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The life expectancy of a Cryocooler (e.g., Stirling and Pulse Tube) is a great concern for space and millitary applications. The performance of a cooler is a strong function of environmental conditions and actual life varies if run conditions are different from that of the life test. Our goal is to come up with a way of estimating cooler life based on life test data that was performed under another environmental condition. A watt-hour approach in estimating cooler life was first proposed by Miskimins, ("Estimating SADA II Cooler Life", private communication). It assumes that coolers of the same build have a constant watt-hour, much like that of a light bulb. Thus operating a cooler a low power will extend its life and vice versa. Yuan et al (reference 1), proposed an integral method, which improves the accuracy of the watt-hour approach.

Figure 1a shows the actual life test data of a half watt cooler. From this, one can generate a plot of cooler life in watt-hour (Figure 1b) for various run conditions. This example assumes a maximum input power of 14 W (specification). The cooler life in watt-hour can be calculated by the area under the life test data and is represented by point 'a' in Figure 1b. If the same cooler is operated in a harsh environment drawing 14 W, the cooler has essentially no life, and is represented by point 'b' in Figure 1b. Thus knowing the input power for one's requirement and the exit criterion (the specification for maximum input power), one can estimate the watt-hour life of the cooler as shown in Figure 1b. Figure 1- The Watt-Hour Approach

To use the information in Figure 1b, simply locate the input power of the cooler in Figure 1b , and look up the watt-hour. For example, if the input power is 6 W, the cooler has approximately 68,000 watt-hr or 68,000/6=11,000 hours of life.

References:

"Cryocooler Life Estimate and Its Correlation with Experimental Data", in the proc. of Adv. in Cryogenic Engineering, vol. 45A, P267, 1999.  (with D.T.Kuo, A.S.Loc and T.D.Lody)         Abstract        Download

BAE's Life Test result on Various Linear Coolers and Their Correlation with a First Order Life Estimation Method, to be published in the proc. of the 11^th International Cryocooler Conference, 2000. (with D.T.Kuo and T.D.Lody)         Abstract        Download



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About the author- Dr. Sidney Yuan is a consultant in the field of Low Temperature Physics and Cryogenics, and has written a Book on Cryogenics and published extensively in the field. E-Mail.

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