1. Liquid-in-Glass Thermometers

Definition: Classic thermometers that use mercury or dyed alcohol sealed in a glass tube.
How It Works: The liquid expands with heat and rises against a calibrated scale.
Used In: Labs, calibration rooms, and field checks where no electronics are allowed.
Notes: Don’t underestimate these—still considered a calibration reference in many labs. Fragile but timeless.
References:

• ASTM E1 – Standard Specification for ASTM Liquid-in-Glass Thermometers
• ISO 1770 – Solid-stem general-purpose thermometers
• ASTM E563 – Standard Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature

2. Bimetallic Thermometers

Definition: Mechanical thermometers using two bonded metal strips with different expansion rates.
How It Works: Temperature causes the strip to bend, turning a needle on a dial.
Used In: HVAC, ovens, tanks—places where ruggedness wins over precision.
Notes: Tough but crude. Good for rough monitoring, not data collection.
References:

• ASME B40.200 – Pressure and Temperature Instruments
• ISO 13163 – Bimetallic strip thermometers — Construction and accuracy classes

3. Thermocouples

Definition: Sensors made by joining two dissimilar metals that generate a voltage proportional to temperature.
How It Works: The Seebeck effect produces a tiny voltage that changes with temperature.
Used In: Kilns, engines, industrial curing ovens—anywhere you need fast response and wide ranges.
Notes: Available in different “Types” (J, K, T, N, etc.), each with a specific range and stability profile. Calibration is key.
References:

• ASTM E230/E230M – Temperature-EMF Tables for Standardized Thermocouples
• IEC 60584 – Thermocouples — EMF Specifications and Tolerances

4. Resistance Temperature Detectors (RTDs) / Industrial Platinum Resistance Thermometers (IPRTs)

Definition: Temperature sensors whose resistance increases with temperature—commonly made of platinum.
How It Works: Typically uses a 100-ohm platinum element (Pt100). Resistance is read via a Wheatstone bridge or digital circuit.
Used In: Lab environments, process control, quality-critical measurement systems.
Notes: Accurate, stable, and more linear than thermocouples, but generally more fragile and expensive.
References:

• ASTM E1137/E1137M – Industrial Platinum Resistance Thermometers
• IEC 60751 – Industrial platinum resistance thermometers and sensors

5. Platinum Resistance Thermometers (PRTs)

Definition: A category of RTDs that use platinum as the sensing material—offering high accuracy and chemical stability.
How It Works: Same principle as RTDs; PRTs are often built with high-purity platinum and better thermal contact structures.
Used In: Industrial process monitoring, environmental chambers, precision thermal control in rubber testing systems.
Notes: The reliable, workhorse RTDs—less extreme than SPRTs, but more refined than budget RTDs.
References:

• IEC 60751 – Platinum resistance thermometers
• ASTM E2877 – Digital Contact Thermometers

6. Standard Platinum Resistance Thermometers (SPRTs)

Definition: The most accurate class of thermometers used to define and calibrate temperature standards.
How It Works: Uses ultra-pure platinum wire in a carefully constructed housing, usually read by precision bridge circuits.
Used In: National metrology labs, accredited calibration services, reference-grade measurements from -200 °C to 960 °C.
Notes: These are the gold (well, platinum) standard—SPRTs are used to realize the International Temperature Scale (ITS-90).
References:

• ASTM E1139 – Standard Specification for Standard Platinum Resistance Thermometers (SPRTs)
• ITS-90 – International Temperature Scale of 1990

7. Thermistors

Definition: Sensors made from ceramic materials whose resistance changes rapidly with temperature.
How It Works: Typically negative temperature coefficient (NTC)—resistance decreases as temperature increases.
Used In: Digital thermometers, medical probes, climate control devices.
Notes: Fast and cheap, but nonlinear and narrow-ranged.
References:

• ASTM E2309 – Thermistor Sensors for General Temperature Measurement
• IEC 60531 – Thermistors for temperature measurement — Direct-immersion type

8. Infrared (IR) Thermometers

Definition: Non-contact thermometers that detect infrared radiation emitted by surfaces.
How It Works: A lens focuses IR energy onto a sensor, converting it into a temperature reading.
Used In: Food inspection, rotating equipment, open-flame applications, and quick surface checks.
Notes: Fast and safe, but sensitive to surface emissivity and environmental interference.
References:

• ASTM E2847 – Calibration and Use of Infrared Thermometers
• ISO 18434-1 – Thermography — General Procedures

9. Digital Thermometers

Definition: Any thermometer that gives a digital output—usually based on thermistors, RTDs, or thermocouples.
How It Works: Measures sensor output and converts it into a number on a display.
Used In: General lab use, industrial settings, food safety, and medical applications.
Notes: The interface is digital, but the sensor inside matters. Don’t judge a thermometer by its screen alone.
References:

• ASTM E2877 – Digital Contact Thermometers
• ISO 80601-2-56 – Medical Electrical Equipment — Clinical Thermometers

Cost & Accuracy Comparison Table

*Approximate 2025 retail cost ranges for industrial or laboratory-grade units (USD).