The voltage for anything should match the supply voltage.
Answer: Hair dryers are made not to damage your hair because they do not get that hot. You should sit under the dryer until you feel your hair is dry depending on how long or short your hair is you have to make that decision yourself. So don't worry about how long you have to sit under it. It will be safe enough.
Hair dryers typically reach temperatures of 140-170 degrees Fahrenheit when set on high heat.
If the voltage is completely unknown, a voltmeter should be set to the maximum range first, then stepped down to an accurate level.
Make a guess of what the voltage will be and set the range accordingly. If needed, change the range after measuring the voltage, to get a better reading.
Are you referring to a meter?.... if so, set it to the closest higher voltage on the meter (usually 600VAC but sometimes 300VAC depending on the brand and model)
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Paul Mitchell make an amazing hair care set for damaged hair. In fact, it makes a number of them. First you need to identify why your hair is brittle. If it is because of dying your hair a lot then you should get the hair care set for people with colored hair. If your hair is just naturally that way then get the intensive moisture set.
You must set the range so that the maximum value is greater than the conventional voltage levels in your house. Like, in the United States, the common voltage level is 120 volts RMS, although many homes are equipped with 240-volt service, to accommodate certain appliances, like clothes dryers. (The 240-volt service is really two 120-volt legs.) It's a good idea to set the range higher than expected initially.
A few are the Super Solano, the BaBylissPro, Vidal Sassoon, Revlon, Rusk CTC, Bespoke Lab, and Conair. Another set of hair dryers include the Chi, the T3, as well as the STNY (the Salon Tech), Sedu and finally there is the Solia.
With an unknown voltage you should start out with the highest voltage setting on the meter. Doing this reduces the risk of "pegging" out the needle on an analog gauge or overloading the input of a DMM. Most DMM's are auto-ranging though.
While choosing your hair, there should be a padlock icon at the top-right. If it's locked, then your chosen hair will be set for all clothing. If you click on that, it should unlock, so you can individually set hairstyles for different outfits. You have to go back and forth the clothing section and the hairstyle section if you want to set hairstyles individually, though.