Reading voltage from 2 wire thermocouple

Hello,

I have a thermo couple probe, with is bi-metal. it looks somthing like this at the plug side http://www.omega.com/Temperature/images/SMPW-CC_m.jpg

As you probably know there is a difference in potential between the two metals because of the temperature, meaning that if i put a multimeter of both jack terminals i will read mVolts and it will change according to temperature that the probe feels.

I was trying to measure that when connecting the (-) to arduino GND and the (+) to A0 pin. i didn't get any reading.

I didn't connect the 5V to anything because i want to measure the Volts that are being generated by the probe.

I think i probably missed something because it's obvious the circuit is not "closed", as for the mulitumeter i get fine reading.

Thanks in advance

The resolution of an Ardunio analog pin is 5V / 1023 steps, or about 5mV per step.

You didn't say what temp you're reading, but according to this graph , no type of thermocouple will provide anywhere near 5mV at ambient and some not even at boiling water temp, so I doubt if you're approaching an analog read value of 1.

Edit... what voltage did you actually read with your meter?

Hi,

I measured 3.8 mV @ 90 degrees C°.

I do belive i'll have to use a chip to control it.

I was looking at ebay and found this one http://www.ebay.com/itm/250814950389

Which is actually this http://www.adafruit.com/datasheets/MAX6675.pdf

What do you think? will it work?

Regards,

T

step 1.

use google image search with http://www.omega.com/Temperature/images/SMPW-CC_m.jpg, get http://www.omega.com/pptst/SMPW-CC.html.

step 2.

Read through above url, it is K thermocouple type. and follow link from same page http://www.omega.com/Temperature/pdf/SMPW-CC.pdf, get Glass Filled Nylon Range -29 to 220°C (-20 to 425°F).

step 3.

google K thermocouple type and omega, and we get http://www.omega.com/temperature/Z/pdf/z218-220.pdf from Thermoelectric Voltage in Millivolts table, we get -1.322 mV (-20°F) to 8.873 mV (425°F).

JimboZA is right, u can not directly read it from Arduino.

tsachi: I measured 3.8 mV @ 90 degrees C°.

90°C =194 °F, from above Thermoelectric Voltage in Millivolts table, correct reading is 3.682 mV, and your 3.8 mV is a way off by either your mV meter or thermometer, might be both.

If I ware u, I will test at 100 °C boiled water which is at least I don't need calibrate thermometer.

The boiling point of water depends on the purity of the water. Water which contains impurities (such as salted water) boils at a higher temperature than pure water, and it also depends on the sea level. Use the chart below to calibrate it.

If I ware u, I will test at 100 °C boiled water which is at least I don't need calibrate thermometer.

Not true.... water only boils at 100C at 1bar... I live at 1800m (6000') ASL and water here boils at about 94.

tsachi: I do belive i'll have to use a chip to control it.

I was looking at ebay and found this one http://www.ebay.com/itm/250814950389

Which is actually this http://www.adafruit.com/datasheets/MAX6675.pdf

What do you think? will it work?

MAX6675 is K-Thermocouple to-Digital Converter (0°C to +1024°C) Yours is Filled Nylon Range -29 to 220°C, as long as you do not care -29 to 0°C range. It will be fine.

JimboZA:

If I ware u, I will test at 100 °C boiled water which is at least I don’t need calibrate thermometer.

Not true… water only boils at 100C at 1bar… I live at 1800m (6000’) ASL and water here boils at about 94.

Did you see the chart I enclosed?

sonnyyu:

JimboZA:

If I ware u, I will test at 100 °C boiled water which is at least I don't need calibrate thermometer.

Not true.... water only boils at 100C at 1bar... I live at 1800m (6000') ASL and water here boils at about 94.

Did you see the chart I enclosed?

Yes of course, and I wondered why you said you use 100C boiled water when you are unlikely to have that. Even if you know the elevation, as you say the impurities and so on make a difference, so even at a known elevation, you will not know the temperature of boiling water without a thermometer. You can't calibrate anything that way since you don't actually know the temperature: even with the same water at the same place, on two different days with different weather and therefore barometric pressures, the water will boil at different temperatures.

JimboZA: Yes of course, and I wondered why you said you use 100C boiled water when you are unlikely to have that. Even if you know the elevation, as you say the impurities and so on make a difference, so even at a known elevation, you will not know the temperature of boiling water without a thermometer. You can't calibrate anything that way since you don't actually know the temperature: even with the same water at the same place, on two different days with different weather and therefore barometric pressures, the water will boil at different temperatures.

Use water boiling point and icing point to calibrate temperature is standard, even is standard for thermometer manufacturer. google it, and first link come out is gov site. for people are interesting. I enclosed it here. http://www.ncagr.gov/meatpoultry/pdf/Thermometer%20Calibration.pdf

I am not interesting further beat up at this topic, It is OP call. He is the person make final decision. :P

Except there are two other important points there: distilled water and a calibrated reference thermometer.

Anyway, whatever, this is detracting from the OP's need to understand that at temperatures available to the man in the street, ie water up to about 100C, his thermocouple is unlikely to register on an Arduino pin because the voltage is below one unit.

sonnyyu: JimboZA is right,

Rule 1. My wife is always right. Rule 2. If I think the my wife is wrong refer to rule 1.

It works for me all the time. :P