What Are My Cut In/Out Pressures? – South-Tek Systems

Applies to: South-Tek nitrogen generators across the beverage, fire protection and industrial lines. Values below are factory defaults — the unit in front of you is the authority.

A nitrogen generator does not run continuously. It makes nitrogen into a storage tank, stops when the tank is full, and starts again when the tank draws down. Two pressures decide that:

Cut-out — the tank rises to this pressure, the generator stops producing and goes into standby.
Cut-in — the tank falls back to this pressure and the generator starts producing again.

The gap between them is the differential. A resting gauge reading anywhere between cut-in and cut-out is normal, and a generator cycling between the two is a generator working correctly.

⚠ Warning — these are not your sprinkler system's pressures Every number on this page is generator pressure — read at the generator or its nitrogen storage tank, upstream of the nitrogen outlet valve. None of them is the fire protection system's supervisory pressure, which is read at the dry pipe or pre-action valve gauge and is set by the sprinkler system design, not by the generator.

Do not set an air maintenance device, a low-air supervisory switch, or any riser setpoint from this page, and do not judge a riser gauge against it. A generator sitting at 100 PSIG cut-out feeding a system supervised at 40 PSIG is entirely normal.

Fire protection — N2-BLAST

Model	Nitrogen cut-in	Nitrogen cut-out
FPS-500, FPS-900	60 PSIG	70 PSIG nominal (65–75)
FPS-1650	60 PSIG	70 PSIG
FPS-3250, FPS-5000	60 PSIG	100 PSIG
FPS-10000, FPS-16500, FPS-22500	60 PSIG	100 PSIG
FPS-650, FPS-1250 (legacy)	About 62 PSIG	70 PSIG nominal (65–75)
FPS-1750, FPS-3000, FPS-6000 (legacy, Type II)	About 72 PSIG	About 80 PSIG
FPS-15000, FPS-20000 (legacy)	Factory set per unit	Factory set per unit

ℹ Note — on the FPS-500 and FPS-900, judge it by watching a cycle Cut-out is 70 PSIG nominal with a working band of 65–75 PSIG, and cut-in sits 7–10 PSIG below whatever the cut-out actually is. Expect 60–70 PSIG on the front gauge at rest.

Record the pressure it enters standby at and the pressure it restarts at rather than reading it once. A cut-out outside 65–75 PSIG, or a differential outside 7–10 PSIG, points at the pressure switch — but a single static reading diagnoses nothing. If it reads low, check power first, then whether the sprinkler system is being tested or drained, then leak-check. See FPS-500 and FPS-900 Preventive Maintenance.

ℹ Note — the FPS-15000 and FPS-20000 have no published figures, by design On those two models the setpoints are entered on the controller and set at the factory for your unit — there is no standard pair to publish. Read them on the touchscreen: Menu → Pressure Transducer Setup, which asks for the password supplied with your system.

Do not adjust them. Changing them can alter both the nitrogen purity and the flow the system was designed to deliver. See FPS-15000 and FPS-20000 Legacy Overview.

ℹ Note — the FPS-650 and FPS-1250 figures were previously published wrong This page used to carry 55/65 for those two models. That came from misreading the 55–75 PSIG nitrogen storage pressure range in the specification table as a cut-in/cut-out pair — they are different things. The operating manual states standby at 70 PSIG, a working band of 65–75, and cut-in at approximately 62 PSIG, 7–10 PSIG below cut-out. Factory preset, with no field adjustment required. See FPS-650 and FPS-1250 Legacy Overview.

Nitrogen cut-in is not bypass cut-in

On the models with automatic bypass — FPS-1650 and above — there is a second, separate pair of setpoints that decides when compressed air takes over from nitrogen. The numbers are close enough to the nitrogen setpoints to be mistaken for them, and they are not the same thing.

Setpoint pair	What it decides	Where it is set
Nitrogen cut-in / cut-out	When the generator produces nitrogen and when it rests. The table above.	N2 Pressure Settings screen
Bypass cut-in / cut-off	When the riser goes onto compressed air instead of nitrogen, and when it comes back off.	Bypass Alarm Settings screen

Bypass cut-in is 50 PSIG on the FPS-1650, FPS-3250 and FPS-5000. The cut-off differs by family, and the whole subject — including forcing bypass on deliberately while a generator is down — is in N2-BLAST Bypass Mode — Automatic, Forced, and Restoring Nitrogen.

Beverage

Model	Cut-in / cut-out
XX-040PPH	60 / 85 PSIG
XX-01KPH	70 / 75 PSIG
XX-03KPH	70 / 75 PSIG
XX-07KPH	70 / 75 PSIG
XX-14KPH	70 / 75 PSIG

Industrial

Industrial units are factory set to the customer's requirement and vary from system to system, so there is no standard table. Read the setpoints on the unit, or call us with the model and serial number.

Reading the setpoints on your own unit

The unit in front of you is always the authority. Setpoints get changed over a system's life, and a machine that is not at its factory default is not necessarily faulty — but a unit you assume is at default when it is not will send you chasing the wrong thing.

On a touchscreen unit — the controller's N2 Pressure Settings screen shows the cut-in and cut-out setpoints directly.
On a pressure-switch unit — watch an actual cycle. Note the pressure it enters standby at and the pressure it restarts at. That is the only honest reading.
Either way, write both numbers down before you change anything, so you know what the machine was doing when it arrived.

⚠ Warning — factory set, and not a field adjustment Cut-in and cut-out are set at the factory for the nitrogen purity and flow the system was sized to deliver. Do not adjust them without contacting South-Tek Systems first. Changing them can alter purity and flow capability, and on fire protection systems the purity is the corrosion protection the system exists to provide. On some products the control programming is proprietary and unauthorized changes affect warranty.

Need help? Have the model and serial number and the pressures you actually observed — the one it stopped at and the one it restarted at. South-Tek Systems Service — Phone: (910) 415-1880 · Open a support ticket at support@southteksystems911.zendesk.com or through the Help Center.