these connection methods are considered to be the most secure and temperature resistant. They require a certain degree of skill and manufacturing know how with a strict adherence to manufacturing parameters. It is important that the weld joint does not taper the outside diameter of the connection wires which could result in breakage. For this reason, Gekon Trading recommends a butt weld type joint.
Corrosive ambient conditions, especially in conjunction with humidity, must beavoided because this could damage the used glass and connection leads. The resistance elements should not generally be exposed directly to the media but need to be adequately protected.
Sufficient heat transfer between the housing on the one side and the resistance element on the other side along with protection against high vibration should be considered in the selection of the mounting method. (For example, vibrated into oxide powder, casting, gluing, immersion into heat conductive compounds or shrinking into suitable shrink hoses.) Special precautions must be taken so that the resistance element is not damaged during these processes.
It is also important that one sided pressure loads are avoided because this could cause a slight bending of the probe resulting in a strain gauge effect.
Thin film resistance elements are generally vibration proof, providing that the mounting has been done homogeneously. The helix of wire wound resistance elements is mounted and sintered to provide a strain relief within the alumina-oxide former.
During the assembly process, measures have to be taken so that the surrounding insulating material does not damage the glaze or the connecting leads. Avoid self supporting leads or leads that are not braced. High purity alumina oxide or magnesia oxide powders can be used as fillers in protective pockets. In the casting process, a compound which is chemically neutral with alumina oxide and high melting glass should be used.
The upper temperature limit of the resistance elements must not be exceeded during the curing process. (For example, during an immersion into a heat conductive compound). This is a very common method of assembly using thermo plastic insulating compounds (e.g. DOW Corning 240).
Please note: The resistance elements have to be protected against mechanical bracing.
Lead wire error: The standard basic values and their permissible deviations apply for resistance elements including the connection leads. All additional resistance in the circuit between the resistance element and the final instrument can cause an error reading if not considered or compensated for by appropriate wiring.
Self heating error: Self heating is an inherent characteristic of resistance elements. (Self heating coefficient = EK). The resistance element is injected with an electrical current. The converted electrical power heats up the resistance element insignificantly (P = I² x R). The self heating of an RTD is dependent on the following factors:
Insulation error: Steps must be taken to insure that the parallel resistance does not interfere with the measuring values. This parallel resistance is dependent upon the insulation resistance between the connecting leads / connecting leads and connecting cable / protecting tube. The nominal resistance and the surrounding temperature are two additional factors that must be taken into account.
In order to maintain proper soldering capacity, resistance elements must not be stored in conditions of high humidity or other hostile environments. The connection wires, composed of Ag5% Pd95%, may oxidize. This does not, however, impair their function. The oxide layer can be removed by a one hour annealing processing at 400°. All Gekon Trading resistance elements are manufactured according to the rules of ISO 9001.
Resistance elements undergo a variety of tests. The electrical values are selectively calibrated at two distinct temperatures.The overall measuring uncertainty of the calibration is 0.02K.
GEKON Trading GmbH
John-F-Kennedy-Str. 88 - D-63457 Hanau
+49 (0) 6181 956-270 -