Looking down at the measurement lines, the 1 and 1 1/2 ounce are standard jigger territory, the 1/2 and 3/4 ounce are sharp lines, and the 1/4 and 1/3 ounce are beveled (or S-shaped) "lines". This last part stressed me since I was unsure of how I was supposed to fill this cup-within-a-cup design -- to the top of the inner cup (so liquid sits on the ledge) or slightly overfill (so liquid just hits the outer cup wall)? I am unsure why OXO chose this style instead of a sharp line except for aesthetic reasons. The aesthetic reason is not in the interior, but on exterior to have a smooth transition from their black plastic gripfast material and the metal.
• OXO Double JiggerAll values are the average of a cumulative 12 pours (i.e.: I did not try to measure each individual pour). I did not test the 1 1/2 or 1 ounce lines as I was more interested in the accuracy of the demarcations inside (and I assumed that they were as accurate as any other quality jigger).
• plastic OXO Mini Angled Measuring Cup
• 1/2 x 1 ounce Double Jigger from Kegworks
• 1/2 x 1 ounce Double Jigger from BarSmarts (part of the kit for taking the class)
• 100 mL graduated cylinder
• 1 funnel
• Tap water (Somerville, MA's finest)
1/4 (0.250) ounce
• Filled to top of inner cup: 0.236 oz (-6.0%)
• Filled to just touching outer cup: 0.283 oz (+13.3%)
1/3 (0.333) ounce
• Filled to top of inner cup: 0.319 oz (-4.2%)
• Filled to just touching outer cup: 0.364 oz (+9.3%)
1/2 (0.500) ounce
• Filled to line: 0.457 oz (-8.6%)
3/4 (0.750) ounce
• Filled to line: 0.708 oz (-5.6%)
1/2 oz on other jiggers:
• OXO 2 oz Cup: 0.488 oz (-2.5%) (* viewed from side)
• KW 1/2x1 oz: 0.497 oz (-0.6%)
• BS 1/2x1 oz: 0.504 oz (+0.8%)
The quick and dirty results are that standard jiggers (filled to the top) are quicker and more accurate in my hands so the all-in-one convenience of the OXO Double Jigger has its set backs. The 1/3 oz jigger is elusive here in the United States although occasionally I have spotted 10 mL jiggers (Boston Shaker had some British ones a few months ago); therefore, this extra measurement line is handy. Moreover, 1/4 oz is only present on the OXO measuring cup or estimated in a larger jigger (i.e.: in a 1/2 oz) or approximated as 2 large-sized barspoons.
The internal line in the plastic OXO measuring cup was easier to be precise since I could view it from the side (I have the clear, not the metal version). Standard laboratory volume measurements are always taken from the side since water forms a U-shape in a vessel as it adheres to the glass or plastic walls. Scientifically, this shape is called the meniscus and it is accurately measured at the bottom point of this U-shape. When I did the experiment looking solely from the top down, I was short by 11.7% for the OXO measuring cup 1/2 oz pour which is consistent with trying to eyeball a clear fluid in the metal OXO Double Jigger.
While these internal measurements in the OXO Double Jigger were not as accurate as regular jiggers, they were close and very handy. More accurate results might be obtained with darker colored liquids (perhaps less tricked in top-down viewing) as well as with increased frequency of use to acclimate to the jigger and compensate for its shortcomings. Overall, not a bad piece of barware, and inaccuracies of 5-10% per measure might not in the end greatly effect the balance of a drink.