Last spring some friends wasted hours trying to find leaks with two kinds of electronic sniffers. One was a Yorkogawa H-10 series, and the other was a Robinair.
The flashing lights and the audible alarm would sometimes work and sometimes not.
Bought a calibration bottle from Johnstone Supply. Bacharach (they took over from Yokogawa) part number 3015-0864. Basically it is a one ounce glass bottle of R-11(???). The bottle lid is glued on (human nature-- you always want to unscrew a bottle)-- with a very tiny hole is the lid's top. The hole gives a leak equal to 1/2 ounce per year of R-134a.
Retail cost is $35. And you use it every time you leak test. If your leak detector doesn't alarm-- then first step is to change the filter. If it still doesn't work, then you might have to change the sensor. At the worst case you send it back to the factory (or just buy a new one).
You might be able to get a cheaper cal bottle somewhere else.
Here are some other manufacturer's part numbers:
Bacharach "reference bottle" Part No. 3015-1416 or 3015-0864.
Dwyer "14 grm/year calibration vial" Part No. S0143.
Robinair "leak standard" Part No. 16705.
TIF "reference leak standard" Part No. TIF5201.
There are a lot of side benefits. Many of the 'modern' sniffers zero out 'background' leaks after 5-10 seconds (with the calibration bottle-- you will know what your tester does); very old sniffers aren't sensitive enough for small R-134a leaks.
"Best electronic sniffers" :
D-TEK and D-TEK SELECT by Inficon.
H-10 series by Bacharach/Yokogawa/Uniweld/General Electric/Westinghouse.