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A Thermocouple Guide Worth Reading!

by | Jan 27, 2014 | 2 comments

Type K, J, Pt1000?? HELP!

The guys at Cropster were kind enough to post some great information on Thermocouples and how to choose the correct one. With all the different types and configurations out there, roasters often get confused and end up buying the wrong sensors. Be sure to read Cropster’s latest blog entry to learn more about the different types and how they are used in machines.

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How does Genio do it?

Genio uses ungrounded 4.7mm Type J thermocouples to take care of the temperature readings. The thinner yet durable stainless steel sheaths allows for quick readouts with good stability. Where Genio goes the extra mile is by adding a second sensor for reading the environment temperature.

Environment Temperature

The Environment probe sits inside the drum right next to the drum’s air outlet. In other words, it senses the air temperature surrounding the beans and tells you exactly how hot the air is that you are using for convection heating. This readout adds a whole new dimension to roasting because it allows you to see how much energy is available in the drum. By gauging the temperature difference between the environment and bean temperatures, one can quickly see if the there is enough energy in the machine to carry the roast through first crack and if the roast is stalling when you rerduce the gas once you reach first crack.

For example: If you are approaching first crack at a bean temperature of around 185 and your environment temp is sitting at 210 degrees, there is almost no way that you will have enough energy in the machine to carry the roast through to full maturity without stalling. An environment temp of at least 240 would ensure that you have enough momentum in the drum to carry the roast through. If, on the other hand, you want the roast to slow down during the development phase so that you can properly caramelise the sugars in the bean before ending the roast, you cannot have too much energy in the drum. An environment temp of over 260 degrees after first crack will amplify the exothermic effect of the roast and cause a second crack only a few seconds after first. There is almost no way to stop this “runaway” reaction if there is too much energy in the drum after first crack.

The optimal curve is to charge the drum with energy before first crack: Enviroment >= 250 and Bean is around 185. At the first signs of first crack, you should dial back on the gas pressure to reduce the Environment temp to around 230 while first crack is in full swing. Maintaining a constant lead of at least 20 degrees between the Environment and Bean temperatures will ensure that the roast is continuing forward without stalling or baking at a rate fast enough to still drive the roast towards full maturity.

Response Times

Another factor that is important to consider is response times between your Environment and Bean probes. While the Bean probe sites inside the bean mass and only reads whatever energy the beans have bean able to soak up, the environment probe sits outside of the bean mass and inside the rapidly moving air stream. This allows the environment sensor to quickly respond to any changes in the roast profile because there is almost no lag in its readouts. The fast response allows the roast master to react to any sudden changes instantly.

If you wanted to react to changes by looking at just the Bean Temperature, your reactions would always be a few minutes behind what is actually happening (in other words: you’re too late!). Bean temperature will always lag well behind Environment temperature but will always follow it given enough time: If your environment temperature is below that of the bean temp, then it stands to reason that the Bean temp will follow this curve in due time since the environment inside which the beans are roasting is cooler than the beans themselves.
If you have any questions about thermocouples and temperature profiles, please contact us and we will try our best to help you out!

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2 Comments

  1. Ralf Hawley

    A great article, thanks Neil. I’ve realised this was a mistake I was making with my 1.5kg Proaster. My environment temp was too low at first crack with the ROR subsequently dropping about half way through. I’ve altered this and am getting much better results. My ET probe is located in the back of the roaster where the air enters the drum.

    Kind regards
    Ralf Hawley

  2. Neil Maree

    Hi Ralf

    Thanks for the comment. Glad to hear it can help you. Our environment temperature sensors are mounted inside the drum on the front panel, similar to the Bean Temperature probe. However, it sits well above the rotating bean mass. This gives us a great indication of the environment in which the beans are roasting.

    However, not all of the energy from the back of the drum makes it into the drum. i.e. You may be getting very high environment temperature readings but all of that energy may not be sucked into the drum by the fan. Perhaps you could more the probe? If it works for you, don’t change it!

    Kind regards
    Neil Maree
    Owner / Design Engineer
    Genio Roasters

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