Thermocouple amplifier always gives same output

Go To Last Post
15 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I have 2 of Adafruit's analog thermocouple amplifiers:

 

Analog Output K-Type Thermocouple Amplifier - AD8495 Breakout

 

and 2 thermocouples.

 

Though I have one in ice water and one in air, they both give me 1.34v. (as close as my meter can show.) Reading them with an A/D on this shamefully non-avr project, they both say 1.6 degrees C. If I disconnect a thermocouple from the amplifier, it reads 39.0 C.

 

Might this be because my "K type thermocouples" aren't?

 

The code:

 

    uint32_t counts = analogRead(Pin)-10;       // seems to be a 10 count offset
    uint32_t mVolts = (counts * 3270) / 4096 ;  // reference measures 3.27v
    uint32_t temp = (mVolts - 1250) / 5 ;                // 5mv / C

 

Adafruit's formula:

 

Temperature = (Vout - 1.25) / 0.005 V. 

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

Last Edited: Wed. Jan 23, 2019 - 10:08 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

From the data sheet applications section:

 

The thermometer mode can be particularly useful for debugging

a misbehaving circuit. If the basic connection is not working,

disconnect the thermocouple and short both inputs to ground.

If the system reads the ambient temperature correctly, the

problem is related to the thermocouple. If the system does not

read the ambient temperature correctly, the problem is with

the AD849x or with the downstream circuitry.

 

The thermometer or measurement mode means that the Sense pin is connected to the Output.

 

With the input shorted I would expect the thermocouple amplifier output to be that for ambient temperature, whether it is grounded or not.

 

Stan

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 1

If you're having touble then simplify your design to pinpoint it.

Is your amplifier working correctly? do you have errrors in your firmware?

 

Start by outputting raw ADC values and decide if those make sense. Debug it and and only then add conversion algorithms in steps.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

+ and - from the thermocouples swapped?

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

 

50K to ground and .1 caps added. microfarads seldom hurt.

 

Brian Fairchild wrote:

+ and - from the thermocouples swapped?

 

That was my first thought.

 

Quote:

Is your amplifier working correctly? do you have errrors in your firmware?

 

1. No. 2. Probably.

 

sbennett: I remember reading that. I'll try. Also, I'll try disconnecting the amplifier output from the uc just to be sure the inputs aren't outputting something at 1.34v.

 

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Interesting: With my meter's temperature probe, my gizmo says I have a temperature of 27C while the meter says 37C, but at least it's not 16C like the food grade probes say everything is.

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Consensus is these are J type thermocouples and K type amplifiers.

 

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Torby wrote:

If I disconnect a thermocouple from the amplifier, it reads 39.0 C.

 

From the chips datasheet...

 

 

...this might suggest something else is going on.

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Since the $11 adafruit boards don't come with the AD8494 chip for J type thermocouple, I just layed out a board and ordered some 8494 chips.

 

Can't seem to find my soldering fry pan. Might have to buy one.

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Wonder if I should put in some ferrite beads to keep digital noise out of the power...

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Last night, I put the K type thermocouple from my meter on it and... got completely crazy results.

 

Today, I repeated last night's failed experiment and got completely reasonable results. Even matched a lab thermometer at any temperature between 0 and 100 C I cared to try.

 

 

Surprised it does so well with just clip leads as we're dealing with VERY low level signals here.

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Brian Fairchild wrote:

Torby wrote:

If I disconnect a thermocouple from the amplifier, it reads 39.0 C.

 

From the chips datasheet...

 

 

...this might suggest something else is going on.

 

Yea. I'm relying on that to say, "Hey, dummy, you don't have the probes plugged in."

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Using sensor conditioning chips with an analog output seems ... old fashioned to me.

Have a look ad ADS1220, It's also an Ali special with cheap breakout boards.

(Cheap as compared to regular western channels, the chip itself is one of the more expensive little critters)

 

Also, what is your knowledge level around thermocouples and cold junctions?

Very short: Thermocouples only measure the temperature difference between the ends of the thermocouple wire.

The temperature of the "hot" end is deduced by adding it to the temperature of the "cold" junction compensation.

Your crocodile clamps and audio cinch connectors are not part of the thermocouple wire and if you have a temperature difference over that part of the circuit it can not be corrected for.

 

 

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Sat. Feb 2, 2019 - 09:12 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

That's why you need the connectors close to the chip with its temperature compensation so those extra junctions are at the same temperature. Clip leads don't belong in the real circuit.

 

Have some J type amplifier chips and made a pc board for them. Need to do some other things before I can try them.

The largest known prime number: 282589933-1

In my humble opinion, I'm always right.