Volume 77, Issue 3, March 2006
Index of content:
- THERMOMETRY; THERMAL DIFFUSIVITY; ACOUSTIC; PHOTOTHERMAL AND PHOTOACOUSTIC
77(2006); http://dx.doi.org/10.1063/1.2176085View Description Hide Description
Fluorescence lifetime detection is widely used for sensing physical and chemical quantities. The measurement accuracy of fluorescence lifetime-based sensing systems, either in time or frequency domain relies on their capability of detection and analysis of low level signal superimposed to noise. In this work a quantitative assessment of several data processing and analysis methods for the estimation of the fluorescence lifetime was carried out by using an experimental arrangement based on a fiber optic temperature sensorsystem. A comparison between the various methods was performed using actual signals from an optical sensing medium. The basic principles of time- and frequency-domain lifetime measurements were also reviewed and discussed in order to point out the limit of the cw frequency-domain approach and to suggest a way to overcome it. The investigated lifetime interval was from 200 to about , corresponding to a temperature span of the sensor of about 300 °C. The results showed that in time domain (such as with Marquardt, integration, and log-fit algorithms) a good agreement, with relative differences from 0.2% to 0.5%, can be reached. Frequency-domain results based on an -point fast Fourier transform (FFT) compare favorably with the previous ones in the long lifetime region (resulting in relative differences lower than 0.2%) with larger differences for short lifetimes. For each data processing method, the uncertainty associated with lifetime estimation was evaluated. Sampling and harmonics effects on the estimation accuracy for -point FFTs were also investigated to trade-off between speed and accuracy of the algorithm in view of its application in real-time detection systems.
Gifford-McMahon/Joule-Thomson cryocooler with high-flow-conductance counterflow heat exchanger for use in resistance thermometer calibration77(2006); http://dx.doi.org/10.1063/1.2185498View Description Hide Description
A cryocooler that consists of a two-stage Gifford-McMahon (GM) mechanical refrigerator and a Joule-Thomson (JT) expansion circuit is developed for use in resistance thermometercalibration. The cryocooler is designed to attain a lower temperature rather than to produce a higher cooling power. A simple but high-performance counterflow heat exchanger is developed for the cryocooler. The heat exchanger has a high flow conductance while maintaining a high heat exchange efficiency. It is an improved type of counterflow heat exchanger composed of a spiral capillary and a thin-wall straight outer tube. The developed cryocooler uses a single counterflow heat exchanger not like a conventional GM/JT cryocooler, which usually has two or three counterflow heat exchangers. is used as the working fluid for the JT expansion circuit. The pot where the condensed collects after the JT expansion can reach in the continuous operation mode and in the single-cycle operation mode. The cooling power of the cryocooler is at with a molar flow rate of . Temperature control of the pot was demonstrated from using two control methods. One method involves controlling the evacuation speed in the JT circuit and the other involves controlling the heat input from a heater to the pot. The temperature of the pot is controlled within the order of magnitude of from peak to peak with either method.
True fluid temperature reconstruction compensating for conduction error in the temperature measurement of steady fluid flows77(2006); http://dx.doi.org/10.1063/1.2186211View Description Hide Description
There are three major types of errors that can cause a temperature probe to read a value different from the true fluid temperature into which it is immersed. One type is the transient temperature lag error in general purpose temperature sensors. Another is an error due to radiation heat transfer from walls at high temperatures. The other type is the error due to thermal energy conduction in lead wires and/or thermowell or other casing from a base on which the temperature probe is mounted. Although much study has been devoted to the former, very little exists on the latter. In fact, any prior research on this subject assumed that the entire sensor was completely immersed in the fluid, which in many cases is not true. For all of these types of errors, the cause is the misunderstanding that all physical sensor types only measure their own temperature, which may or may not be equal to the fluid temperature into which they are immersed. This article focuses on conduction error in temperature measurements and quantifies in simple terms a temperature error and lays out a general method for backing out (reconstructing) true fluid temperature from the sensor temperature measurement. Although only thermocouples are experimentally tested here, the theoretical work is valid for other types of physical sensors as well. The experimental verification for the model shows very good agreement.