Single vs Double Baskets
An important discovery made by early proponents of the coffee refractometer was that single espresso baskets, in the region of 20% extraction yield, are less efficient than double baskets. Where a double basket is delivering 10% TDS from a 16g / 32g recipe, in halving the recipe but keeping the same ratio for a single basket (8g/16g) the anticipated TDS would only be around 8%. This could be due to water channelling down the edges of the single basket. A higher basket-edge-to-coffee-grinds ratio with single baskets will also contribute to channelling.
Double and single basket cross-sections
Although single shots may lead to low TDS shots and under-extraction, using a double shot basket, but splitting the shot with a double spout,