Fixed Orifice, pistons and Capillary tubes

Fixed orifice is a fixed hole that cannot change in size.
This is often referred to as a piston with an orifice size and in other cases its a capillary tube where the length and size of the tube restricts the flow of refrigerant.

These metering devices cannot adjust to changing conditions.
When the outdoor temperature rises, the head pressure also rises. We now have more pressure pushing against the liquid refrigerant and pushing more refrigerant through the fixed hole into the evaporator. We will need the temperature of the air entering the condensing coil.
The indoor air temperature and indoor humidity affects how fast the refrigerant in the evaporator coil boils to a vapor. By collecting the return air wet bulb temperature, it will include the effects of sensible air temperature and latent humidity temperature of the air affecting the refrigerant in the evaporator coil.

By taking the outdoor air entering condensing coil and the indoor return air wet bulb temperature we can find the target superheat for the indoor coil. 

We can enter those numbers into a formula (     ) a chart ( see below), a calculator (ac service tech) or an app ( hvac school app/Measurequick) to find the target superheat, or what the superheat should be.  One day it could be as low as 0 flooding the compressor and another day it could be as high as 35 overheating the compressor so it's less than ideal but many techs like it because it does not break, ignoring the damage it can cause to the compressor.

Copy of Target Superheat new1.xlsx - Sheet1(1).pdf