How to remove periodic noise in temperature logger?

[A. Ocellaris]

I am using a 4-channel thermologger (AZ Instruments model #88598) that uses K-type thermocouples. The data is fine for about half of the day, but between about 8pm and 9am (overnight), all 4 channels rise and fall in a sine pattern with a period of about 40 minutes and an amplitude of ~5 degrees F. Three of the thermocouples are in salt water (marine aquariums), and one is in the air measuring room temperature. The data logger is running on 4 AA batteries and is not near any electrical wires.

Does anyone have any idea what might be causing this noise?

 

[MacGyverS2000]

Let's try another experiment... coil up all of the probe wires and place the probes within the same insulation box as the main unit. Wrap the entire thing (unit and probes) in a wrap or two of aluminum foil.

Let's see if all of the probes measure the same temp and the same swing amplitude when it's all in a close-knit bundle.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Does the entire length of every thermocouple need to be insulated?

No.

Any ideas on what is going on here?

I'm not seeing anything as you've described in the T/Cs themselves that would be any particular issue.

Thoughts?

hmmm We have a brain twister here.. Dan's experiment is worth a try. What's going to happen is after enough data lands here something will jump out at us.

All these various tanks... Since you're measuring different ones I'm going to assume they are not running in a series flow loop and that you are individually heating them? How? With standard thermostatic aquarium heaters?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianE22]

It's a plastic enclosure? Judging from where it comes from I'd guess there isn't much shielding. If you build a shield you might try attaching the shield to the (-) of the battery with a wire. Aluminum foil would help with the electrical part of any airborne noise. To stop the magnetic portion I'd try some sheets of mu-metal.

Thermocouple extension wire does come in shielded and/or twisted wire. You might want to try that.

I also doubt these units use low pass anti-aliasing filters.

 

[waross]

We had an instance where an oil heater would go into high temperature shutdown several evenings a week.
It coincided with the railway picking up rail cars on a track adjacent to the heater.
When the train was backing in to couple onto parked cars, the brakeman would be checking the distance remaining to contact.
He would be holding the transmit button down and counting off the distance to the engineer.
Our operators did some testing with our radios and found that a a radio signal would drive the indicated temperature on the controller slowly upscale until it tripped off on high temperature.
You may want to try looking for any intermittent source of interference.
My first response will still be to look for some component that is common to the compensation circuits of all four chanels, but I would be aware of and watching for any possible source of interference.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[A. Ocellaris]

Let's try another experiment... coil up all of the probe wires and place the probes within the same insulation box as the main unit. Wrap the entire thing (unit and probes) in a wrap or two of aluminum foil.

Thank you. I will try this next and post the results.

All these various tanks... Since you're measuring different ones I'm going to assume they are not running in a series flow loop and that you are individually heating them? How? With standard thermostatic aquarium heaters?

Thermocouples #1 and #2 are in single 10-gallon tanks that are part of 2 separate series flow loop systems (you can sort of see how the systems are set up in the 2nd and 3rd pictures of my 4 Aug 21 13:43 post). Both of these systems are cooled by chillers at their respective sump tanks. The chillers will kick on when the water temperature reaches 1 degree F above the set point, and they turn off when the temperature drops to the set point. Each system is about 400 gallons in total volume. Heating is not necessary because the pumps and UV filters contribute more than enough heat, necessitating cooling even when the room temperature is below the target system temperatures.
Thermocouple #3 is in an isolated 200L tank that is heated by a standard thermostatic aquarium heater.
Thermocouple #4 is measuring room air temperature.

It's a plastic enclosure? Judging from where it comes from I'd guess there isn't much shielding. If you build a shield you might try attaching the shield to the (-) of the battery with a wire. Aluminum foil would help with the electrical part of any airborne noise. To stop the magnetic portion I'd try some sheets of mu-metal.

Thermocouple extension wire does come in shielded and/or twisted wire. You might want to try that.

I also doubt these units use low pass anti-aliasing filters.

The units are relatively inexpensive - something like $96 each. I'm going to try placing everything in a foil-encased box as MacGyverS2000 suggested.

If this removes the noise, then one question I will have will be why is the EM interference only affecting 2 of the 4 channels? For example, channels 2 and 4 are using identical thermocouples, yet channel 2 has no noise, while channel 4 has a lot.

 

[BrianE22]

Swap the thermocouples. See it the noise stays with the thermocouples or the channel number.

 

[RedSnake]

I would think that the reason for sensor 1 and 2 not having as musch "noise" is that you have a cooling systems there that keeps the temperature in check.
What are the base temperatur you wont keep?

And I would guess that the air temp sensor is in the same room as sensor 1 and 2 and the 10/400 gallons tanks?

And my only explanation for why the 200 gallons tank has a musch higher temp values then the other 3 sensors is that it is in a room with afternoon and and evening sun.

And that it is the ventilation that is doing this.

A sinus wave that peaks ones every hour, is hard to achieve with electrical noise I would think.
And they do not have exactly the same amplitud or hertz either.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[waross]

How about a simple test. Put two probe in a crushed ice bath for a couple of hours during the time that the fluctuations occur.
One ice bath adjacent to the tank and one close to the data logger on short leads.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!