Metals like brass, constantan etc. can't be made directly in a Tinker's forge so they have be melted in separate melters and then mixed in a mixer. I'm doing it like this:

Nuggets / dusts / ingots of the origin metals go into chests, which are hoppered into melters. Copper pump and pipes then move the two molten metals into a basin where they are mixed by a mixer, the basin then dumps the molten alloy result into another basin.

I've checked the recipes and for e.g. brass, the input amounts are the same - 16 mb of copper and zinc produces 16 mb brass (feel like it should be 32 but anyway). But at some point, bits of molten fluid go missing. Inputting the same amount of solid metals into the hoppers produces a random loss of either one of the molten metals at some point, e.g.:

At first I thought it was just me mistakenly putting in uneven amounts of copper and zinc. But the giveaway is that I am often left with 8mb of one of the two fluids - that shouldn't be possible unless random bits of fluid are being lost because one nugget is 16mb and dusts and ingots are a whole number of nuggets.

I feel like the issue is with copper pumps and pipes, that something is being destroyed under some condition, but I can't figure it out the exact reproduction.

I also get this issue. Due to 16mB per tick of the crafting recipe, the first craft using 1 bucket each leaves 8mB in the output (this issue would be avoided by doing the crafts in 8mB increments instead, for 125 crafts per bucket vs 62.5 crafts per bucket). Pouring two more buckets in the drains left 8mB left over again, but also only 960 mB in the spout output:

EDIT: for clarity:

1. Poured 1 bucket molten zinc 1 bucket molten copper into basin via item drains
2. Result: 984 mB molten brass, 8mb molten zinc, 8mb molten copper. (This is working as intended, due to the 62.5 crafts per bucket)
3. Poured 1 bucket molten zinc 1 bucket molten copper into basin via item drains
4. Spout poured 1 bucket of molten brass at this point into bucket on depot. Mixing continued.
5. Result: 960 mB molten brass, 8mb molten zinc, 8mb molten copper.

2000mB/16mB = 125 crafts, so this should have been a lossless conversion at the end of the operation.

In creative mode now, singleplayer, for better testing: at 256 rpm this is the end result of 2000mB zinc + copper in tanks being pumped into the basin: clutch and lever ensures both pumps engage simulataneously.

This is the result of 128 RPM, same conditions:

The issue is almost certainly in the pumps or the tanks. Only 8mB of zinc and 8mB of copper was lost in this trial. My hypothesis is that as there is 1000mB of each liquid in the basin, 992 processed in the output tank, and 8mB left in the input tanks, the 8mB gets deleted somehow, via rounding in the tank or getting shoved into a basin that didn't have free space.

More fluid is lost at faster pumping speeds, so it is probably something about the pumps.

After separating the pumps and mixer, running the pumps at 1 RPM and the mixer at 256, no liquid was lost, seeming to confirm that fast pumps will delete fluid, probably via pushing more fluid than can actually fit into a basin.

After separating the pumps and mixer, running the pumps at 1 RPM and the mixer at 256, no liquid was lost, seeming to confirm that fast pumps will delete fluid, probably via pushing more fluid than can actually fit into a basin.

That fits with what I experienced - my pumps were running at at least 192 RPM, probably 256. Using seared channels to fill the basin, nothing is ever lost.

Probably never noticed this in "vanilla Create" because I don't recall there being many fluid mixing recipes, or if there are, mostly likely only highly replenishable stuff like water where you wouldn't notice the loss.

That fits with what I experienced - my pumps were running at at least 192 RPM, probably 256. Using seared channels to fill the basin, nothing is ever lost.

Probably never noticed this in "vanilla Create" because I don't recall there being many fluid mixing recipes, or if
there are, mostly likely only highly replenishable stuff like water where you wouldn't notice the loss.

Yea, seems about right. As a workaround I made a completely passive fluid flow setup, with seared tanks, lever enabled faucets and channels, with the mixing basin draining into a secondary basin, and that basin draining into an ingot tank via a seared faucet: no need for the pump and filter pipe output.

PS. Congratulations on an exceptional modpack. I particularly appreciate the reskinned machines/pipes to match Create style.

Follow-up later: using this alternative set-up which reduces the use of copper pipes and pumps to a minimum (melter > seared faucet > seared channel > mixing basin > smart pipe > seared drain > smeltery), I'm not losing anything at all.

I can confirm, the same thing is happening on my end with copper pipes and pumps, some 8mb are left on either metal, which shouldn't be possible

I've managed to replicate the issue with only Create stuff, no TiC involved. Seems easy to replicate if you keep pumping one input while the other runs out, seems to always happen. If you pump both inputs at the same pace, the problem doesn't seem to happen.