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EnviroMon temperature problems

EnviroMon temperature problems

Postby superadmin » Mon Nov 28, 2005 9:05 pm

Hello,

I have been using for more than a year now the EL005 data logger with the EL041 4-channel converter to monitor four water baths maintained at 37 C.

Last month I replaced the thermocouples (they were type K that came with the unit) because the unit was taking high readings beyond our specification (37 +/- 2 C). In addition, the readings seemed to peak daily for all four baths at around the same time. This was only rarely observed on weekends when the temperature readings usually held steady around 37 C. It was felt at the time that perhaps the thermocouples were wearing out. However, due to the strange daily temperature fluctuations, it was also felt that perhaps there are electrical interference issues.

There are two HPLC instruments sharing the bench with the water baths as well as the mounted electrical outlets that run the length of the bench above the water baths. In this area of the lab there are also stir plates, a sonicator, magnifying lamps, and two additional HPLC instruments.

When the thermocouples were replaced last month, we used heavier gauge type T thermocouples that appeared to monitor the water baths consistently. However, the same daily fluctuations (and OOS readings) have begun to appear again.

Our metrology department qualifies the calibration on the data logger every three months using their own equipment to perform a 24 hour temperature study on all four baths. Additionally, calibrations on the baths themselves are checked every 6 months. Consistently, their numbers are very tight around 37 C. It is my feeling then that the data logger is not recording the correct temperature.

Given this information, my questions are these:
1.) Is this issue familiar?
2.) Does the problem seem related to electrical interference or grounding?
3.) Does the data logger need re-calibrating?
4.) Should the thermocouple tip be an exposed tip or not?
5.) What is the optimum thermocouple (Type, wire gauge) for 37 C water bath temperature measurement?

Your help is greatly appreciated.

Mark
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Postby Michael » Tue Nov 29, 2005 4:15 pm

Hello,

Thermocouples rely very small changes in voltages therefore are susceptable to noise and grounding problems.

Small fluctuations are indicative of interference whereas larger fluctuations are indicative of grounding issues.

Try shielded thermocouples or isolated tips. Also, test one thermocouple outside of the bath to confirm the interference is airborne rather than an earth problem.

Best regards,
Michael
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