lucav
New Member
Posts: 7
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Post by lucav on Jan 19, 2021 5:53:00 GMT -5
Hi, I need some help to understand the LS240-8p accuracy performance. I have the following application: 8 DT-670-CU-70L calibrated sensors, approximately 25m cable (2m cryo cable provided by LS), LS240-8P module. I have a specification of 100 mK accuracy around 80K.
the Electronic accuracy quoted in the user manual is +-320 uV+-0.01% rdg at 25 degC. Considering the temperature response @77k of 1.73 mV/K it can be derived that the accuracy in the interval is of the order of +-180 mK. Am I correct? As a matter of fact, I tested these 8 calibrated sensors sinked together in LN and the measured output (calibrated sensors, curve loaded) on the LS240 8P shows values with approximately +-200 mK dispersion.
Since the sensor intrinsic calibrated accuracy is quoted as +-22 mK @77k, how can I improve my setup? Can I have the LS240-8P calibrated to a higher accuracy (since the sensitivity is quoted as 20 uV)? If this is the case, how often shall I recalibrate the module? As an alternative: shall I use a different temperature monitor? Will this be compatible with 25m cables (I am aware of the prescription for such long cables, as per appendix E. thank you in advance.
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Post by Lake Shore Jeff M on Jan 19, 2021 13:22:52 GMT -5
Hello Lucav,
Your accuracy calculation is a little off. For a DT-670 sensor, at 77 K, it is nominally around 1.03 V with a sensitivity of 1.73 mV/K. The 240 diode accuracy is 320 µV + 0.01% of reading. So the overall accuracy, in V for the 240 with this sensor is 1.03 * 0.01% + 320 µV = 423 µV. Then the error in temperature based on the sensitivity of this sensor is 423 µV/1.73 mV = 245 mK. Then add in the accuracy of the sensor and you are at around 267 mK. Their results are matching that.
This is about as good as the 240 is really expected to do. We do not offer a better calibration for this equipment, however, if you want to attempt to do your own calibration, I can send you our test and calibration theory document and you can attempt to perform your own calibration. If you want this information, please send me an email at support@lakeshore.com and I will send you the documentation.
Our Model 336 or 224 for the monitor version does have better specifications for the 2.5 diode range. Maybe that is good enough for your use.
Finally, regarding the long lead length, you will begin to be limited by thermal EMF voltages in the wiring. Since it is a diode, the excitation current cannot be reversed to remove the affects of this. This can be 10 µV or so per joint.
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