Pressure Variation on Hydrotesting

What pressure difference?

It was ~20-22 PSI pressure rise in 20 mins at 67 F metal temperarure.

And pressure was rising.( Initial pressure was 450PSI)

Sorry, misread your post.

I'd look at thermal expansion of the test fluid vs the metal plumbing.

It likely has a higher thremal expansion rate than the metal and could account for the pressure difference.

The fact that the pressure is holding or rising with temperature and that there is no leak indicates a successful test.

Presure also drops at lower temperature..

Look at the thermal expansion profile of your test fluid and compare that to expansion rate of the piping.

Fluids shrink when cooled and expand when heated.

This isn't rocket sceinec.

Almost certainly is due to thermal expansion. I am guessing your volumes are quite small if you are using oil? Mineral Oil expands roughly 1% for 13 Deg C. If you know your volumes you can calculate. As long as you stay above test pressure for the specified period your test is good.

How long are you testing for? Some tests (B31.4, 24 hours tests) will allow you to bleed off pressure if it rises too much during the day, as long as you record the volume bled off and record the temperature.