Using the HMK15 Saturated Salt Calibration Device: A Practical Guide

In the world of humidity calibration, sometimes the simplest tools can offer the most practical solutions. The HMK15 is a saturated salt calibration chamber—a compact, low-tech device that provides stable humidity references for calibrating hygrometers. While it might appear basic, its reliability and independence from a power supply make it a valuable asset in many metrology labs.

The Working Principle

At its core, the HMK15 relies on the predictable behavior of saturated salt solutions. Each salt creates a known relative humidity (RH) environment when equilibrated in a sealed chamber. For example:

•    Sodium chloride (NaCl) provides approximately 75% RH
•    Lithium chloride (LiCl) provides approximately 11% RH

These humidity points are commonly used in calibrations and offer a dependable reference without complexity.

Benefits of a “Primitive” Solution

Although it may seem very simple, the saturated salt method offers key advantages:

•    No power required – Ideal for field or bench use without electrical connections
•    Cost-effective – Minimal components, long service life

However, it’s not without limitations. The stabilization process is slow, and results are sensitive to ambient conditions. Calibrations must be performed in a stable temperature environment to ensure accuracy.

Preparing the HMK15 for Use

Here’s how to set up the HMK15 calibration chambers:

1.    Sodium Chloride Chamber

• Add 10 ml of distilled water to the chamber
• Gradually mix in sodium chloride while stirring
• Once fully saturated, seal the chamber with the lid to prevent contamination

2.    Lithium Chloride Chamber

• Add 12 ml of distilled water
• Stir in lithium chloride gradually
• Be aware: this reaction generates heat, which is normal
• Allow the solution to cool and stabilize before using the chamber for calibration

Cross-contamination between salts is a risk so, always ensure the lid for each chamber is screwed on after mixing the water and salts to avoid introducing foreign substances.

A Reliable, Low-Maintenance Option

For metrologists who require stable humidity references without the need for complex systems, the HMK15 is a dependable and easy-to-use option.  It may not be the high-tech tool in your calibration lab, but its simplicity and reliability make it ideal for many routine humidity calibrations.