Fixes mistake in scrubbing calculation

code/ATMOSPHERICS/atmospherics_helpers.dm

@@ -1,7 +1,14 @@
+/*
+	Atmos processes
+	
+	These procs generalize various processes used by atmos machinery, such as pumping, filtering, or scrubbing gas, allowing them to be reused elsewhere.
+*/
+
+
 //Generalized gas pumping proc.
 //Moves gas from one gas_mixture to another and returns the amount of power needed (assuming 1 second), or -1 if no gas was pumped.
 //transfer_moles - Limits the amount of moles to transfer. The actual amount of gas moved may also be limited by available_power, if given.
-//available_power - the maximum amount of power that may be used when moving gas. If null then the transfer is not limited by power, however power will still be used!
+//available_power - the maximum amount of power that may be used when moving gas. If null then the transfer is not limited by power.
 
 /obj/machinery/atmospherics/var/last_flow_rate = 0	//Can't return multiple values, unfortunately...
 
@@ -17,7 +24,7 @@
 	if (available_power && specific_power > 0)
 		transfer_moles = min(transfer_moles, available_power / specific_power)
 	
-	if (transfer_moles < MINUMUM_MOLES_TO_PUMP)
+	if (transfer_moles < MINUMUM_MOLES_TO_PUMP) //we check this repeatedly so that we do as little processing as possible
 		return -1
 	
 	last_flow_rate = (transfer_moles/source.total_moles)*source.volume //group_multiplier gets divided out here
@@ -34,33 +41,36 @@
 	
 	return power_draw
 
-//Generalized gas filtering proc.
-//Filters the gasses specified by filtering from one gas_mixture to another and returns the amount of power needed (assuming 1 second), or -1 if no gas was filtered.
-//filtering - A list of gasids to be filtered from source
-//total_transfer_moles - Limits the amount of moles to filter. The actual amount of gas filtered may also be limited by available_power, if given.
-//available_power - the maximum amount of power that may be used when filtering gas. If null then the filtering is not limited by power, however power will still be used!
-/obj/machinery/atmospherics/proc/filter_gas(var/list/filtering, var/datum/gas_mixture/source, var/datum/gas_mixture/sink, var/total_transfer_moles = null, var/available_power = null)
+//Generalized gas scrubbing proc.
+//Selectively moves specified gasses one gas_mixture to another and returns the amount of power needed (assuming 1 second), or -1 if no gas was filtered.
+//filtering - A list of gasids to be scrubbed from source
+//total_transfer_moles - Limits the amount of moles to scrub. The actual amount of gas scrubbed may also be limited by available_power, if given.
+//available_power - the maximum amount of power that may be used when scrubbing gas. If null then the scrubbing is not limited by power.
+/obj/machinery/atmospherics/proc/scrub_gas(var/list/filtering, var/datum/gas_mixture/source, var/datum/gas_mixture/sink, var/total_transfer_moles = null, var/available_power = null)
 	if (source.total_moles < MINUMUM_MOLES_TO_FILTER)
 		return -1
 
 	filtering &= source.gas		//only filter gasses that are actually there.
 	
-	//Filter it
-	var/total_specific_power = 0		//the power required to remove one mole of filterable gas
+	//Determine the specific power of each filterable gas type, and the total amount of filterable gas
 	var/total_filterable_moles = 0
-	var/list/specific_power_gas = list()
+	var/list/specific_power_gas = list()	//the power required to remove one mole of pure gas, for each gas type
 	for (var/g in filtering)
 		if (source.gas[g] < MINUMUM_MOLES_TO_FILTER)
 			continue
 	
 		var/specific_power = calculate_specific_power_gas(g, source, sink)/ATMOS_FILTER_EFFICIENCY
 		specific_power_gas[g] = specific_power
-		total_specific_power += specific_power
 		total_filterable_moles += source.gas[g]
 	
 	if (total_filterable_moles < MINUMUM_MOLES_TO_FILTER)
 		return -1
 	
+	var/total_specific_power = 0		//the power required to remove one mole of filterable gas
+	for (var/g in filtering)
+		var/ratio = source.gas[g]/total_filterable_moles //converts the specific power per mole of pure gas to specific power per mole of filterable gas mix
+		total_specific_power = specific_power_gas[g]*ratio
+	
 	//Figure out how much of each gas to filter
 	if (!total_transfer_moles)
 		total_transfer_moles = total_filterable_moles
@@ -71,7 +81,7 @@
 	if (available_power && total_specific_power > 0)
 		total_transfer_moles = min(total_transfer_moles, available_power/total_specific_power)
 	
-	if (total_transfer_moles < MINUMUM_MOLES_TO_FILTER)
+	if (total_transfer_moles < MINUMUM_MOLES_TO_FILTER) //we check this repeatedly so that we do as little processing as possible
 		return -1
 	
 	var/power_draw = 0

code/ATMOSPHERICS/components/unary/vent_scrubber.dm

@@ -128,7 +128,7 @@
 		//limit flow rate from turfs
 		var/transfer_moles = min(environment.total_moles, environment.total_moles*MAX_FILTER_FLOWRATE/environment.volume)	//group_multiplier gets divided out here
 		
-		power_draw = filter_gas(scrubbing_gas, environment, air_contents, transfer_moles, active_power_usage)
+		power_draw = scrub_gas(scrubbing_gas, environment, air_contents, transfer_moles, active_power_usage)
 	else //Just siphon all air
 		//limit flow rate from turfs
 		var/transfer_moles = min(environment.total_moles, environment.total_moles*MAX_SIPHON_FLOWRATE/environment.volume)	//group_multiplier gets divided out here

code/ZAS/_gas_mixture_xgm.dm

@@ -119,8 +119,7 @@
 		. += ratio * specific_entropy_gas(g)
 	. /= total_moles
 
-//Returns the ideal gas specific entropy of a specific gas in the mix. This is the entropy per mole of /pure/ gas.
-//It's important not to get that mixed up with the mixed entropy, which takes into account mole ratios (I did, it was bad).
+//Returns the ideal gas specific entropy of a specific gas in the mix. This is the entropy due to that gas per mole of /that/ gas in the mixture, not the entropy due to that gas per mole of gas mixture.
 /datum/gas_mixture/proc/specific_entropy_gas(var/gasid)
 	if (!(gasid in gas) || gas[gasid] == 0)
 		return SPECIFIC_ENTROPY_VACUUM	//that gas isn't here