Thanks to everyone who responded.  The summary is:
   Figaro makes sensors.
   Buy one, it will be tested and certified.
   Buy a canary:)

Actually a year or so ago electronic ones were close to $100 and the cardboard
 ones were $5 and wouldn't wake up a ....canary.

Just got back from Target, they had a Nighthawk, battery backup, continuous
 digital read out, no replacement sensor required for $55.

The only thing that changes faster than the price of electronics is the stock
 market.

Thanks all

Steve