temperature sensing a bunch of coffee beans - Page 1
Forum section: electronics / projects

author: Etesla
author_rank: Regular Contributor
timestamp: September 09, 2020, 01:31:14 am
content: Hi all,I've got a tricky problem that I'm not sure is even possible to resolve. Here's the issue.To go from green fresh coffee beans to coffee, you have to roast the beans.Roasting the beans means making them hot in a rotating drum, with the highest important temperature being around 400 F, and the lowest being room temperature.It's nice to monitor the temperature of the beans, in order to make adjustments to the roasting process for the sake of consistency.Normally, the temperature is just read from a thermocouple shoved into the pile of sloshing beans. Since the internal bean temperature is really the information I would like to have, the idea that a thermocouple shoved into the pile of beans will give me a good measurement doesn't sit well with me.I was thinking that placing some sort of small wireless temperature sensor with similar thermal properties to that of a bean in WITH the beans would be a far better way to measure the internal temperature of the beans. The problem is that making a small wireless device that does this over the temperature range of interest seems impossible.With that as background information, my question is this: How can I better estimate the internal temperature of the beans? Any ideas are welcome. Thanks.

author: phil from seattle
author_rank: Super Contributor
timestamp: September 09, 2020, 01:42:41 am
content: Hmmm, it seems to me that doing what you don't like (just shoving the probe into a bunch of beans) is not that bad. The temperature that registers won't be the internal bean temperature but it should be a fairly consistent fraction of the actual temperature. I think you get there via trial and error. If you want to get more technical, you could measure the temperature drop over a period of time. Beans that haven't reached the right internal temperature will cool off faster than ones that have.The problem of a wireless sensor is the temperature. You would need electronics and batteries that will withstand 400F.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 09, 2020, 02:24:27 am
content: Drill a small hole in a bean, put a small thermocouple in it, and heat it up with a heat gun (in conditions approximating the roasting process) to get some data. Repeat with a few more beans to check for variation, and also repeat with a bean from a different batch. If you really want to be scientific, also note the size and weight of the beans used in the test, before and after roasting.At some point, you'll be able to estimate the internal temperature in relation to the outside temperature over time. Maybe all beans of that type will be similar enough that one profile will work for all, maybe you'll have to do some size and weight measurements to tweak the process.

author: DrG
author_rank: Super Contributor
timestamp: September 09, 2020, 02:51:14 am
content: I wonder how a non-contact thermal sensor might do in that situation, e.g.https://www.melexis.com/en/product/MLX90614/Digital-Plug-Play-Infrared-Thermometer-TO-Can

author: ejeffrey
author_rank: Super Contributor
timestamp: September 09, 2020, 04:45:10 am
content: I'm not sure what the best approach is, but I found this paper on thermal properties of coffee beans:https://pdfs.semanticscholar.org/1613/18efd160b933f507b45676ec24e37ffe76eb.pdf

author: AndyC_772
author_rank: Super Contributor
timestamp: September 09, 2020, 07:29:11 am
content: A sensor within the pile of beans will give a good indication of the temperature of the air surrounding each bean.If the beans start off at a lower temperature, or there are more of them, then they'll tend to bring that temperature down. Fewer or warmer beans will bring it up.Since at the end of the day you're looking to achieve a consistent process, IMHO the air temperature is a perfectly reasonable choice of variable to monitor and control. You might not know the temperature that the inside of each bean has reached, but you could certainly determine the temperature-time profile for the air which results in a perfect roast.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 09, 2020, 07:32:36 am
content: Quote from: NiHaoMike on September 09, 2020, 02:24:27 amNo good, the thermocouple wires are thermally conductive and will draw heat out. Even if you could get good thermal contact between the spherical joint and the bean, it still wouldn't read anything close to accurately.

author: KT88
author_rank: Frequent Contributor
timestamp: September 09, 2020, 11:04:16 am
content: I won't expect large temperature gradients iside of the bean. Together with the thermal conductivity of the TC (as mentioned by AndyC_772) you would get more or less the temperature of the bean surface...I think you could use a temperature probe which doesn't hamper the flow of the beans that is submerged in the bulk of the beans. This helps with two challenges:1. Keeping good contact to the beans for effective heat transfer.2. Shielding the probe from heat radiation from the walls of the roasting drum.

author: rpetitho
author_rank: Contributor
timestamp: September 10, 2020, 07:48:57 am
content: I have built 2 air rosters and even with a large amount of hot air being pumped in, the probe in the pile of beans works great. I get consistent measurements, roasts, and control. With the probe in the pile of beans I get first crack at 400 degrees almost like clockwork. As long as lots of beans are contacting the probe you should be good.-Bob

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 10, 2020, 09:15:23 am
content: Putting a thermometer inside the drum of beans is an excellent way of managing process parameters. Especially if it's done right, which means it's being touched by a mayority of the beans.Unless your beans burn like charcoal and such, the outside of the beans will always be hotter then the inside during heating, and once the heat has soaked into them, the temperature will be pretty much constant throughout the whole bean, and also the air between the beans.During cooling it will be the other way around of course, the center of the bean is hotter then the outside, it's simple thermal conductivity.Even if the absolute temperature of the thermometer is different than from the beans itself, it will be very repeatable, and you can adjust process parameters to fine tune the process.I do have doubts about using a thermo couple though.These generate very small voltages, and also rely on external compensation of the cold joint.Your time and effort may be better served by researching accuracy and repeatability of temperatures sensors themself. Resistive temperature sensors have a much bigger signal output, and are therefore easier to measure accurately and don't need the exterlan compensation of a thermocouple. These may be better if they can work at the intended temperature.

author: rpetitho
author_rank: Contributor
timestamp: September 10, 2020, 05:03:39 pm
content: Thermocouple is fine. A MAX31856 will have 0.15% error with a thermocouple. There could be better options but it will get expensive, granted those max chips are not really cheap either.

author: rvalente
author_rank: Frequent Contributor
timestamp: September 10, 2020, 08:13:38 pm
content: If you're aiming for precision maybe a tiny PT100 or PT1000 (thus they can respond faster) is a better solution in terms of accuracy.

author: wizard69
author_rank: Super Contributor
timestamp: September 10, 2020, 08:32:23 pm
content: Having worked on validation activities with ovens and sterilizers, you really need to be able to verify the temperature profile in the roaster. This requires a large number of thermocouples and the ability to track all of them over the cycle time of the oven. It can actually be a bit tricky to get a flat profile within your working space. If you don't have a flat profile then you will never get equal impact across all items (beans). In other words the temperature of one bean or bean simulator, doesn't mean a lot until you have some confidence that all beans are being processed equally.As for a bean simulator I don't think such a thing is realistic. You would be better off modeling the process and verifying results post roasting. With a reasonable amount of testing you should be able to get good results.