| Decision | Options | Note |
|---|---|---|
| Panel cut-out | 48×48, 96×48, 48×96, 72×72, 96×96 | Measure the existing hole before ordering a replacement |
| Input | TC, RTD, mV, mA, universal | Universal covers everything and costs little more |
| Control output | Relay, SSR drive, 4-20 mA, valve motor drive | Relay wears on fast cycling — use SSR drive for heaters |
| Alarm relays | 1 to 5 | Two is usually enough; five suits interlocked plant |
| Ramp-soak | No / 16 segment | Only if you run heat treatment or sterilisation profiles |
| Communication | None, RS485 Modbus, Ethernet, Wi-Fi | RS485 is the safe minimum for future SCADA |
| Zones | 1, 2 or 3 loops | Multi-zone saves cut-outs and wiring on furnaces |
| Enclosure | Panel, field weatherproof, flameproof | Only specify Ex d if the area classification demands it |
On a replacement job this is the first constraint, not the last. A 96×96 controller does not fit a 48×48 hole, and enlarging a hole in a live panel door is not a five minute job.
The price difference is small. The day someone changes a thermocouple to a Pt100, or adds a 4-20 mA transmitter, the controller does not have to be replaced.
A relay output switching a contactor is fine. A relay cycling a heater every few seconds will wear out in months — use an SSR drive output there. A modulating valve or a VFD needs 4-20 mA. A motorised valve without a positioner needs valve motor drive output.
High, low, deviation and a common trip is four. Buying a five-relay controller and using one is waste; buying a two-relay controller and needing four means a second device.
Heat treatment, sterilisation, ceramic firing and composite curing need a segment programmer. A plain PID cannot run a profile, and retro-fitting means a new controller.
It costs little and it is the difference between a panel you can integrate later and a panel you have to rebuild. If the plant already has a LAN, an Ethernet controller with on-board logging is worth the step up.
48×48 universal input with two relays. Fits the standard hole, covers every sensor a customer might use.
A three zone controller with independent auto-tune per zone — one cut-out instead of three, and each zone tunes to its real behaviour.
A flameproof field controller with a CCoE certificate, so control sits at the process instead of a remote safe room.
Match the panel cut-out you have. 48×48 mm is the most common in Indian machine panels; 96×96 mm is standard on process panels.
Auto-tune runs a controlled disturbance and calculates PID constants from the response. It works well on most heating loops. On very slow or interacting loops, manual tuning still beats it.
Whenever the output switches faster than about once a minute — typically heater control. Mechanical relay contacts wear out under fast cycling.
Running a temperature profile — ramp to a temperature, hold for a time, ramp to the next. Heat treatment, sterilisation and kiln firing all need it.
Not today, but it costs very little and it is what lets the machine be monitored later without replacing the controller.
Send the panel cut-out, the sensor type and what the output has to drive. We will match it and tell you if a cheaper model does the same job.
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