A full loop takes glycol by displacement, so the answer is two numbers: what to drain and what to add. Freeze point and burst point both read off the manufacturer's curve for the fluid in the drum.
Model 05 · Open to everyone
Displacement300.0 galOut of the pail, in from the drum
Blend0.0% to 30.0%Loop tint follows the target
Freeze point8 °FFrom 32 °F today
Drain, then addDose required
Diagram controlsInspect a live quantity
Supporting readoutsDose, protection, and the loopOpen
Drain first300.0gal
Freeze today32°F
Percent needed30.0%
FluidPropylene
Operating curveFreeze point versus percent30.0% target
Freeze point vs percent
Drag the plot to set the target. The dashed curve is the fluid you did not pick, and the gap between them is what the food safe choice costs in gallons.
Reference
How the glycol dosage model works
A loop that is already full has nowhere to put more fluid, so glycol goes in by displacement: drain some of what is in there, replace it with the same volume of concentrate, and the percentage climbs. Topping up without draining lands short of the target every time, because the fluid leaving carries glycol out with it.
Two temperatures come back, and they are not the same promise. The freeze point is where slush starts. The burst point, ten to fifteen degrees colder, is where the slush finally expands hard enough to split a pipe. Design to the freeze point for a system that has to keep running, and to the burst point for one that only has to be intact in the spring.
The instrument above runs the dosing math live. The governing relationships are written out below.
The gallons drained out of a full loop and replaced with concentrate to reach the target. The denominator is the gap between the drum and the loop, not the gap to one hundred, because premix is weaker than neat glycol and a loop being topped up carries glycol out with the fluid it loses.
Volume
Loop volume (gal)
Target
Glycol target, by volume (%)
Current
Glycol in the loop today (%)
Concentrate
Strength of the drum going in, neat or premix (%)
Assumes
Drain-and-replace displacement, not addition into an expanding volume.
A concentrate weaker than the target can never reach it.
Freeze point
Freeze=ffluid(Target)
Interpolated from the fluid's published freezing point table, not from a fitted equation. Propylene and ethylene run separate curves, so the fluid in the drum changes the answer at the same percentage. The burst point, which is colder, comes off the same manufacturer's second table.
Target
Glycol in the blend, by volume (%)
Freeze
Where slush begins to form (°F)
Assumes
Propylene: Dow DOWFROST technical data sheet, Form 180-01314-1019.
Ethylene: Dow DOWTHERM SR-1 technical data sheet, Form 180-01312-0417.
Freeze tables keyed on volume percent glycol, which is what a refractometer reads.
Worked example
Take the room's default case: a 1,000 gallon loop with no glycol in it, a 30 percent propylene target, and a drum of neat concentrate.
Drain, then add back
300.0 gallons
Percent needed
30.0%
Freeze point at target
8°F
Burst point at target
−13°F
Switch the fluid to ethylene and watch the same target buy several more degrees. That gap is the price of the food safe choice, and it is paid in gallons.
Common questions
How much glycol do I need for a closed loop?
Drain and add volume times the target percent minus the current percent, divided by the concentrate strength minus the current percent. With neat glycol that denominator is one hundred minus the current percent. With a premix it is the premix strength minus the current percent, which is why premix takes more.
What is the difference between freeze point and burst point?
The freeze point is where ice crystals start to form and the fluid turns to slush. The burst point is colder, and it is the temperature below which the slush expands enough to rupture pipe. Between the two the loop is stiff but intact.
Propylene or ethylene glycol?
Propylene is the low toxicity choice and the default anywhere the fluid could reach potable water or food. Ethylene protects better at the same percentage and transfers heat better, so it survives in industrial loops where contact is not a concern. At 30 percent the difference is several degrees.
Where do the freeze point numbers come from?
The manufacturer's published tables, interpolated rather than fitted: Dow's DOWFROST technical data sheet for propylene and DOWTHERM SR-1 for ethylene. The freeze tables are keyed on volume percent glycol, which is what a refractometer reads. Dow states the burst tables as percent of the inhibited fluid, which runs a point or two higher.