Brew Temperature and the Chemex

I remember reading early in my coffee days that someone had done an experiment where they had compared heat loss in porcelain espresso cups of different thicknesses and found that the heat retention of the porcelain didn’t much matter because all the heat went out of the top anyway. Like a lot of what I read on the internet at the stage I didn’t question it, didn’t want to see data but instead found it interesting, tucked it into its little trivia box in the brain and moved along.

After some comments in the thread about using scales to brew my Chemex on brew temperature I decided to perform a few rounds of experiments. Simple really, I just placed a probe (K-type, not ideal but it will do) into the bed of coffee, poured over the water and waited to see what would happen. The first test gae me an unexpected result.

Brew temperature of a Chemex

Brew temperature of a Chemex

What I expected was a steady decline in temperature during the extraction, and over a couple of minutes losing a fair few degrees, due to the area of steeping coffee being quite large. When the probe was in at the top of the brew the total heat loss was about 2C. Not much at all. What was interesting also was the temperature gradient throughout the liquid. At the start it was a couple of degrees from the top of the brew to the point of the filter cone and over brewing that stretched out to about 4 or 5C.

I’ve only done this experiment a few times so if you have the kit and a few extra minutes to spare I’d love to see it replicated – the coffee coming out isn’t much affected by the probe so it isn’t a wasted cup in the morning.

To completely and utterly hypothesize – it seems the floating crust of grounds does wonders for insulation, possibly with the help of CO2. In the next round I’d be tempted to stir and skim like a cupping bowl to see if this accelerates the cooling.

Thoughts anyone?


  1. I wonder how significant accurate temperature is with a pour-over as opposed to on a clover. A temperature variation on the clover of 2 degrees can completely alter the character of the cup, but I wonder if this is mainly due to the short dwell time. Perhaps a longer infusion time with a pour-over somehow softens the importance of a strictly controlled temperature?

  2. OK, anything to get me away from this week’s heavy work load with college … I made a Chemex brew while monitoring it with a Fluke 54 II and K -type probe. Once I added the water after the initial “wetting”, the temp dropped a total of 1.8 degrees Celsius. I did not agitate the grounds during the brew. The majority of the temp change (about 1.5 degrees C) was during the initial pour. After that, the temp was rock solid. Pretty amazing actually. I thought that the wide mouth of the Chemex would provoke huge heat loss. In contrast, it’s more stable than the press. I think you are right on about the crust’s ability to insulate. I believe that fact combined with the geometry and quality of the glass holds the temp well.

    By the way, the coffee was Stumptown’s Guat. El Injerto … it’s phenomenal.

    Further aside, is that cloth or paper that you’re using?

  3. I notice a drop of around 5 or 6 degrees when brewing in the Chemex, but I’m one of those weirdos that doesn’t pour until my water is down to 85 degrees.

  4. The whole 80-85 degree thing, whether as brewed or as espresso is interesting but whilst the cup is balanced enough I can’t help but feel it is an underextraction and we aren’t getting some of the more interesting notes.

    Would be interested to cup out 95,90,85,80,75 and 70C to see how it changes. Just need half a dozen chemexes. (Is that spelt right)

  5. Well, with respect to insulation factor, I can add that with tonight’s press of Costa Rica Flecha Roja (thanks Emma at Manic!) I started out with an infusion temp of 189.5F that held totally solid for about a minute. After a minute I stirred with my thermometer and the temperature dropped by about 0.1F per second from that point on. The result was a slightly sour cup, finishing at about 175.5F =(

  6. Sorry to spam the thread but it is interesting to me… I definitely agree that you get an underextraction with a temperature of less than 90C. This new ceramic press I’m using at the moment sucks a ton of heat from the brew water, no matter how much I pre-heat it. I’m always getting infusion temperatures around 87-88C and the cups always offer great taste balance with good acidity and body characteristics, but are dulled in terms of aromatic nuance. Also, to bring in the clover as an example, I’ve experienced a similar ‘dull’ aroma from a clover set to 203F (infusion probably 198 or 199) but when the temperature was reset to 205F (200-201 actual) the cup really came alive.

    Thinking about this makes me realize how rudimentary coffee technology still is. It would be nice to stir the coffee in order to avoid saturation of the water at the top of the brew, but this leads to huge temperature loss. It’s also a concern that the highest temperature is also in the part of the brew with the greatest risk of saturation.

  7. The vast majority of any heat loss during extraction, will in the chemex, be via convection/radiation.

    Convection will occur from the top of the liquid if the grounds are removed, but as we are measuring the temperature of the liquid beneath the grounds, any heat loss will be from the grounds itself rather than the liquid. It would be interesting to see the temperature difference between the top layer of the grounds and the liquid beneath

    The filter would provide sufficient insulation to stop heat loss in to the glass via conduction, although this will occur as the brew enters the bottom bulb as the laws of thermodynamics state that heat flow will occur until medium A and medium B are at an equal temperature. beyond this this glass itself will radiate heat providing the gradual cooling that we know will happen.
    This does take time though as glass is a far better conductor of heat that air is.

    in a nut shell, the simple experiments you’ve conducted have already proved that to maintain a constant brew tmpereture, you must leave the grounds on top when brewing.

  8. Another quick thought, if the Chemex is pre-warmed with hot water, then the initial temp change should drop considerably, allowing you to regulate the brew temp much easier.

  9. Interesting thread. Haven’t measured temps myself, but i tend to stir the slurry and hadn’t given much thought to the effect stirring has on brew temperature. Maybe I should.

    Having said that was also concerned re heat loss from convection, while brewing and almost since day one with the Chemex got into the habit of covering the Chemex with a paper towel/sheet of kitchen roll after finishing adding the water and waiting on the remainder of the brew to filter through.

    As I said I haven’t done any temp measurements, but at least psychologically I feel I have covered the brewing coffee and hopefully reduced some of the heat loss!

  10. I did a lot of temp measurements in the chemex a while ago to find out how long to let the water sit after boiling. Ending up with opening the kettle and letting it sit for 45s. This gives me ~94degC, dropping to 92degC.

    Another problem I wrestled with for a while was brewing different amounts of coffee in my rather large chemex. I usually brew 3/4l for my self in the morning. But for guests I want to use the full 1.5l capacity. Initially that meant dialing in the grind twice, and making too much coffee for a few days every time I changed coffee.

    I now use the following solution: I start the watch when the kettle shuts off, start adding water to the grounds after 45s, and then keep topping it up until 4min on the watch. After all the water has dripped through I usually end up with a fraction less than 3/4l. I just add a bit of the remaining water in the kettle directly to the coffee to get 3/4l.

    For a full chemex, I do exactly the same. Just add twice the weight of grounds, and add approx. 3/4l additional water after brewing.

    This gives the same steep time for 3/4 and 1.5l, using the same grinder setting, resulting in very similar extractions.

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