Di vessels ppm reading

[gwindows]

I use two vessel system and the first one is only reading 4  but the second in l one keeps coming out at 1/2  .Is there anything inside them that can go wrong and needs to be replaced 

 

[Nudel]

Try a larger sample size, fill a clean 10 litre bucket and measure in that. 

When I measure from a cup I sometimes get 1ppm reading, but measured in tank or 25l barrel it's always 0.