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Wet beans - Estimating bean humidity with environmental sensor
Does anyone have experience measuring bean humidity using an air humidity sensor? In principle it should work (somewhat), but there are calibration issues and temperature issues. What bean humidity settles in equilibrium with what room humidity, and at what temperature? If someone can give me some rule-of-thumb assistance that would be appreciated.

Sub-question: Any experience with using silica gel with greens? Again, what bean humidity settles with what level of rehydration of silica gel? (On the internet it is claimed that 100g silica gel will absorb 40g water - but obviously that does not mean what it appears to mean at face value. In an infinitely humid room it will no doubt absorb 40g water before it ceases to have any dehydrating effect, but in a sealed environment with limited total water content it is bound to come into equilibrium at some point - after reaching a certain air humidity level and silica gel rehydration level combination, it will presumably remain in equilibrium).
The long version:
In early March I bought some green beans at source, at about 900m above sea level. Temperature was about 29C at that time, and humidity a little bit high.

I know very well that the beans were not well dried, they cannot dry adequately in those conditions. I was planning to sun dry them after reaching the city, but found the humidity was sky-high and feared the bean humidity would only INCREASE, so I decided on a drastic solution. At that point the beans were smelling quite wet (at 34C).

For 11kg greens I got 1kg silica gel with cobalt chloride indicator (initially dark blue, turns first pink and finally almost transparent when rehydration is complete). Eventually the beans ended up in plastic bags with about 1.1kg beans per bag, and a paper envelope containing 100g silica gel. The bags were sealed with a heat welder, i.e. completely sealed (the first 2 weeks was a less meticulous version). Today after another 2 weeks in the completely sealed condition I opened one bag to check the condition of the silica gel and reseal with a room-humidity sensor inside. I was slightly surprised by the results.

The beans were no longer smelling wet (not at all). The silica gel was still quite dark in colour, quite blue, only slightly pink - i.e. only a little bit used up. However the air humidity reading inside the bag after sealing was quite a bit higher than expected at 52% (temperature 35C).

My reasoning is that the silica gel will pull moisture out of the air surrounding the beans, which will be replaced by moisture diffusing out of the beans. Given the colour of the silica gel, this should imply that after two weeks there should be a moderately settled equilibrium between bean humidity/air humidity/silica gel water absorption. So why should the air humidity be so high as 52% inside the sealed bag? Or is silica gel highly ineffective at a temperature of 35C? (Recommended oven temperature for recycling the silica gel is 160C).

Does anyone have some informed comment on these figures?
Why measure air humidity inside bag, not so precise because when opening bag escapes and mix with ambient... instead measuring beans mass itself, then and now ?
The difference should give the moisture loss, I guess...

As a sidenote, I found that skywalker roaster is awesome for drying stuff... set the power to 10-15%, and playing with airflow, you can establish inside a very stable 50-70C environment. Grin
Edited by renatoa on 04/08/2024 9:39 AM
Because the sensor is designed to measure air humidity not bean humidity. It just has a simple probe stuck in a plastic tube with slots to allow air exchange. The probe is obviously stuck in the mass of coffee beans, but it can still only measure the humidity of the air between the beans and inside the probe tube, not the bean humidity, because that is what it is.

Why seal in the bag? (1) so that the air in the bag comes into equilibrium with the beans and silica gel, and is uninfluenced by the much higher environmental humidity; (2) in theory so that as more water is absorbed from the beans the surrounding air will settle to a lower humidity level (however there is a hint that it does not seem to want to go below 50%).

The industry meters seems are using a different approach, an interesting lecture below:

Ben N
I would take the most accurate scale you have, measure out something like 100gm of beans. Put the beans in an oven at 225°f for 4 hours. Remove the beans, weigh immediately. The amount of weight lost is your moisture content.
Then use same beans to calibrate a capacity meter, to mimic a commercial $$$ meter Grin


renatoa wrote:

Then use same beans to calibrate a capacity meter, to mimic a commercial $$$ meter Grin

Can you elaborate on this? Is this just a Multi meter on capacitance mode?
Measuring is the easy job... for me harder is to design a measurement container acting as a capacitor, in a consistent way, i.e. the beans count and size not influence measurement... that much.

LE: after posting the above, a fellow pointed me to a sensor that I would never have considered suitable for this task, because of a single word in the description: soil Grin This is the sensor: https://www.dfrob...-1385.html

Big mistake... because further research revealed that a very similar sensor, if not the same, was been subject of a research project, intended to measure ... drums... grain moisture. https://www.resea..._of_Things

I have the feeling this could be the beginning of a new great project, like the grounds colorimeter Grin
Edited by renatoa on 04/09/2024 12:11 PM
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