Sample thermocouple calibration

The present work involves measurement of k in a 0.1 atmosphere, stoichiometric CH -Air flame. All experiments were conducted using 3 inch diameter water-cooled sintered copper burners. Data obtained in our study include (a) temperature profiles obtained by coated miniature thermocouples calibrated by sodium line reversal, (b) NO and composition profiles obtained using molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis and (c) OH profiles obtained by absorption spectroscopy using an OH resonance lamp. Several flame studies (4) have demonstrated the applicability of partial equilibrium in the post reaction zone of low pressure flames and therefore the (OH) profile can be used to obtain the (0) profile with high accuracy. 

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

The underlying heat flow signal is calibrated by the use of standards with known melting temperatures and enthalpies of fusion. A series of such samples is run over the operating temperature range of the instrument. The sample thermocouple has a nominally known relationship between its output and temperature. Any observed differences between measured (by the sample thermocouple) and expected melting... 

When accuracy greater than the tolerances in Table 16.9 is required, the wires must be calibrated. Normally, this requires comparison calibration of sample thermocouples taken from each spool to account for spool variability. Typically, two thermocouples—one fabricated from the beginning and the other from the end of the spool—are calibrated to determine an average calibration for the entire spool. If the deviation between the two calibrations is not within the required uncertainty, a third thermocouple fabricated from the center of the spool should be used. If the results are still unsatisfactory, then each thermocouple should be calibrated individually, or a different spool should be used. 

The calibration and use of base metal thermocouples at temperatures above about 300°C will produce inhomogeneities in the wires, which can change the calibration itself [43]. The usual practice to overcome this dilemma for application at high temperature is to calibrate sample thermocouples to obtain the calibration for the remainder of the spool of wire and discard the calibrated thermocouples. 

A thermocouple in the probe can cause a variety of problems. For example, the rf pulse may disturb the temperature controller hooked to the thermocouple. A more common problem is that the thermocouple can act as an antenna for spurious signals. The thermocouple should be located as close as possible to the sample without introducing these problems. If it is necessary to locate the thermocouple well removed from the sample, a calibration is needed to correct for the temperature difference between it and the sample. 

The two most crucial differences between the two techniques are (a) in DSC, the sample and reference have their own heaters and temperature sensors as compared to DTA where there is one common heater for both (b) DTA measures AT versus temperature, and, therefore, must be calibrated to convert AT into transition energies, while DSC obtains the transition energy directly from the heat measurement. The confusion is also partly due to the fact that there are at least three different types of DSC instruments a DTA calorimeter, a heat-flux type (Fig. 2c), and a power compensation (Fig. Id) one. This, in turn, arises from the fact that some define calorimetry as quantitative-DTA. As opposed to conventional DTA, the thermocouples in a DSC instrument do not come into contact with either the sample or reference. Instead, they either surround the sample (thermopiles) or are simply outside the sample (thermocouples). Furthermore, the sample and reference weights are usually under 10 mg. 

Exhaust gas temperature measurements are made with a fine-wire R-type thermocouple connected to an Omega model 660 digital readout. Gas samples are extracted using a 6.4-millimeter (0.25-inch) O.D. water-cooled stainless-steel suction probe and then filtered, dried, and analyzed for CO, CO2, O2, UHC, and NOj . Instrumentation includes a Beckman model 864 NDIR CO2 analyzer, Beckman model 867 NDIR CO analyzer, Siemens OXYMAT 5E paramagnetic O2 analyzer, Siemens FIDAMAT 5E-E FID total hydrocarbon analyzer, and a Beckman model 955 Chemiluminescent NO/NOj, analyzer. Certified span gases are used for instrument calibration. PC-based data acquisition is available during experimentation. All of the emissions data reported here were obtained approximately 24 pipe diameters downstream of the fuel injector and represent average exhaust concentrations. 

Several techniques are available for thermal conductivity measurements, in the steady state technique a steady state thermal gradient is established with a known heat source and efficient heat sink. Since heat losses accompany this non-equilibrium measurement the thermal gradient is kept small and thus carefully calibrated thermometers and heat source must be used. A differential thermocouple technique and ac methods have been used. Wire connections to the sample can represent a perturbation to the measurement. Techniques with pulsed heat sources (including laser pulses) have been used in these cases the dynamic response interpretation is more complicated. 

Furnace temperatures were measured by two Pt—10% Rh thermocouples. One was mounted next to the suspended substrate samples and the other next to the vapor source. The rate of flow of the dry air through the furnace was determined by measuring the air pressure upstream from a capillary restriction in the air line. The pressure was measured by an oil manometer which had been calibrated against known flow rates of air through the capillary restriction. 

This instrument was designed to yield information intermediate between the ARC and the DSC. A sample of 0.2-0.5 g is loaded into a tube-like container and placed into the device (larger sample sizes may be used at slower scan rates). A thermocouple is connected to the outside of the tube and the cell is fitted with a pressure transducer. A similar, empty cell in the same oven with thermocouple serves as a thermal reference. The oven is heated at a slow, linear rate (0.5 to 1 °C/min), and the pressure and differential thermal data are collected. The data are presented in a fashion similar to DSC - Heat Rate (mW) vs. Temperature (°C). The thermal data are enthalpically calibrated by means of a series of standards (cahbration at high heat rates may be non-linear). Detection of thermal events approaches the sensitivity of the ARC. 

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