Thermocouples K type display wrong readings when an AC component (heater) is connected to Solid State Relay Module

[tran88]

Hello, I have K-type thermocouples connected to DAQ-6211, the readings are fine until I connect a 2000W heating element into an outlet. 

I am using LINX function to send ON/OFF Signal to Arduino Uno. Arduino connects Relay Module (5V). Relay Module connects to Solid State Relay (40A, 24-320V) which then turn ON/OFF Heater. As soon as I plug in the heater's electrical cord into an outlet, the thermocouples reading are going to negative values. 

[GerdW]

Hi Tran,

your sketch is missing all power supplies, grounding and shielding…

[tran88]

Hello, yes forgot to draw the power supplies for the Relay Module and Solid State Relay. When you talk about shielding, you mean shield the AC Heater from the other DC components? Because I am thinking the AC heater is taking too power and that is why it is causing my thermocouples readings to be wrong. 

[RTSLVU]

On top of what Gerd said...

Sounds like there is voltage on your thermocouple!  

Most DAQ's have a differential input, but you still have to be concerned about each input's voltage in reference to ground. 

[tran88]

yes, there is voltage on the thermocouples, that is why I said, each time I plug in the heater, its taking taking away power and causing a shortage in power supply to the DAQ-6211, therefore I got wrong readings from the thermocouples. Is there anyway to prevent this from happening?

[RTSLVU]

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@tran88 wrote:

yes, there is voltage on the thermocouples, that is why I said, each time I plug in the heater, its taking taking away power and causing a shortage in power supply to the DAQ-6211, therefore I got wrong readings from the thermocouples. Is there anyway to prevent this from happening?

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1. Isolate your thermocouple form the voltage source.
   1. Safest method 
2. Isolate your DAQ and it's power source from ground.
   1. Not safe because now the entire DAQ is floating and could have voltages present that could create an electrocution hazard.

[tran88]

1) To isolate thermocouple from the voltage source, I think it is not quite ideal since this type K thermocouple only has 2 wires, positive and negative, it is not like the other thermocouple for example the DS18B20 thermocouple has 3 wires, (data, positive, and ground). And both wires from this type K has to be connected to DAQ-6211 analog input to receive data. Unless there is another way to isolate. Please suggest...

2) To isolate DAQ's power source from ground: are you suggesting that I cut the USB cable that is connected from the DAQ to computer and take the power source wire and hook it up to a power supply? 

[RTSLVU]

How about putting a "sil-pad" or some other material that has a high dielectric strength while being thermally conductive between your thermocouple and heater?

[tran88]

Hi, thanks for replying....But these thermocouples are not measuring water that is being heat up by the heater. They are measuring the water temp before and after it goes into the Heat Exchanger. I have another thermocouple (DS18B20) this one is connected to Arduino to measure the temp of water in the tank that is being heat up by the heater. This thermocouple (DS18B20) is working just fine. 

So, How do I supply enough voltage to the DAQ-6211 or protect it so that the heater will not have any signal interference against it? 

[RTSLVU]

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@tran88 wrote:

Hi, thanks for replying....But these thermocouples are not measuring water that is being heat up by the heater. They are measuring the water temp before and after it goes into the Heat Exchanger. I have another thermocouple (DS18B20) this one is connected to Arduino to measure the temp of water in the tank that is being heat up by the heater. This thermocouple (DS18B20) is working just fine. 

So, How do I supply enough voltage to the DAQ-6211 or protect it so that the heater will not have any signal interference against it? 

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If you have high voltage on your thermocouple you have a shock hazard and you are risking damaging your DAQ.

There is no way to fix it besides electrical isolation.