I think the one Tracy sells is as good and inexpensive as they come.  All of the ones that I am familiar with use the same basic design (except for the latest ones that use the new (and expensive) so-called broad band sensor).  I personally like the ones that use the 3/4 wire sensor (these have their own heating element) as you do not have to wait for the exhaust gas to warm up the sensor before it start working.  They are a bit more expensive that the 1/2 wire sensor may $10-20 more.
 
I built my own LED A/F indicator  and integrated it with my fuel monitoring system.  But regardless of whether you build or buy, they are well worth having, in fact I would rather lose my EGTs than my LED A/F indicator.
 
Ed

    ----- Original Message -----
    *From:* Bulent Aliev <mailto:atlasyts@bellsouth.net>
    *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
    *Sent:* Monday, December 06, 2004 3:18 PM
    *Subject:* [FlyRotary] Re: Nice to Have Re: ECU oxygen sensor

    On 12/6/04 2:20 PM, "Ed Anderson" <eanderson@carolina.rr.com
    <mailto:eanderson@carolina.rr.com>> wrote:

        I certainly would not draw that conclusion, Bulent.  I have
        found theLED  A/F indictors such a good indication of what my
        engine is doing that I would hesitate to fly without it now.
         If the engine starts to stumble you immediately know whether
        it because its too much or too little fuel - sort of nice to
        know, trust me. {:>)

        Ed Anderson

    Thanks Ed,
    By any chance can you recommend one A/F indicator?. I have
    installed the single wire O2 sensor that came with the engine. I
    may even replace it?
    Bulent