Trouble reading Nitrate tester

[CaysE]

I have the API nitrate tester. My problem is, if I hold the test tube vertically up against the card, it looks yellow (0ppm nitrate). If I hold it horizontally, it looks a little orange (5ppm). And if I take off the cap and look straight down, it's even more (10ppm).

What's the right way to read it?

[zacusmaximus]

As long as it is like the API kits I know and love, just hold it over the blank white area of the card in a well lit room.

[CaysE]

That's exactly what I'm doing. But what's the proper orientation to hold the tube against the white card? I get three different readings depending on which way the tube is oriented against the white background.

try up and down.

[zacusmaximus]

I've always held it with the length of the card. The only other test I've ever seen done differently is copper.