All resistance thermometers require the following considerations in their manufacture. Wire-wound sensors must be supported on mandrels closely matching the wire in thermal expansion to minimize strain effects. Additional assembly materials, such as cements, should not introduce additional strain in the operating temperature range. The final assembly must be in a stable, annealed condition, trimmed to the required resistance tolerance. Only high-purity materials and clean assembly methods should be used to avoid sources of contamination that might degrade the sensor. All internal connections should be welded, and connecting leads should be chosen for the required temperature capability and avoidance of thermoelectric junctions. To realize the ruggedness of fully supported elements in the total sensor assembly, all internal connections should be anchored and isolated from effects of thermal and mechanical strains, including shock and vibration. The same requirements apply when deposited film or foil-type resistance elements are used (Figure 4.10c). For equivalent performance in their respective temperature ranges, base metal RTDs cost the same as 100 ohm platinum RTDs. Construction requirements and materials cost are similar. Base metal RTDs have a materials cost advantage at higher resistance values compared to wire-wound platinum sensing elements. Thin film platinum elements erase this advantage.In the case of platinum RTDs, the full supported rugged construction using ?reference grade? wire has a temperature coefficient (alpha) over the interval 32 to 212 of between 0.00387 and 0.003915 depending on the manufacturer. Compared to 0.003927 C on a SPRT, the reduction in sensitivity is insignificant. For best accuracy, the user should be aware of or specify the actual temperature coefficient. One common value available from most manufacturers C. This is the result obtained for windings on pure alumina mandrels. The relationship between the resistance change of an RTD vs. temperature is referred to as its alpha curve. The most commonly used is the European curve that has an alpha of 0.00385 The American curve has an alpha of 0.00392 The instrument used with the RTD must be configured to use the same alpha curve as the RTD or serious errors will occur.