A capillary thermostat does two things: it reads a temperature through its sensing bulb, and it switches the heater on and off at the temperature the dial is set to. When the dial says 180°C and the oven actually holds 200°C, the thermostat is out of calibration — and the food, the fryer oil and the energy bill all pay for the difference. Calibration is the check that closes that gap: comparing what the dial promises against what a calibrated reference thermometer actually measures, correcting the dial where the design allows it, and knowing when the drift means the thermostat should be replaced instead. This guide works through the check, the adjustment, and the line between the two.
What calibration can and cannot fix
Calibration corrects a dial offset: the switch still works, it still cuts in and out cleanly, but it does so at the wrong temperature — the dial says one thing and the chamber does another, by a steady amount. That is a calibration job. What calibration cannot fix is a failed switch, a damaged capillary, or a bulb that has lost its charge: a thermostat that never switches off, that switches erratically, or that drifts further every week is not out of calibration, it is failing, and no dial adjustment will save it. The same distinction applies to the safety limiter on the same equipment: its trip point is factory-set and must never be adjusted in the field, as covered in our safety limiter guide.
The usual causes of a dial offset are ordinary wear: the knob or pointer slips on its shaft, the capillary takes a knock during cleaning or service, scale or grease coats the sensing bulb and insulates it, or the mechanism simply ages after years of cycling. A thermostat that has just been fitted should be checked too — a new control reads true only if the bulb is seated where the equipment maker intended, and the seating rules are in our installation guide.
The reference check, step by step
Check against a calibrated reference thermometer, not against another dial on the same equipment and not against a domestic probe: the reference is the one instrument in the check whose reading you trust. Place its sensor next to the thermostat's sensing bulb, in the same air or liquid, so both instruments measure the same temperature. Let the equipment settle at the setpoint — a reading taken while the temperature is still climbing tells you nothing — then compare the dial setting with the reference reading, and record both the cut-in point (where the heater switches on) and the cut-out point (where it switches off). The gap between the two is the switching differential, a property of the control; what you are checking is whether the pair of points sits where the dial says it should.
Check at two or three working setpoints, not one: a thermostat can read true at 100°C and be out at 200°C, and a single-point check will miss it. For water and steam duty, boiling water is a useful reference point — 100°C at sea level, lower at altitude — and for ovens and fryers, check at the setpoints the equipment actually cooks at. Write the readings down: a record of the offset at each setpoint is what tells you, months later, whether the thermostat is holding steady or drifting. The same check-first approach applies when a thermostat misbehaves in other ways — our troubleshooting guide works through the symptoms before any part is replaced.
Adjusting the dial and re-checking
Where the thermostat's design allows it, a steady dial offset is corrected by realigning the knob or pointer to the true switching point: set the control to a known reference temperature, note where the switch actually cuts in and out, move the pointer to match, and re-check at a second setpoint to confirm the correction holds across the range. Only adjust by the maker's procedure for that model — never force a shaft, bend a pointer, or open a sealed adjustment — and always re-check after the adjustment, because a correction that holds at one setpoint and fails at another is a sign of a deeper fault.
If the offset is larger than the adjustment can correct, if it drifts again within weeks, or if the switching itself has gone erratic, stop adjusting and replace the thermostat: a control that cannot hold its setpoint is a control that will sooner or later cook at the wrong temperature or fail to switch off. When you order the replacement, the range, the electrical rating, the capillary length and the bulb size all have to match the equipment — our specification guide covers the full ordering checklist, and the table below covers the common capillary models.
| Model | Rating | Range | Reset |
|---|---|---|---|
| AG16-K | 16A / 20A · 110V / 250V | 50–400°C | Automatic |
| TY316 | 16A · 110V / 250V | 50–380°C | Automatic |
| AG32-1K | 16A / 20A / 32A · 110V / 250V / 380V | 50–450°C | Automatic |
| AG16-S | 16A / 20A · 110V / 250V | Trip 380 / 400 / 430°C (fixed) | Manual reset |
Technical data is a catalogue reference. Confirm final ratings, dimensions and approvals for the selected model before ordering.
Limiters: verify, never adjust
A safety limiter is not a control and it is not calibrated in the field. Its trip point — 380 / 400 / 430°C on the AG16-S, up to 400°C on the AG32-2S, up to 380°C on the waterproof TY316-8 — is factory-set to the equipment maker's specification, and the only field check is the one the equipment maker prescribes: confirm that the limiter trips when it should and that it stays tripped until it is manually reset. If a limiter trips, the question is never how to adjust it, but why the equipment overheated — a failed control, a scaled or misplaced bulb, or a boiler run dry. Find the cause before resetting, and have the control tested if the trips keep coming.
Keep the calibration record with the equipment: the setpoints checked, the reference instrument used, the offset found and the correction made. The next service visit starts from that record instead of from scratch, and a drift that shows up in the record is caught before it becomes a batch of spoiled food or a tripped limiter.
Certifications
Our thermostats are certified by TÜV Rheinland, UL and KEMA-KEUR — see our Certifications page for the certificate number and verification route for each model. Confirm the approvals carried by the specific models you select.
FAQ
How often should a capillary thermostat be calibrated?
There is no fixed interval: check it when the readings look wrong, after fitting a new control, after any work on the capillary or the sensing bulb, and on the equipment maker's service schedule. A thermostat that holds its setpoint and switches cleanly needs no adjustment; one that drifts between checks should be watched, and replaced when the drift keeps growing.
Can I calibrate a thermostat with a kitchen thermometer?
No — the check is only as good as the reference. Use a calibrated reference thermometer, with its sensor placed next to the thermostat's sensing bulb so both measure the same temperature. A domestic probe is not a reference, and a reading taken while the temperature is still climbing tells you nothing.
My thermostat reads 10 degrees out — adjust it or replace it?
If the switch still cuts in and out cleanly and the offset is steady, adjust the dial by the maker's procedure and re-check at a second setpoint. If the offset is larger than the adjustment can correct, if it drifts again within weeks, or if the switching has gone erratic, replace the thermostat — a control that cannot hold its setpoint will sooner or later cook at the wrong temperature.
High-Current Control Thermostats · Manual Reset Safety Limit Thermostats · Capillary Control Thermostats