Despite proper use, different results

Have the antifreeze tester used in my C-Class for measuring the content of antifreeze my cooling water, on different days. Each received a different result. Have the unit properly used, ie:
- Held device only vertical, although it can be in any position Fairs
- Move the pointer to leave her alone leveling off
- Several measurements carried out in succession by the same cooling water and also here different results
- After using the device, it neatly flushed with clear water
- After cleaning the appliance, leave this alone and drying on the second day re-measurements are performed and again different results
Every time I had been a different result from -15 to -35, and more. Can I definitely do not know, although it is so highly praised by the car magazines.