| Choice | Options | Rule of thumb |
|---|---|---|
| Element | Pt100, Pt500, Pt1000 | Pt100 unless a long cable or a specific instrument demands otherwise |
| Accuracy class | Class B, Class A, 1/3 DIN, 1/10 DIN | Class A for process control; higher classes cost and drift-check more often |
| Wiring | 2, 3 or 4 wire | 3 wire cancels cable resistance and covers 95% of duties |
| Simplex / duplex | One or two elements | Duplex gives a spare or a second loop with no extra pocket |
| Sheath material | SS316, SS310, Inconel 600 | SS316 to 600 °C, Inconel above |
| Sheath diameter | 3, 4.5, 6, 8 mm | Thinner responds faster; thicker survives longer |
| Insertion length | Made to order | Tip should reach the middle third of the pipe |
| Head | Weatherproof, Ex d, DIN B | Ex d only if the area classification requires it |
Class A Pt100 is specified to about ±0.15 °C at 0 °C, Class B to ±0.3 °C. A 1/3 DIN element is tighter still. Buying a tighter class than the loop can use is wasted money, because the transmitter and the wiring add their own error.
Two wire adds the resistance of the cable straight into the reading — on a 50 metre run that can be a full degree. Three wire cancels it. Four wire is the laboratory answer and is only worth it when the whole loop is that accurate.
SS316 to about 600 °C. Inconel 600 above that. In chloride service, SS316 pits — consider Inconel or a coating. The sheath is the part that fails first, so it deserves more thought than the element.
The tip should sit in the moving stream, roughly the middle third of the pipe. Too short and it reads the pipe wall; too long and it can vibrate. Add the thermowell lagging extension if the line is insulated.
A spring loaded assembly pushes the tip against the bottom of the well. Without it there is an air gap, and an air gap slows the reading by tens of seconds and biases it low.
A duplex element costs a little more and gives you a spare inside the same pocket. On a point that needs a shutdown to access, that is the cheapest insurance you can buy.
A weatherproof die-cast head with a proper double compression gland keeps water out. Water in the head is the single most common cause of drifting RTD readings in Indian plants.
It converts to 4-20 mA at the sensor, so ordinary copper carries the signal and cable resistance stops mattering. It also lets you use a two wire loop instead of three or four cores.
Tri-clover RTD with polished wetted parts and no crevice — a threaded sensor is not acceptable in hygienic service.
Mineral insulated RTD, or a thermocouple. A conventional RTD element does not last under heavy vibration.
Flameproof Ex d head with a CCoE certificate, and an Ex d double compression gland on the cable entry.
Tolerance. Class A allows about ±0.15 °C at 0 °C, Class B about ±0.3 °C. The gap widens as temperature moves away from zero.
Four wire is more accurate because it cancels all lead resistance, but three wire cancels enough for process control and needs one core less. Use three wire unless the loop is a precision one.
Enough that the sensing tip sits in the flowing stream — as a rule, the middle third of the pipe bore, plus the nozzle and any lagging extension.
A spring in the sensor assembly pushes the tip firmly against the bottom of the thermowell. Without it, an air gap forms, and the reading becomes slow and biased low.
With an Inconel sheath, up to about 650 °C in some designs, but the element drifts. Above that, use a thermocouple.
Send the process temperature, the pipe bore and the head you need. We will fix the class, the wiring, the sheath and the insertion length.
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