In my opinion, needle valves in this application are a guaranteed migraine. We ditched them 20 years ago and went to fixed orifice plates. Just like a 2505 spray
nozzle will flow .05 gpm at 40 psi, so will a fixed orifice plate. No guess work, no swollen valve seats etc etc. Set a regulator to a pressure that you can maintain, we use 40 psi. Then decide how many oz/min you need to weep to not freeze. We weep hot water and therefore can use less the 16-20 oz/min that 2biz has settled on. We have found that 12 oz/min works for us, but like I said, we are weeping 110-115 F water. Yes there is an expense to heat the water as well as soften it, but, we can use less of it, it keeps our trough toasty and it doesn't freeze on the bay floor. Just my two cents.
These are the parts we use for the fixed orfice....
you need a teejet body, an orfice plate with the correct size hole (the hole size is in .000" and I believe we are using a 27 or .027"), a strainer if you want and a female adapter cap.
https://www.kleen-ritecorp.com/p-18237-spraying-systems-cp1322-14-teejet-body-brass-male.aspx
https://www.kleen-ritecorp.com/p-18...4916-39-stainless-steel-orifice-plate-39.aspx
https://www.kleen-ritecorp.com/p-18...53-100-ss-teejet-strainer-brass-100-mesh.aspx
They make this next part in brass but I couldn't find it in
Kleen-Rite's catalog
https://www.kleen-ritecorp.com/p-31...female-adapter-for-diaphragm-check-valve.aspx
You can use this website to calculate flows. Our primary PSI is 40 and our orifice size is 27 or 28. When you plug in 40 psi and .027 into the calculator you'll get a flow of .0961468 gpm or 12.3 oz/min. Dultmeier is actually where we ordered the plates years ago.
https://www.tlv.com/global/US/calculator/water-flow-rate-through-orifice.html