Managing Relative Humidity

Tendo Systems Museum

It’s all relative… humidity

In many industries the management of Relative Humidity (%rh) is important. My family and I recently visited a Museum where I noticed Relative Humidity and Temperature sensors placed around many exhibits. The readings displayed on the sensors seemed somewhat unusual. This prompted me to write this article.

What problems are caused by humidity?

In a museum, for example, it is generally accepted that an RH of between 40% and 60-65% is acceptable. Objects and artifacts can be effected by:

Unstable RH (Fluctuations over +/-3% within a 24hr period)

An unstable level of humidity can cause material to swell (in high humidity) and contract (in low humidity). This swelling and contraction of the material causes stress on the connecting fibres, gradually weakening them causing embrittlement.  Flaking oil paintings are a good example of the effects of this expansion and subsequent contraction. Some materials may show some efflorescence of salts as they crystallise after being dissolved. Fluctuations should not be more than +/- 3% relative humidity within a 24 hour period.

High RH

As well as allowing hygroscopic materials to swell, high RH can also lead to mould or fungal growth at levels over 70%, and encourage pests. Emulsions on photographs can adhere to sleeves or other photographs. Metals can rust as a result of high moisture content. High moisture levels are also linked with the crisseling of glass, producing small cracks in the surface which appear to haze the finish.

Low RH

Low RH causes materials to dry out and contract. Glues and repairs may dry out, become brittle and break. Veneers may lift on wooden objects. Stability is an important factor in RH management and provided the environment is in the accepted 40%-60% range, your collection should be safe.

What is Relative Humidity?

Understanding Humidity is the first step in solving these problems.

Relative Humidity is the amount of water vapour in the air, expressed as a percentage of the maximum that the air can hold at any given temperature.

When RH reaches 100%, the air is fully saturated and cannot hold more water vapour. The temperature at this point is called the dew point temperature. If more moisture is added, or if the temperature falls, the water vapour starts to condense.

How can we monitor humidity levels?

This is where Tendo Systems can help you. We have solutions from pocket-sized USB Data Loggers to enterprise grade Environmental Monitoring Systems (D3).

Each solution will record data and provide alarms. Our D3 system will produce automated reports and send email/SMS alerts when the RH or Temperature goes outside of your pre-set limits.

D3 Environmental Monitoring System

USB Temp and RH Data Logger

USB Temp and RH Data Logger

Where should RH be monitored?

There is a wide range of industries where RH should be measured and controlled. These include:

  • Pharmaceutical
  • Paper mills
  • Computer/Server rooms
  • Healthcare
  • Livestock
  • Food Manufacturing
  • Electronics Production
  • Museums and Heritage

How can I control RH?

Dehumidification can lower the moisture content and the dew point temperature.

Dessicants, such as silica gel, can absorb moisture in the air.

Managing Temperature is a very effective way to control RH.

As a rule;

  • If the temperature rises, the air can contain more water vapour, so the RH will fall.
  • If the temperature falls, the air cannot contain as much water vapour, so the RH will rise.

Therefore, RH can be effectively controlled by temperature. Managing a stable temperature will make the management of RH levels far more straightforward.

Use of our Environmental Monitoring System or our Humidity Data Loggers will help you to monitor and manage both temperature and relative humidity. At Tendo Systems we have solutions to meet your business needs.

Get in touch

If you would like more information on our Environmental Monitoring Systems, please call or email. We can organise a no-obligation Demo Visit or provide an Estimate. We are here to help.

 

Recommended further reading

If you want to understand more of the science behind RH; The Mollier Diagram shows the relationship between humidity, temperature and moisture content click here.

We recently published an article on the digital monitoring of Food Processing environments. Read here.

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