Then you have the heated diode type detectors. They are less prone to false alarms from moisture and shop chemicals. More importantly, heated diode leak detectors are also much more sensitive to R-134a. Other leak detectors are suitable for finding R-12 and R-22 leaks, but we've found that leaks in R-134a systems are best located using the heated diode detectors.  In fact, right after R-134a came out, most of our electronic leak detectors became obsolete.  Leak detectors which once worked well with R-12 and R-22 just weren't doing the job.  It's not that electronic leak detectors don't work with R-134a, it's just most don't work well enough.

Here's our recommendation. Look for a leak detector that uses a heated sensor.
Years ago there were few models that incorporated the heated diode.  Today, there are many heated sensor units to choose from, and most are very economical to own.  When researching which detector to buy, consider sensor life expectancy, and unit sensitivity during comparison .  These specifications should be mentioned in sales literature. 

Other factors to consider when choosing a leak detector:

• sensor replacement cost

• warm-up time

• power supply.