{"id":3246,"date":"2023-05-02T06:34:08","date_gmt":"2023-05-02T06:34:08","guid":{"rendered":"https:\/\/thermibel.be\/faq\/what-is-an-rtd\/"},"modified":"2023-06-27T12:21:34","modified_gmt":"2023-06-27T12:21:34","slug":"what-is-an-rtd","status":"publish","type":"faq","link":"https:\/\/thermibel.be\/en\/faq\/what-is-an-rtd\/","title":{"rendered":"What is an RTD?"},"content":{"rendered":"\n<p>In the same year that Seebeck discovered thermoelectricity, Humphrey Davy (1778-1829) claimed that the <strong>resistivity of metals showed a marked dependence on temperature<\/strong>. Fifty years later, William Siemens used platinum in a resistance thermometer. The propitiousness of this choice has been proved by the use of platinum as the main element in high precision resistance thermometers. In fact, the Platinum Resistance Temperature Detector or <strong>PRTD<\/strong> is used today from the triple point of hydrogen (-259.34 \u00b0C) to the freezing point of silver (961.78\u00b0C). <u>Platinum<\/u> is particularly suitable as it can withstand a wide range of temperatures while remaining highly stable and suffering almost no deterioration.    <\/p>\n\n\n\n<p><\/p>\n\n\n\n<figure class=\"wp-block-image size-full is-resized fl-left\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.1.png\" alt=\"image\" class=\"wp-image-282\" width=\"251\" height=\"224\" srcset=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.1.png 335w, https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.1-300x267.png 300w\" sizes=\"auto, (max-width: 251px) 100vw, 251px\" \/><\/figure>\n\n\n\n<p>In 1932, C.H. Meyers proposed building a resistance temperature detector (RTD) comprising a platinum wire wound in a helix round a cross-shaped mica support, all encased in a glass tube. This system minimises the tension on the wire while maximising the resistance. Although this construction produces a highly stable element, the quality of the thermal contact between the platinum and the point to be measured is poor and consequently the thermal response time is slow. The fragility of the structure restricts its use today to that of a laboratory instrument.   <\/p>\n\n\n\n<p class=\"cl-both\"><\/p>\n\n\n\n<figure class=\"wp-block-image size-full fl-right\"><img loading=\"lazy\" decoding=\"async\" width=\"235\" height=\"162\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.2.png\" alt=\"image\" class=\"wp-image-283\"\/><\/figure>\n\n\n\n<p>A different laboratory instrument has replaced Meyer\u2019s design. This is the \u201c<u>birdcage<\/u> structure proposed by Evans and Burns. The constraints on resistance caused by the time and temperature are thus minimised and the \u201cbirdcage\u201d became the norm in the laboratory. <u>Its fragile structure and sensitivity to vibrations mean that it is not suitable for industrial environments.<\/u><\/p>\n\n\n\n<figure class=\"wp-block-image size-full fl-left\"><img loading=\"lazy\" decoding=\"async\" width=\"235\" height=\"158\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.4.png\" alt=\"image\" class=\"wp-image-284\"\/><\/figure>\n\n\n\n<p>More robust construction techniques are shown in the figures opposite: a bifilar platinum wire is wound round a glass or ceramic coil. The bifilar winding reduces the magnetic induction and associated noise. Once the wire has been wound round the <u>coil<\/u>, the structure is <u>sealed<\/u> with molten glass. Unless the coefficients of expansion of the platinum and the coil are perfectly matched, expansion of the wire will result in a change in resistance with the consequence of a possible permanent change in the resistance of the wire.   <\/p>\n\n\n\n<p class=\"cl-both\"><\/p>\n\n\n\n<figure class=\"wp-block-image size-full fl-left\"><img loading=\"lazy\" decoding=\"async\" width=\"265\" height=\"61\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.5.png\" alt=\"image\" class=\"wp-image-285\"\/><\/figure>\n\n\n\n<p>There are RTD versions which offer a compromise between the birdcage and the sealed spiral. This approach uses a helicoidal <u>platinum spiral<\/u> wound round a ceramic cylinder and held in place by sintered glass. <\/p>\n\n\n\n<figure class=\"wp-block-image size-full is-resized cl-both\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.3.png\" alt=\"image\" class=\"wp-image-286\" width=\"265\" height=\"45\"\/><\/figure>\n\n\n\n<p class=\"cl-both\"><\/p>\n\n\n\n<figure class=\"wp-block-image size-full fl-left\"><img loading=\"lazy\" decoding=\"async\" width=\"265\" height=\"182\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.6.png\" alt=\"image\" class=\"wp-image-288\"\/><\/figure>\n\n\n\n<p>Modern manufacturing techniques use a <u>platinum or metal film<\/u> on a flat, etched ceramic substrate, guided by laser and sealed. The film RTD offers a significant reduction in assembly time and has the advantage of increased resistance for a given size. The manufacturing technology means that the devices are small with low thermal inertia; they can respond rapidly to changes in temperature.  <\/p>\n\n\n\n<p class=\"cl-both\"><\/p>\n\n\n\n<figure class=\"wp-block-image size-full fl-right\"><img loading=\"lazy\" decoding=\"async\" width=\"265\" height=\"120\" src=\"https:\/\/thermibel.be\/app\/uploads\/2023\/05\/faq-19.7.png\" alt=\"\" class=\"wp-image-287\"\/><\/figure>\n\n\n\n<p>Film RTDs are slightly less stable than wire RTDs but the benefits in terms of size, production cost and ease of use mean that they are very widely used.<\/p>\n\n\n\n<p class=\"cl-both\"><\/p>\n","protected":false},"featured_media":0,"template":"","format":"standard","class_list":["post-3246","faq","type-faq","status-publish","format-standard","hentry"],"acf":[],"aioseo_notices":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>What is an RTD? - Thermibel<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/thermibel.be\/en\/faq\/what-is-an-rtd\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"What is an RTD? - Thermibel\" \/>\n<meta property=\"og:description\" content=\"In the same year that Seebeck discovered thermoelectricity, Humphrey Davy (1778-1829) claimed that the resistivity of metals showed a marked dependence on temperature. Fifty years later, William Siemens used platinum in a resistance thermometer. 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