Humidity and Temperature

Measuring relative humidity

Being able to measure relative humidity is not absolutely necessary when controlling fluctuations or extremes; but it is helpful in identifying problems.

If the environment is very dry or very damp, you can generally feel it—your skin responds to these conditions. We feel dry and sometimes a bit itchy in very dry conditions, and clammy in humid conditions. But feeling the extremes doesn't tell us how extreme the conditions are; and it gives us no indication of how rapidly the conditions are fluctuating.

To gather information about the levels, you need to measure the relative humidity and temperature; and to get information about the rate of fluctuation, you need to monitor the environment.

Measuring the conditions involves taking readings at a specific time—a snapshot of the conditions.

Monitoring conditions involves continuous or repeated measurement, so that you get a changing picture or series of snapshots over a period of time.

There are a number of devices which can be used to measure relative humidity.

Whirling hygrometer

A whirling hygrometer—also called a sling psychrometer—measures relative humidity and temperature directly.

It has two matched thermometers. One thermometer is called the dry bulb and the other the wet bulb. The wet bulb thermometer has a cotton sleeve wrapped around its base. Distilled water from a small reservoir is used to keep the sleeve wet.

The hygrometer is whirled around in the air. While this happens, water from the sleeve of the wet bulb thermometer evaporates, and the wet-bulb temperature shown by the thermometer goes down.

A whirling hygrometer or sling psychrometer. Photograph courtesy of Artlab Australia

The amount of water which evaporates depends on the capacity of the air to contain water. The decrease in temperature shows how much water has gone into the air; this shows the capacity of the air to contain water.

The dry bulb gives the temperature of the air.

By comparing the two temperatures after the instrument has been whirled, the relative humidity can be calculated. This is made easier by using published tables listing the relative humidity against the dry and wet-bulb temperatures. These tables should be provided with the hygrometer.

The advantages of using a whirling hygrometer are:

• they are relatively cheap;

• they don't require ongoing maintenance; and

• if you purchase another measuring device, you will still need a whirling hygrometer, as a reference device to calibrate the other device.

The disadvantages of using a whirling hygrometer are:

• they provide a snapshot only. They do not continuously monitor the environment;

• to monitor the environment with a whirling hygrometer, you need to take readings in the same places at regular intervals, and record when and where you took the readings; and

• whirling a manually-operated hygrometer can make your arm tired. Battery-operated whirling hygrometers are available.

Thermohygrograph

A thermohygrograph allows for continuous measurement of relative humidity and temperature over a period of time. It monitors the environmental changes.

Thermohygrographs work on the principle that organic materials expand and contract as the relative humidity changes. In this case, the organic material is human hair.

The hairs are bundled together and stretched between a fixed pin and a moveable pin. The moveable pin is attached to a series of levers, which amplify the movement of the hairs.

A pen is attached to the end lever; and this pen plots the movement of the lever on a chart, which is mounted on a rotating cylinder. The rate of rotation can be altered, so that the relative humidity is plotted over a day, a week or a month.

A thermohygrograph also has a temperature sensor—which records the temperature on the same chart.

The chart should be changed at the end of each recording period. When you change the chart, set the pen on the correct time, and you will have a record of the times when changes occur.

The advantages of using a thermohygrograph are:

• the thermohygrograph chart contains information about temperature and humidity, as well as the relationship between the two; and

The thermohygrograph Photograph courtesy of Artlab Australia

• the chart also shows when fluctuations occurred, so you can relate fluctuations to events in the area being monitored, for example, the arrival of a bus-load of tourists on a wet day!

The disadvantages of using a thermohygrograph are:

• they require ongoing maintenance, to ensure they are recording accurately and to ensure that the cylinder is rotating at the right rate;

• they need to be calibrated periodically, and the hairs need to be re-conditioned; and

• thermohygrographs give you information only if you look at the charts. Most people look at the charts only at the end of the recording period, so they don't respond immediately to problems as they arise.

Dial hygrometers

Dial hygrometers work on the same principle as the thermohygrograph—using human hair to operate a lever which moves a dial.

A dial hygrometer Photograph courtesy of Artlab Australia

The advantages of using a dial hygrometer are that it is small and can be placed in display cases and on shelves.

The disadvantages of using a dial hygrometer are:

• they measure relative humidity, but not the temperature; and

• they measure continuously, but don't record the information—you have to look at them continuously if you want to use them to monitor changes.

Electronic hygrometers

Electronic hygrometers are generally used to provide a snapshot of conditions. They measure relative humidity and temperature, and need to be calibrated periodically. Before they are used, they need to be allowed to acclimatise to the area they will be monitoring.

Data loggers

Systems which monitor relative humidity and temperature, and download data to computers are now available—they are called data loggers.

The advantages of using data loggers are:

• they can be linked to alarms so that when conditions move outside the recommended levels, action can be taken; and

• the remote sensors can be placed in display cases, storage boxes and crates.

The disadvantages of using data loggers are:

• you need a computer to access the information; and

• they are expensive—although they are likely to become cheaper as time goes by.

Humidity indicator cards

Humidity indicator cards are also available. These use moisture-sensitive salts which change colour as the relative humidity alters. They can be very useful for low-cost monitoring—especially within display cases and storage boxes—provided you check them regularly.

Separate temperature cards are needed if you want to check temperature variations.

Calibration

Thermohygrographs and dial and electronic hygrometers do not remain accurate. Ideally, they should be calibrated against an instrument such as a whirling hygrometer once a month and if they have slipped out of calibration, they should be recalibrated. The hairs in thermohygrographs and dial hygrometers must also be reconditioned regularly.

We will not give detailed instructions for the calibration of individual instruments because there will be slight variations, depending on the type of instrument you have—whether a thermohygrograph or a dial hygrometer. The instrument will come with instructions. If it does not, ask the supplier for clear instructions. If you don't follow the instructions, your readings won't be accurate and can't be relied on.

If you buy a thermohygrograph, you will need to buy a whirling hygrometer as well. If funds are limited, the whirling hygrometer would be a wiser investment.