Tolerances of basic values for platinum temperature sensors are specified in DIN EN 60751. The following applies:
Class B: Δt=±(0.3°C + 0.005 Itl)
Class A: Δt=±(0.15°C + 0.002 Itl)
and according to our own definition:
Class 1/3 DIN: Δt=±1/3 (0.3°C + 0.005 Itl),
Class 2B: Δt=±2(0.3°C + 0.005 Itl)
Long-term stability
The ageing effects on thin-film sensors as a result of long-term operation or temperature shock may have a negative influence on the precision and reproducibility of the sensor signal. Long-term stability is therefore of the greatest importance.
Due to the chemical stability and the relative chemical inertness of the platinum, platinum thin-film sensors are the most stable thin-film sensors.
Depending on the operating conditions, the resistance changes after 5 years of operation at 200°C are typically less than 0.04 %. The standard test conditions include 250 h, 500 h and 1000 h. However, shock tests and long-term tests can be customized to the customer's requirements.
Climate and humidity
A double glass layer and glass ceramic fixing drops protect the sensor element reliably from environmental influences. Measurements show that climatic and humidity variations do not have an impact on the measuring accuracy according to DIN EN 60751 of the sensor elements.
Circuit design
Platinum thin-film sensors are often under continuous current, as standard in 2-lead circuitry. For energy-saving reasons (accumulator or battery operation), a switched measured current can also be used. The voltage output signal is a function of the Rt resistance.
Because of the simple quadratic function of the platinum thin-film sensor characteristic curve, as well as the possibility of a simple linear approximation, the linearization of the measuring symbols poses no problem.
Connection
Standard 2-lead circuits may result in a loss of precision. Therefore, 3 or 4-lead circuits are recommended:
for longer cables, where the resistance and the temperature-dependent resistance of the cable may achieve significant values
for platinum thin-film sensors with narrower tolerances
if significant electromagnetic interferences exist
Storage
Platinum thin-film sensors should not be subjected to caustic and corrosive media and atmospheres. The specific storage information for each type is to be followed.
Cleaning
Platinum thin-film sensors are cleaned before packing and further cleaning is normally not required. Should cleaning be required after mounting, most conventional industrial processes can be used, including immersion in a liquid or bath. We recommend that residue-free cleaning agents be used.
Handling
Platinum thin-film sensors are precision components, and should therefore be carefully handled during mounting. Metal holders, clamps or other rough gripping devices may not be used. Plastic tweezers are recommended for working with elementary sensors. The supplied leads should not be bent near the body of the platinum thin-film sensor. Frequent repositioning of the supplied leads should be avoided.
Connection technology
The best results are achieved with welding (resistance welding, laser welding etc.) or soldering (soft, hard solder). When using hard solder, it should be ensured that the platinum thin-film sensor body is not heated above its maximum rated temperature. In general, the soldering times for hard solder should be less than three seconds.
Crimping and ultrasonic welding is also possible.
- When crimping is used, it must be ensured that any electrical resistance is eliminated at the connecting point.
- With ultrasonic welding, the leads are to be bent out of the level of the platinum thin-film sensor, in order to avoid interior damage.
- For the SMD and SOT223 series, we recommend automated processing with wave or reflow soldering processes.
Adhesion and embedding
When adhering, embedding or coating platinum thin-film sensors, it is important that the coefficients of thermal expansion of the different materials are closely matched, in order to avoid mechanical tensions that may affect the sensor signal. The embedding materials should be chemically neutral and remain elastic after drying. The position of a connected platinum thin-film sensor should under no circumstances be subsequently corrected by sliding its body. Heraeus Sensor Technology's MR series is already repackaged in a ceramic casing. The SOT223 and TO92 series are housed in plastic.
