Temperature measuring instruments can be broadly classified into several categories based on their operating principles, accuracy, range, and application. Here's a general classification:

1. Thermocouples: Thermocouples consist of two dissimilar metal wires joined at one end. They generate a voltage proportional to the temperature difference between the measurement junction and the reference junction.

2. Resistance Temperature Detectors (RTDs): RTDs use the principle of electrical resistance changes with temperature. Typically made of platinum, nickel, or copper, RTDs provide a more linear response compared to thermocouples.

3. Thermistors: Thermistors are semiconductor devices whose electrical resistance changes significantly with temperature. They offer high sensitivity and accuracy in a limited temperature range.

4. Infrared (IR) Thermometers: IR thermometers measure temperature by detecting the infrared radiation emitted by an object. They are non-contact and suitable for measuring surface temperatures of objects.

5. Bimetallic Thermometers: Bimetallic thermometers consist of two different metal strips bonded together. When heated, the strips expand at different rates, causing the instrument to bend, which can be calibrated to indicate temperature.

6. Liquid-in-Glass Thermometers: These traditional thermometers use the principle of thermal expansion of a liquid (such as mercury or alcohol) to indicate temperature changes. They are often used for laboratory and household applications.

7. Pyrometers: Pyrometers are specialized instruments used for measuring high temperatures, typically above the range of other thermometers. They utilize various principles such as optical, radiation, or thermal conductivity to measure temperature.

8. Fiber Optic Temperature Sensors: Fiber optic sensors use the principles of light reflection or absorption to measure temperature. They are suitable for harsh environments and remote sensing applications.

9. Semiconductor-based Sensors: These sensors use semiconductor materials and electronic circuits to measure temperature. They offer high accuracy, fast response times, and are commonly used in electronic devices.

10. Digital Temperature Sensors: Digital temperature sensors incorporate analog-to-digital converters (ADCs) to provide a digital output representing temperature. They are often integrated into microcontrollers and used in various consumer electronics and industrial applications.

These are some of the general classifications of temperature measuring instruments, each with its own advantages, limitations, and suitability for different applications.