Trying to get myself to wake up from alarms on the CGMS at night drives me nuts. Last night I went to bed with a blood sugar of 75. I ate a small snack at bedtime, and I raised the low alarm threshhold to 80, with a snooze of one hour, thinking I could wake up in an hour if I was still below 80, and treat again, but not wanting to overtreat.
I woke up around 7 AM. It was beeping. It said I was 67, and my meter said 61. The Dexcom showed my blood sugar as having been in the 50-75 range THE ENTIRE NIGHT. Did it wake me up? Nooooo. The Guardian wasn't very good at it either. I know I should get some sort of amplifier. Blech.

Fact:
1 international unit of insulin is equivalent of 1/22 mg of pure cyrstaline insulin. In the United States, you can buy insulin at a strength of 100 units in one mL (U-100 insulin) or at a strength of 500 units in one mL (U-500) insulin. You can also buy a dilutant to be used with some insulins and make insulin at any lower concentration.
Historically, U-30, U-40 and U-80 insulin were prevalent on the US market.
Using a higher concentration helps with absorption of larger doses (U-500 is recommended for those with a sustained need for more than 200 units per day), and a lower concentration helps with measuring lower doses (particularly for those who frequently need doses under 1/2 unit), but having different concentrations has sometimes caused dangerous mix-ups.
Historically the lower concentrations were due to the difficulty of concentrating the insulin.