Re: DS1820 temp sensor odd results




Eric Yancey wrote:
Hello,

I recently installed a temperature monitoring system for my home workshop.
The primary goal is to monitor the heat capacity and dissipation of a
concrete slab floor with radiant heat tubing. When I installed the heating
tubes before the concrete was poured, I embedded a fairly short (5 feet or
so) piece of PEX tubing to house a temperature sensor.

I built kit 145 from here: http://kitsrus.com/kits.html to interface with
four dallas DS18S20 sensors. One sensor measures ambient air temp, two
measure the temperature of the fluid entering and exiting the floor, and the
fourth is inserted in the terminated PEX tube as described above.

The problem I'm having is that the 4th sensor is reporting flaky results. I
replaced the original sensor but the replacement is exhibiting the same
behavior as the original. Here is some sample data:

07:45:00 78.35
07:44:30 74.07
07:44:00 71.15
<snip>

Does anyone have any ideas on why this may be happening?

I apologize for the length of the message and thanks for any help!

Eric Yancey



If your test of the sensor outside the slab shows as I suspect that it is OK, I think your issue may be due to temperature fluctuations in the still curing concrete. IIRC, concrete, when it is curing generates heat.
This heat will gradually fall off towards the ambiant temperature in time, but as you said that the slab was recently laid, maybe that has not had time to complete in the center. You could double check by
drilling a hole (remember where the pipes are?) and inserting a mercury
thermometer to see if the same thing happens.
As the fluid is in motion it will not exhibit the swings as not enough time will have occured between entry and exit for transfer. As it is a heating system, I guess the liquid's temperature will be higher than that of the slab anyway, so heat transfer will be away from the liquid.


.

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