Calibration of Dataloggers Using Saturated Salt Solutions

Calibration of Dataloggers Using Saturated Salt Solutions

Introduction

Relative Humidity (RH) sensors can drift over time and have a limited lifetime. It is very important that the accuracy of RH sensors be checked on a regular basis. Since datalogger manufacturers charge significant fees for recalibration it is cost effective to be able to check them yourself before sending them in for servicing.

This article provides guidance on setting up calibration salt chambers that are an easy and inexpensive way to check loggers when they first arrive after purchase to make sure they are functioning properly before they are used, as well as to check calibration over time. The chambers are reported to have limited accuracy but if you run several dataloggers and logger types at once for cross comparison, they can be a reliable tool.

Lithium Chloride (LiCl), Magnesium Chloride (MgCl) and Sodium Chloride (NaCl) which each hold a different RH are used in the chambers. Conducting a three point calibration ensures that a logger is accurate across its entire measurement range.

Salt	RH for a saturated solution
Lithium Chloride	11%
Magnesium Chloride	33%
Sodium Chloride	75%

General Procedure

Figure 1. Saturated salt solutions that hold specific RH levels are placed in small well sealed plastic containers. The lids of the containers are fitted with a Gortex window that allows vapor exchange but prevents liquid spills and salt migration.

Figure 2.This small container is then placed in a larger container that can hold several loggers.

The RH of salt solutions can be affected by temperature fluctuations so it is best if the temperature is as constant as possible. Try placing the chambers in a gasketed cabinet in a climate controlled store room.

Figure 3. The loggers, set to take readings every two minutes, are placed sequentially in each chamber for two-three days, moving from low to high RH.

At the end of the monitoring period the data is downloaded and any which show values outside the expected range of accuracy are sent back for refund or recalibration as necessary.

Supplies

• Polypropylene containers – widely available at home and kitchen stores or online
• Goretex or Sympatex non-permeable fabrics – Talas (www.talasonline.com)
• Salts - – widely available from scientific suppliers
• Glass dishes – widely available from scientific suppliers

Contributors

Samantha Alderson Conservator American Museum of Natural History Anthropology Department Conservation Lab

Rachel Perkins Arenstein Conservator A.M. Art Conservation LLC