I changed my technique a few months ago to weighing every shot to add consistency but I've noticed something. I use a Robur E and the grider dosing can vary by a gram or more at times. Other times it remains very close. I think the humidity of my house has an affect as well as the density of each bean. I wouldn't mind doing an experiment to find out why my weights vary. I was thinking about grinding ten doses and then let them dry in an oven to remove moisture and then weigh each shot to see if that removes some of the difference. I could also check the grounds in a column to see if the volume of grounds remains constant?

The question I want to answer is; Am I better off using the E's dose without weighing (once my dosing is set) or should I continue to weigh each shot, adding or removing grounds as necessary to keep the weight the same?

Ask yourself how much water is required to effect a 1 gram change. One mole of water weighs 18 grams, and in the vapor phase occupies around 22 liters, which is over 5 gallons of volume for you SI impaired folk. At reasonable room temp and humidity (25C and 30% RH), the water vapor concentration in the air is around 10000 ppm or 1% of the total volume. So that means that you have to extract all of the water from 2200 liters, or 500 gallons of air in order to gain 1 gram in the weight of your coffee due to humidity, even if the concentration gradient between the coffee and the air would support it, which is probably not the case.

Based on the above, humidity is prolly a very small direct driver of density changes, although presence of water could cause expansion / contraction of the beans. My understanding is that the E grinds by time, not by volume or weight, so this expansion would not be accounted for.

Here's a question for you. What happens if you set the grinder to grind an excess of coffee, use some sort of distribution scheme like Stockfleths, then strike off the excess with a putty knife instead of your finger. Then weigh the result. Do you get better weight consistency? That might give you a better handle on whether the cause was density related or dosing scheme (elapsed time) related.

-greg

Weighing for my measley two or three shots a day is not a problem at all. I guess the value of the E version is for the busy shop to keep some consistency and limit overgrinding waste.

I'll just keep on weighing the shots!!

In areas where RH rapidly changes drastically, volume dosing is next to useless.
Yesterday RH was 84% and today it was 20% @ shot 1.

As of late, I've been single charging the grinder. Yesterday, 9.8g filled a single basket with the grind level with the rim after a gentle side to side shake. Today 9.8g came to about 2.5mm below the rim. The coffee was less fluffy, clumped slightly and much more compact. Shot 1 was also moderately more ristretto than yesterday. Taste led me to opt to back off the grinder ½ notch rather than adjust the dose. If I'd dosed by volume, it'd been a no flow.

jammin wrote:What have you been doing differently since August 8th when you stated you were getting extremely consistent dosing?

What happened is I removed the static screen. I was getting clumping from the screen and since I live in Florida, I was able to remove the screen without much mess. With the screen in place, there is resistance to the grounds leaving the chute. Without the screen, the grounds can just fall out. If grounds do fall out, the shot weighs more, if the grounds stay in the chute, the shot weighs less. I don't bother trying to clean out the chute with every shot as I'm too lazy and also, there's more mess (grounds flying) when the chute is empty and the grounds can just fly out. Even with the screen removed, there is some resistance to grounds leaving the chute.

I think that's where humidity may play a bigger role, as opposed to actually making the grounds weigh more (as Greg effectively debunked): when the air is more humid, it may cause the grounds to stick together more, so that sometimes a chunk will fall out or stay in the chute because it's clumped with other grinds, thereby decreasing the consistency of the dose weight from a timed doser compared to more dry days. Pure speculation, of course. But +1 anyway for grinders that dose by weight, and I'll throw in a +1 for espresso machines dosing by weight too, as many coffee folk have started asking for.

If you leave the grid in place, you'll get much more consistent dosing, that is how it was designed to be used. By having it there you have a uniform level of coffee 'compaction' in the chute. It follows then that a timed grind will push out a pretty constant volume of coffee. With no grid, its not a uniform level of coffee in the chute and thats where you get your variation from. The clumps you mentioned aren't of any significance, they are just some mildly compacted coffee that has been cut like cheese as its been pushed through the grid and not quite fallen apart on its run down the funnel into the pf. Just tamp it and you are good to go.

gyro wrote:The clumps you mentioned aren't of any significance, they are just some mildly compacted coffee that has been cut like cheese as its been pushed through the grid and not quite fallen apart on its run down the funnel into the pf. Just tamp it and you are good to go.

I've heard that before but my clumps were really dense (maybe humidity?). At any rate, I removed the screen and love the end product. I'll just continue to weigh my doses. I think I have less waste since removing the screen because there is less retention of grounds in the chute after dosing.

Thanks!!