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Thermocouple Amplifier - MAX31850K for gas turbine
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Thermocouple Amplifier - MAX31850K for gas turbine

by dbeacroft on Wed May 11, 2022 10:14 am

Hi
Im new here and am looking for a quick overview of the Thermocouple Amplifier - MAX31850K in relation to its ability to react quickly to various temperatures. I have built a Controller ACU for a gas turbine but am not able to get the temperature rise quick enough
Currently as the temperature rises e.g. 100deg C per second or more up to around 1000dec C the current sensor is slow to react about 10 deg per second and takes forever to show the correct temp (see UI dial). The current Thermocouple is a 2 wire K type (Grove - I2C Thermocouple Amplifier (MCP9600) running off an Arduino Uno, the display is via a Nano every connected to the Arduino Uno via SPI

this gives an idea of my project and why it needs the fast temperature curve
https://youtu.be/1u4YC3RMUoY


Full workshop details
https://finishthoseprojects.blogspot.com/

Small_20211009_185326.jpg
Control box
Small_20211009_185326.jpg (43.35 KiB) Viewed 61 times

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by adafruit_support_bill on Wed May 11, 2022 10:39 am

The temperature conversion time for the thermocouple amp is in the 70-100ms range. See the thermal characteristics on page 4: https://datasheets.maximintegrated.com/ ... X31851.pdf

Other reasons for slow reaction time could be the related to how well the thermocouple junction is thermally coupled to what it is measuring. Also, any noise filtering being done in software would also tend to slow things down.

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by dbeacroft on Thu May 12, 2022 10:31 am

The Slowness is that it slowly increases, eg first 10 seconds the temperature goes from 0 to 800deg
the the thermocouple is at say 50 and is still slowly catching up, almost as if its sequentially reading the temperature changes,

Using the MAX 31850K can i assume this is not the case, Unfortunately the cold junction between the thermocouple and the controller is not continuous, im to worried about to much accuracy +/- 15degrees is probably fine, just i need it to react quickly. Read time of around 80ms should be fine.

is there any example code for the MAX 31850K on Arduino ?

Derek

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by adafruit_support_bill on Thu May 12, 2022 12:18 pm

The thermocouple amp is just measuring the voltage at the cold junction end and calculating the temperature from that. That cycle takes 100ms. The one-wire communication protocol back to your controller is pretty slow (slower if you have more one-wire sensors on the bus). But the data returned should be up-to-date with the last reading.

Try measuring the actual voltage at the cold end of the thermocouple probe. If that is changing too slowly, the problem is thermal coupling at the hot-end. Otherwise, the problem is likely filtering in your controller software.

Here is the Arduino library: https://github.com/adafruit/MAX31850_DallasTemp

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by dbeacroft on Fri May 13, 2022 11:56 am

That sounds like the issue, i will attempt to take some readings with my meter,
Measurement in millivolts ?

I think i will try the MAX31850K

As an extra question to this, i have also built a diagnostics unit that emulates the main engine, one part a got stuck on was the thermocouple, to emulate that (which is relative to the above) could i just push a few millivolts etc down to the thermocouple sensor to pretend heat is generated ? again would this be millivolts ?

Thanks

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by adafruit_support_bill on Fri May 13, 2022 12:13 pm

You should be able to simulate the thermocouple millivolt output. Here is a chart of K-type millivolt output: http://www.thermocoupleinfo.com/pdf/typ ... -table.pdf
If you don't have a variable voltage source that works in the millivolt range, you can always divide it down with a resistor voltage divider.

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by dbeacroft on Fri May 13, 2022 1:03 pm

Brilliant
Many Thanks for That

Derek

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by petespaco on Fri May 13, 2022 10:46 pm

Tell us about the actual thermocouple.
I use a lot of type K thermocouples, by the way.
-Is it sheathed?
-What is the diameter of the sheath?
-If sheathed, is the T/C grounded to the sheath?
Any sheath will certainly slow the whole change down. And--- of course, the thicker the sheath, the longer the delay.
Way back in the 1950's and 1960's we where instrumenting gas turbines. We simply spot welded the T/C junction right to the shell of the device we were testing.
A lot more recently, I have been using them (Type K T/C's and Max T/C amplifiers with Arduinos) to measure gas temperatures in woodgas reactors. Temps approaching 1000°F with pretty fast results. For this, I usually use the thinnest sheathed, SS, non grounded T/C that I can find ---IN the gas stream. But I still use bare T/C's wherever I can for the fastest response.
One other thought: how about your code? (I haven't looked at it) Is it possible that you have some delays in the loop that are causing slow response?

--Just a few thoughts,
Pete Stanaitis
-------------------

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by dbeacroft on Mon May 16, 2022 5:14 am

It is a K type, it sits in a 6mm shroud with a collar (see image)

Thermocouple.jpg
Thermocouple
Thermocouple.jpg (51.93 KiB) Viewed 15 times


its on a 1 meter length but then i have had to connect to a 6m cable before entering the sensor, this may affect the accuracy but should not affect the reaction time
the probe slides down into the housing with holes to let the gas pass in/through to touch the probe sleeve.

I tried using a blow torch on the probe as a test and that was slow too, i got it red hot within a about 20 seconds but the readings were still slowly increasing.
im hoping the MAX31850K will do better and have to check the voltage output yet.

Exit.jpg
Exit Gas
Exit.jpg (47.69 KiB) Viewed 15 times

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Re: Thermocouple Amplifier - MAX31850K for gas turbine

by adafruit_support_bill on Mon May 16, 2022 6:48 am

What about the code? Aggressive noise filtering tends to slow down the reported rate of change.

How does it perform when you run the simple test code from the library?

https://github.com/adafruit/MAX31850_Da ... Tester.pde

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