Steripen - clean water, but fairly temperamental

I've been using a Steripen Classic for all my water purification needs when out bush, but it has proven to be pretty unreliable.  Firstly it seems to be very particular about batteries: if you didn't charge your NiMh batteries within the past few days they probably won't work.  It is also very picky about how long you hold the button down for: 2 or 3 seconds seems to work best for me.  If you give the button a few goes and nothing happens, the FAQ says you should wait 15 seconds, which works most of the time:

Pressing the button too quickly may result in an unrecognized electronic signal. Wait 15 seconds to allow the SteriPEN to reset and then start again. Push the operating button slowly (hold for 1/2 second) and release. If selecting the 0.5 L dose, pause for a half a second before the second push of the button. If this doesn’t work, please check the batteries.

 I was also getting a problem where the UV light would switch off mid-dose and go to steady red, which means the Steripen thinks it isn't in water any more.  Ironically this happens when the water is too pure:

SteriPEN water sensors look for electrical conductivity to indicate the presence of water. In most cases water has enough trace mineral content to allow for a small amount of electrical current. On occasion though, very pure water may be encountered – most often in areas where water is from recent snow melt. Since very pure water is a poor electrical conductor, the sensors may only function marginally resulting in an incomplete UV dose. This can easily be corrected by adding a small pinch of salt, or a drop or two of some kind of electrolyte beverage to the water.

 Turns out the Canberra tap water I was testing with was pure enough to fool it - adding some salt made it work reliably.  Apparently newer models have been improved (and there is a new model with an optical water sensor):

Since late 2008, all SteriPEN models have had their water sensors enhanced by doubling the sensor electrode voltage, thereby increasing electrical current flow in water with low conductivity to allow proper function in snowmelt and even mineral-free distilled water

 Despite all these issues, I think it is still the best bet for purification (combined with some tablets for backup).