Resurrected NixiSat GPS clock...

[Nick]

Whats the life of the Nixies? If finite then maybe at the almost irreplaceable cost you shouldn't run it 24/7 ?

Later nixies with mercury doping have a life of about 200,000 hours (nearly 23 years continuous) if driven correctly - overdriving a nixie exponentially shortens its life... without Hg, tubes typically last about 5,000 hours.

The concept of "life" with respect to nixies is a complex one - the generally accepted "end-of-life" is when one of the following occurs:

• Emitted light falls to 50% of its peak value (caused by silvering on the inside of the envelope)
  
• "Cathode poisoning" results in significant sections of digits no longer illuminating
  
• Erosion of cathode material resulting in incomplete digits
  
• Mechanical failure causes the device to cease functioning normally
  

The first item is largely dealt with by the use of Hg doping - that also helps the second issue.
The second issue can be reversed by driving the digit at several times its normal current for an hour or so to restore the surface of the cathode.
The second & third issues can be avoided by cycling the digits in a tube on a regular basis (typically at night) with a higher-than-usual current. In addition, physically/mechanically rotating the tubes from right to left every once in a while so that those digits which are over-used, e.g. the tens of hours "1" & "2" and digits "0" through "5" generally, are more evenly used.
The last point is covered by just being careful!

The Hg doping has a side effect - a purple haze around the digits - the red filtering on some nixies removes the purple resulting in a clearer display. Clear nixies are exactly the same as those with the red coating but are designed to operate behind a flat red filter, e.g. in test equipment...