Choosing between an electromechanical pressure switch and an electronic transmitter: fixed safety cut-outs versus continuous control signals, adjustability and covers (XMA/XMX), high-pressure Nautilus switches, and the Carel SPKT / Danfoss AKS ratiometric and 4–20 mA transmitter ranges in stock.
A pressure switch opens or closes a contact at a set pressure — simple, self-contained, functional with no controller, and the right device for hard safety functions (HP cut-out with manual reset) and simple pump/compressor cycling. A pressure transmitter streams the measurement continuously (4–20 mA or ratiometric 0.5–4.5/0–5 V) into a controller that does the deciding — mandatory for electronic superheat control, floating head pressure, VFD control and data logging. Modern plants carry both: transmitters for control, switches as the independent last line.
| Range | Output | Stocked |
|---|---|---|
| −1…4.2 bar | 0–5 V ratiometric | Carel SPKT0053R0 |
| −1…9.3 / −1…12.8 bar | 0–5 V | SPKT0013R0, SPKT00E1S0 |
| 0…10 / −0.5…7 bar | 4–20 mA | SPKT0011C0, SPKT0021C0, SPK1000000 |
| 0…30 / 0…34.5 bar | 4–20 mA / 0–5 V | SPKT0031C0, SPKT0033R0, SPK3000000 |
| 0…44.8 / 0…45 bar (CO2 subcritical) | 4–20 mA / 0–5 V | SPKT00B1C0, SPKT00B6R0 steel |
| 0…60 / 0…150 bar (CO2 transcritical) | 0–5 V / 4–20 mA | SPKT00G1S0, SPKT00D8C0 |
| −1…12 / −1…159 bar | Ratiometric | Danfoss AKS 32R, AKS 2050 |
| 3000 psi, 0.5–4.5 V | Ratiometric | XMEP3K0PT130 |
Ratiometric vs 4–20 mA: ratiometric (0.5–4.5 V or 0–5 V of the 5 V supply) is the refrigeration-controller convention (Carel, Danfoss inputs) over short runs; 4–20 mA wins on long runs and electrically noisy plants because the signal is immune to voltage drop. Match the controller input first, then the range so the normal operating point sits at 30–70 % of span, then the connector (female/male M12-style per listing).
Mount transmitters above the tapping where condensate could collect, use a valve or Schrader access for service (core tool), and on CO2 pick the range that covers standstill pressure, not just running pressure. Verify annually against a calibrated gauge at two points; log zero offsets. Switches get a functional trip test — drive pressure through the setpoint and record actual trip value and differential; a safety HP cut-out that has never been tripped on purpose is an assumption, not a protection. Controllers and the rest of the loop are covered in the controls guide.