Immersion Temperature Sensor (PT100)

Function

A PT100 platinum element changes its resistance linearly with temperature — 100 Ω at 0 °C, approximately 175 Ω at 200 °C. The controller reads this resistance through a three-wire connection that compensates for cable resistance error, which is significant when the cable run from the tank-mounted sensor to the control panel is 3–6 m (10–20 ft). Sensor drift or bitumen contamination of the element — from a cracked thermowell — will cause the controller to heat or cut out at the wrong temperature, directly affecting bitumen grade integrity and spray quality.

Construction & quality

Element: platinum thin-film or wire-wound PT100, Class B tolerance to IEC 60751 (±0.3 °C at 0 °C, ±0.8 °C at 200 °C). Protection tube or thermowell: 316 stainless steel, 12 mm OD, insertion length 150–300 mm selected to reach the mid-tank bitumen level. Process connection: BSP ½" or G½ threaded fitting with lock nut. Terminal head: aluminium or stainless steel, IP65 rated, Pg-thread cable entry. Internal element spring-loaded against the thermowell tip for positive contact and vibration resistance. Cable: PTFE-insulated three-core screened, rated to 200 °C (392 °F).

When to replace

• Controller displays an open-circuit or short-circuit error code — sensor element or cable failure
• Displayed temperature is stable but does not respond to changes during burner cycling
• Displayed temperature reads consistently high or low relative to an independent calibrated thermometer
• Bitumen visible inside the thermowell when it is removed — well has corroded through
• Terminal head shows moisture ingress, corrosion or loose terminal screws on inspection