BC, the key to substitution is to PRACTICE!. Then you know what the temp of the sample will be at any given time under any given condition. Get the 2oz bottle, fill it with warm water, crotch it and start practicing. Use a fever thermometer to check the temp. You'll find it works as advertized, without and additional heating source. Just body heat, but the practice will give you confidence and knowledge. Hope that covers it, but if you have more questions, just ask. Oh, and read the substitution guidelines, if you haven't, linked to in my signature. N2