Most warmer packs by themselves take approximately 1 hour to heat the sample bottle to 98-100 degrees. For this reason, your sub should be preheated prior to activating the warmer pack. A few ways come to mind:

a) Microwave - If youre using synthetic urine as a sub, stick it in the mic for 10 sec. For high-wattage mics, you may have to cut the heating time down by half or more. If youre using human urine, microwaving is not recommended because human urine is a biohazard and should not be in an area where food and utensils are involved.

b) Hot water - Place the sub container in a ziplock baggie and submerge it in a sink filled with hot water. Typically takes more time than a microwave, but its good for both synthetic and human urine subs.

c) Vehicle heater - Place the heater control switch to "floor heat" mode and use the floor ducts to heat the sample to proper temp.

These are some of the methods that Ive personally utilized. Other posters can chime in on other methods they have used.