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SR630 Thermocouple Monitor

Compensation Without Reference Junctions negative lead is always red, while the rest of the world usesred to designate the positive lead. Often, the standard thermocouple types are referred to by their trade names. For example, K-type is sometimes called Chromel-Alumel, which is the trade name of the Ni-Cr and Ni-Al wire alloys.It is important for a good thermocouple to have a large, stableSeebeck coefficient, wide temperature range, corrosion resistance, etc. Generally, each wire of the thermocouple is analloy. Variations in the alloy composition and the condition of the junction between the wires are sources of error in temperature measurements. The standard error of thermocouple wire varies from ±0.8 °C to ±4.4 °C, depending on the type of thermocouple used.Voltage vs. temperature measurements have been tabulated byNISTfor each of the seven standard thermocouple types.These tables are stored in the read-only memory of the SR630. The instrument operates by converting a voltage measurement to a temperature, with the internal microprocessor interpolating to achieve 0.1 °C resolution.The K-type thermocouple is recommended for most generalpurpose applications. It offers a wide temperature range, low standard error, and has good corrosion resistance. The K-type thermocouples provided by SRS have a standard error of±1.1 °C  half the standard error designated for this type. The second approach to the problem relies on the fact that thevoltage across the junction A − C plus the voltage across thejunction C − B junction (at the measured temperature). In theSR630, the temperature of the A − B is the same as the voltage across a junction ofA − C and B − Cjunctions, and subtracting the voltage which we would expect for an A − B. As long as all the junctions are at the same temperature,the presence of an intermediate metal (C) has no effect. This allows us to correct for the voltage seen by the voltmeter in Figure 2 by measuring the temperature at the A − C and C − B junctions ismeasured with a low-cost, high-resolution semiconductordetector, and the subtracted voltage is the tabulated voltage ofthe A − B thermocouple at the measured temperature of theA Any two dissimilar metals may be used to make athermocouple. Of the infinite number of thermocouplecombinations which can be made, the world has standardizedseven types which exhibit a range of desirable features. These thermocouple types are known by a single letter designation: J, K, T, E, R, S or B. While the composition ofthese thermocouples are international standards, the color codes of the wires are not. For example, in the USA, the − C and C − B junctions. The advantage of this method is thatany type thermocouple may be used without having to change compensation junctions or maintain ice baths. Characteristics of Thermocouple Types Type B E J K R S T Positive Material Pt/Rh (30 %) Ni/Cr Fe Ni/Cr Pt/Rh (13 %) Pt/Rh (10 %) CuNegative Material Pt/Rh (6 %) Cu/Ni Cu/Ni Ni/Al Pt Pt Cu/Ni Positive Color (USA) Grey Purple White Yellow Black Black Blue Negative Color (USA) Red Red Red Red Red Red RedLowest Temperature 50 °C − 200 °C0 °C − 200 °C0 °C0 °C − 200 °CHighest Temperature 1700 °C900 °C750 °C1250 °C1450 °C1450 °C350 °C Minimum Std. Error ±4.4 °C±1.7 °C±2.2 °C±2.2 °C±1.4 °C±1.4 °C±0.8 °C Figure 4: Thermocouple reference data

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