(MM) wild temp sensor readings

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Senior Member

Joined: 17/10/2011
Location: United States
Posts: 187
Posted: 01:47am 13 Jan 2012      

(MM) Wild temp sensor readings LM335Z

I’ve been having some difficulty with wildly extreme temperature readings from LM335Z temperature sensors.

I’m doing the trial runs of the wood fired boiler and have noticed that two of the four temperature circuits fluctuate as much as 5 degrees C (10-12 degrees F) in as little as five minutes. Sometimes this fluctuation is in as little as two minutes and it is not uncommon for the first minute at MM boot to fluctuate as much as 8 degrees F or 4 degrees C.

All temp sensor pins are set using SETPIN(x),1

The MM SM1 3.3 volt circuit reads at 3.26 volts so I use 3.26 in formula. For example: OUTDOORTEMPCENT=(PIN(2)/3.3 * 3.26-2.73)*100.

I’ve swapped the outdoor temp sensor with the household hot water sensor and the fluctuation is extreme (beyond factual) regardless of which sensor I use.

All four of my sensor circuits use only the + & - legs of the LM335Z. I have them soldered to 22 gauge 2 conductor red/black insulated wires with an outer layer of gray plastic. The hot water and outdoor temperature circuit wires are both about 8 meters long. The one for the indoor house temperature is similar, but only 3 meters long. This shorter one does not fluctuate nearly as much, probably reads more accurately is my guess.

I have tested the sensors on different sensor circuits to make sure the problem was not on the circuit board. The results are that the sensors with the longer wires have the most fluctuation regardless of which sensor connection point they are plugged into.

I wondered if I had too high of a current load on the MM SM1 power regulator, which I may have since it was warm when I was running the VGA to Composite converter off the 5 volt leg. But I disconnected the VGA converter and the temperature fluctuation problem is still there. I tested the setup using two different MM SM1’s too, so it is not machine dependent.

Does anyone know what the heck is going on?

I built an earlier MM project, a temperature sensor and data logger using a 10 meter wire to a LM335Z which is still running. I use it to view and log the current, high and low daily temperatures. It does not seem to fluctuate within a single minute much at all, maybe 1 or 2 degrees F (1 degree C). But these other circuits in my Wood Fired Boiler project sure do.

Today I even tried replacing the Outdoor Temperature Sensor, one of the 8 meter 22 gauge stranded wires sensors, with a new 8 meter 20 gauge solid wire with a brand new LM335Z on the end. It has the same problem as the 22 gauge one it just replaced.

So, I’m wondering, is it possible that these sensors might be picking up transient RF voltage on these longer wires? I can’t think what else it could be. The argument against that though is that my longer sensor on the other MM temperature logger does not manifest the wide range fluctuation problem.

I would have taken the long 10 meter sensor off my daily temperature logger to test on the Wood Fired Boiler project, but that one is soldered in whereas all the others are connected using 2 conductor header pin connectors off the component circuit board. I don’t think those connectors could be the problem.

I do run my outdoor sensors along a place on the back corner of the house where about a dozen CCTV camera signal and power wires are running into the house. But the other earlier project sensor wire that does not fluctuate much also runs along this corner of the house near these wires.

Does anyone have any ideas on this? Has anyone else experienced wild fluctuations of temperature readings using LM335Z sensors? Should these longer runs of wire to sensors be shielded? Would this solve the problem?

Pete in Hyder
We're all here 'cause we're not all there.