Measurement and Control of Humidity

Technical reference 10th June 2026 By Richard Wench

In this temperature and humidity transmitter technical reference guide we explain basic theory and show available products.

Contents:
1. What is humidity and why do we measure it?
2. Humidity and temperature transmitters.

What is humidity and why do we measure it?

Humidity is the concentration of water vapour present in the air. The maximum amount of water vapour that can exist in air is depended on temperature. One cubic metre of air at 30°C can hold around 30g of water vapour.

The most common humidity measurement is Relative Humidity (RH) which is a percentage of the actual amount of water vapour in the air compared to the total amount of vapour that could exist at its current temperature.

A reading of 50%RH would indicate the air is holding 50% of water vapour it could hold at its current temperature. I.E., 15g per m³ at 30°C.

So, to indicate relative humidity we need to measure both the concentration of water vapour and the temperature. Most transmitters have outputs for both relative humidity and temperature measurements.

There are many reasons why the measurement and control of relative humidity is important for many aspects including protection of moisture sensitive goods in storage, electrical safety, process control and, of course, ability to measure and control RH not only helps to create comfortable environment inside buildings but also optimizes the efficiency of HVAC systems.

Humidity and Temperature Transmitters

Carlton Thermal Systems Ltd offers temperature and humidity sensors with or without built-in displays and with a wide choice of signal outputs for compatibility with new and existing control and monitoring systems.

These include 4-20mA, 0-10V dc, RS-485 MODBUS, 12C and CAN/CANopen – CiA DS 301.

Sensors are available with different enclosure styles and levels of protection to suit most industrial, commercial and lab applications. Each of these styles have models with ATEX certification for use in potentially explosive environments.

Wall mount temperature and humidity indicators and transmitters for use in internal spaces that are not exposed to water or corrosive atmospheres. These units are designed to be integrated seamlessly into majority of interiors. Available in white and grey finishes.

Programmable temperature and humidity sensors for outdoor use are rugged temperature and humidity transmitters that have ingress protection of IP65 for the main electronics enclosures and IP40 for the sensor probe. They are designed for use in internal industrial applications or outdoors.

Duct mount temperature and humidity transmitters are designed for use in monitoring air conditioning, ventilation, and air handling units.

Temperature and Humidity Sensors with cable and digital output are miniature temperature and humidity transmitters, just 8 mm in diameter and 40 mm long.

Their small size and IP65 protection make them suitable for measurement in previously inaccessible areas. The I2C (IIC) output has a maximum communication speed of 1 MHz.