Establish the residual gap first, then open the drum label when it matters. The loop stays visible while the dose answers the same known volume.
Model 06 · Open to everyone
Product to add1.8 galPail fills against a 55 gallon drum
Residual path200 to 1,000Gauge shows the gap to close
Active nitrite16.7 lbBefore the drum's density
One dose, two labelsDose required
Diagram controlsInspect a live quantity
Supporting readoutsThe dose and what it stands onOpen
Residual path200 → 1,000ppm
Product activity40%
Product density9.5lb/gal
Freeze protectionGlycol room
Reference
How the closed loop model works
A closed loop barely loses water, so treatment is a matter of holding a concentration rather than chasing a rate: a nitrite residual high enough to keep the steel passive, held against a volume that never changes. The dose is a mass balance against that one number.
The answer comes back in gallons, not pounds, because gallons is what gets pumped and what gets ordered. Getting there takes two numbers off the drum label rather than one: the percent active, and the density. Treating an inhibitor as though it weighed what water weighs underdoses a typical product by ten percent or more.
The instrument above runs the dosing math live. The governing relationship is written out below.
Gallons of inhibitor product to raise the residual to target: a mass balance against the loop volume, grossed up for the product's active fraction, then divided by the product's density so the answer reads in the unit that is actually on the truck.
Volume
Loop volume (gal)
ΔNitrite
Nitrite gap to close (target − current) as NaNO2 (ppm)
Activity
Nitrite product active content (%)
Density
Product density off the label (lb/gal)
Assumes
Nitrite reported as NaNO2.
8.34 lb per gallon of water.
Density from the product data sheet, not assumed equal to water.
Worked example
Take the room's default case: a 1,000 gallon loop, a nitrite residual of 200 ppm against a 1,000 ppm target, a 40 percent active product, and a density of 9.5 lb per gallon.
Active nitrite required
16.7 lb
Product to add
1.8 gallons
Change the loop volume in the instrument and watch the dose move with it. In a closed system the volume is the number every other number stands on.
Common questions
How is a nitrite dose calculated?
A mass balance against the loop volume: the residual rise in ppm converts to pounds through 8.34 lb per gallon of water, then grosses up for the product's active fraction, then divides by the product's density to land in gallons. Nitrite is reported as sodium nitrite.
Why does the loop volume matter so much?
Every dose in a closed system is volume times concentration. If the volume is wrong, the residual lands wrong with it, which is why the system volume room exists.
What does a falling nitrite residual mean?
Nothing leaves a tight loop, so what you add stays until something is wrong. A residual that keeps dropping is a leak detector wearing a chemistry disguise, and the makeup meter usually confirms it.
Where did the glycol calculation go?
To its own room. Freeze protection and corrosion protection are different jobs with different inputs, and the glycol math needed a fluid selector and a concentrate strength this room had no business carrying.