Resistance Sensors

Resistance Temperature Detectors (RTD) Overview

Principle

Resistance Temperature Detectors (RTDs) work on the principle that certain metals, particularly platinum, change resistance with temperature fluctuations. This change in resistance is used to precisely measure temperature.

Advantages

Highly accurate and stable
Lower converter costs compared to thermocouples

Disadvantages

Higher sensor cost than thermocouples
Fragile sensor tips, prone to damage from vibration
Not suitable for extremely low or high temperatures
Slower response time compared to thermocouples

Styles

RTDs come in three main types, with coiled elements being the most common. Coiled elements consist of platinum coils housed in insulating material like ceramic. Though accurate, they offer the least protection against vibration. Wire-wound sensors are more robust but less accurate due to limited coil movement. Thin-film sensors are the most durable, with a thin layer of platinum on an insulator, offering excellent vibration resistance but reduced accuracy.

Types and Considerations

Number of Wires

The length and thickness of RTD cabling affect resistance, potentially reducing sensor accuracy. Three-wire sensors minimise this error by measuring the resistance across two red wires on the same side of the sensor. Four-wire sensors, often used in lab settings, offer even greater accuracy by ensuring equal resistance across both sides of the cable.

RTD Base Resistance

PT100 is the most common RTD, offering 100 Ohms at 0°C. For higher sensitivity, PT1000 RTDs offer 1000 Ohms at 0°C, providing ten times the resistance changes per degree. NTC sensors are also available but are suited for narrow temperature ranges, with resistance decreasing as temperature increases.

RTD Class

Standard RTDs are Class B, accurate to 0.12% at 0°C. Class A RTDs are more precise, accurate to 0.06% at 0°C, and are used in demanding applications.

Construction: Building a Contact Temperature Sensor

Designing a Sensor

Off-the-shelf RTDs rarely meet specific application needs. Most temperature sensors are custom-made, consisting of three main sections:

1. The Tip

The sensor tip houses the junction or RTD bulb and endures the harshest conditions. It also determines how the sensor attaches to the monitored item.

2. The Middle

The middle section can include various lengths of tubing, wire, or conduit, with fittings, brackets, and protective materials tailored to the application. Options include PVC for cost-efficiency, PFA for chemical resistance, fibreglass braid for high temperatures, and stainless-steel armour for mechanical protection.