Resistance Thermometer Styles
Stepping back in time for a moment, a variety of construction methods have been used in the development of RTDs over the last century. They include: Callendar’s original, with its mica cross around which the platinum wire was wound (problems included dehydration and embrittlement of the mica for exposed sensors, and condensation in gas filled and sealed versions); porcelain cross varieties with coiled wire (heavy and introducing time lag difficulties); twisted silica strip units forming a helical support for the coil; machined ceramic formers with grooves for the coil; and so on.
For laboratory standard instruments today, the element may be a thin wire (typically 0.07mm) wound in a helical form and supported by frictional contact in a closely fitting thin walled glass, silica or alumina tube. This may be U shaped, or two separate tubes twisted together for mutual support, with platinum coils in each, connected at the bottom by a thick platinum wire sealed into the glass and welded to the coils. Four platinum connection leads are sealed into the top - two in each limb - and the whole assembly may be provided with a silica outer sheath (see Figure 6.1). All designs are aimed at producing a strain-free thermometer which can expand and contract on heating and cooling without the wire rubbing or being scratched by its support.
For high precision thermometry above -189°C, the resistance element is cleaned and mounted in the glass or silica tube with the leads passing through a glass seal at the top. The device is then evacuated and back filled with dry air or high purity argon with a few percent of oxygen - to ensure that the platinum operates under oxidising rather than reducing conditions, such that remaining contaminants are preferentially oxidised during operation. Also, to maximise resistance between leads at higher temperatures, the leads are insulated from one another using mica, silica or sapphire.
Meanwhile, for very low temperature purposes, capsule type designs are favoured. Here, a thin walled platinum tube, about 50mm long by 5mm OD with a glass head, contains the resistance coil wound on a former. After evacuation, this type of device is filled with helium, for good thermal contact, before sealing.
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