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The temperature tool is our oldest production logging sonde. Several types of sensors have been used in modern temperature tools; the three most common are briefly discussed below. Most premium tools contain a platinum Resistance Temperature Detector (RTD*). RTDs exploit and optimize the tendency of metallic conductors to exhibit a change in resistance with temperature. The RTD will perform over a very wide temperature range, and is inherently relatively linear. Its major drawback is a rather small change in resistance per degree, mandating special signal processing (amplification) in the "front end" of the tool. RTDs are available in the newer "thin film" or the older, and increasingly less common, "wire wound" constructions. The reduced mass of thin film elements results in a faster response time than wire wound RTDs. RTDs are made in different resistances, with the resistance usually specified at 0°C (32°F), known as the R0 value. 500 and 1000 ohm R0 platinum RTDs have been commonly used in temperature logging tools over the years. The GO / MLS Differential Temperature Tool (20 counts per degree) used a nickel RTD rather than platinum presumably because the resistance change per degree is somewhat larger; here is the Resistance versus Temperature Table (pdf file) for this oddball RTD. AnaLog Services, Inc. stocks RTD probes as well as replacement elements or capsules in a number of configurations and resistance values, both in thin film and wire wound constructions for repair or modification of customer's tools. Some tools contain thermistor sensors, metal oxide semiconductor devices. Both positive (PTC) and negative (NTC) temperature coefficient devices are manufactured. Thermistors provide a large change in resistance per degree, but they are highly nonlinear devices. There are a number of linearizing schemes, but in logging tools, where a dynamic range of 300 degrees Fahrenheit or more may be needed, thermistors are problematic. AnaLog Services, Inc. was asked to rework some old thermistor tools for use in shallow wells requiring a relatively narrow temperature range. We developed an Excel spreadsheet template based on the Steinhart & Hart equation, and using the classic three point single resistor linearization formula. Old thermistor based temperature tools, especially the old GO unijunction models, can thus be resurrected for some applications. Diodes have been used as temperature sensors in a few tools; Comprobe and Bell manufactured diode based tools. Diodes produce a relatively high change in voltage per degree, but there are probably limitations with respect to higher temperatures. Burr-Brown's Application Bulletin AB-036, Diode-Based Temperature Measurement contains an interesting and useful discussion of diode thermometers. |
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For an educational site on temperature, see www.temperatures.com. 12-99 |
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