The dryer operates on R-134a, so the blue temps are the ones you'd read. The temps are in °C rather than °F which makes it a bit confusing.
The hot gas bypass (hgbp) puts a load on the system to keep it from freezing when there's no air flow or less flow than is required to keep the suction pressure in the desired range. When you adjust the hgbp you're adjusting the load on a spring. As the suction pressure approaches the lower end of operating range, the hgbp will begin to open to keep the pressure up. The hgbp modulates to maintain the pressure rather than being a simple open or closed device. Works similar to the thermostat in an engine where it attempts to maintain just enough of an opening to hold a steady temp rather than just flying open or slamming shut.
You would adjust the hgbp with the dryer running but with no compressed air going thru it. Make small adjustments and wait a couple minutes for the pressure to stabilize before making another adjustment. HF's specs call for 32-39 psi which would give you an evaporator temp ranging from 35°F to 46°F. I'd shoot for the middle of that range for the hgbp setting.
In the pic above with the dryer running, the evaporator temp is about 10°C or 50°F. If there's no air flowing, the hgbp should be adjusted to bring the pressure down into the correct range. If air is flowing, the pressure could indicate the unit was just turned on and hasn't reached steady state operating conditions, or the hgbp could be out of adjustment, or the dryer could be overloaded by too much air flow or too high an entering air temp.
