A pressure-control question can look like arithmetic when the real test is sequence. “Cut-out” is the pressure event that opens the control's running circuit; “cut-in” is the event that closes it again. Before calculating anything, identify whether the drawing shows an open-low or open-high control and whether reset is automatic or manual.
What does an open-low control do as pressure falls?
For the SPST open-low P70A/P70B/P170A models in Johnson Controls' catalog, falling pressure reaches the cut-out and opens the line-to-M1 circuit. On rising pressure, the cut-in closes it. The switch action matters: the same pressure number means something different on an open-high model. Read the model's wiring diagram and nameplate before interpreting a control on actual equipment.
How do you calculate the differential?
Imagine an automatic-reset, open-low control specified to cut out at 20 psig and cut in at 40 psig. On a falling-pressure cycle, it opens at 20 psig. After pressure rises to 40 psig, it closes again. The differential is 40 − 20 = 20 psi. These numbers are an educational example, not settings for a particular system.
If the question gives cut-in and differential, subtract the differential to find the open-low cut-out. If it gives cut-out and differential, add the differential to find cut-in. Say the direction aloud before doing the subtraction.
Why can two control faces show different numbers?
Johnson Controls' P70/P72/P170 documentation distinguishes all-range low-side controls, whose scale shows cut-in and cut-out, from MICRO-SET models, whose scale shows cut-in and differential. Do not treat the second pointer as cut-out without checking which model is in front of you. The manufacturer also says to verify real operating setpoints with reliable gauges and the equipment maker's specified settings.
What changes with a manual-reset lockout?
A manual-reset model remains locked out after the cut-out event; pressure recovery alone does not restart it. Johnson Controls instructs the technician to find and remedy the cause before resetting. In an exam study case, identify the reset type before predicting whether a rising pressure will close the circuit automatically. On the job, never use the example numbers above to set or bypass a safety control.
This is a controls-reasoning topic within refrigeration service and commissioning. Compare it with the compressor terminal-reading example when practising a second measurement sequence. The free sample contains original, unofficial draft questions; it does not reproduce the certifying exam.
Sources
- Johnson Controls: P70/P72/P170 low-pressure control functionalityOpen-low switching and reset action; revision 2026-04-28, checked 2026-09-24
- Johnson Controls: P70/P72/P170 low-pressure control setupCut-in and differential scale; revision 2026-04-28, checked 2026-09-24
- Johnson Controls: P70/P72/P170 manual reset procedureLockout, pressure recovery and reset sequence; revision 2026-04-28, checked 2026-09-24
- Red Seal: Refrigeration and Air Conditioning Mechanic trade pageCurrent-exam standard link; checked 2026-09-23
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.