As stated in About ambient conditions, the purpose of measuring and assessing ambient condition is to make sure the temperature and relative humidity are favourable for surface preparation, paint application and drying and curing, as well to avoid condensation to form on the substrate at critical stages during the surface treatment.

Modern electronic inspection equipment suppliers, such as Elcometer and DeFelsko, offer multi-purpose instruments which can easily and quickly measure air and substrate temperatures, establish relative humidity and calculate dew point. As long as the battery provides sufficient charge and the electronics works as designed, these instruments are very convenient. They are simple to operate, just follow the manufacturer’s instruction, and may produce electronic lists of results and even digital reports. However, if batteries dies or the electronics stop working, a professional surface treatment inspector should also be familiar with how older and simpler equipment work, equipment which is not depending on advanced electronics.

Everybody knows how to read the temperature on a thermometer. This could be a simple thermometer used for reading the temperature in a room or in the garden, or it could be the dry bulb thermometer of a whirling or sling hygrometer. There are two main types of thermometer:

• Spirit thermometer, identified by having a bulb and rising column of coloured spirit (most common red or blue)
• Mercury thermometer, identified by having a silver-coloured bulb and raising column containing mercury. Note: mercury thermometers are not allowed to be transported by air

Magnetic thermometer

This consists of a spring thermometer connected to a graded dial and is contained in a housing which has a magnet at its base. It can be attached to steel substrates in any horizontal or vertical position / orientation. It is cheap and stable, not battery depending, but need some time to adjust to the temperature of the substrate.

Digital contact thermometer

These come in many different versions. They all have a probe which is put in contact with the substrate and its temperature can quickly be read in a digital display window. They have a fast response and are accurate. They are more expensive than the magnetic type and is battery depending.

Infrared (laser) thermometer

Just point the laser on the substrate and read the temperature on its digital display. There is no need for contact with the substrate, so it can be useful for areas difficult to reach. It responds very fast and is fairly accurate, although there have been reports of results being influenced by conditions between the instrument and the substrate (for example the presence of smoke). Battery is required.

Sling / Whirling hygrometer

Anybody having been involved in washing cloths will have noticed that the time they take to dry will vary from one day to the other. In warm and dry weather the cloths dry quickly, while in wet and misty weather it takes longer time. We are using this principle in order to determine the relative humidity in the air. When water evaporates it requires energy, leading to a lowering of the temperature of the wet cloth. If there is little moisture in the air the wet cloths will dry quickly, meaning their temperature will fall noticeably. With a lot of moisture in the air the cloths will dry slowly, so the cooling effect will be smaller.

A sling or whirling hygrometer contains two thermometers, the bulb of one is covered by a cloth wick and the other is exposed. After we make the cloth wick wet by water, we will see that the thermometer with wet bulb shows a lower temperature than the one with a dry exposed bulb. To speed up the effect we spin (whirl) the two thermometers in the air around an attached handle, ensuring the evaporation and cooling effect takes place. The difference in temperature between the wet and the dry bulb is directly influenced by the existing humidity in the air. A large difference in temperature between the two thermometers indicates a low humidity, while a small difference signifies a high humidity in the air.

Procedure

1. Fill the reservoir with distilled water
2. Check that the wick covers the bulb of the wet thermometer fully and is saturated with water
3. Hold the handle of the hygrometer and whirl it for 20 to 40 seconds away from your body
4. Quickly read the wet thermometer and whirl the hygrometer again
5. Repeat steps 3 and 4 until consecutive readings are the same
6. Immediately read and note the wet & dry thermometers
7. Calculate the wet bulb depression, i.e. the difference between the dry and wet bulb temperatures
8. Use the accompanying table or slide rule to find Relative Humidity (%RH)
9. Alternatively align the dry bulb temperature and dew point temperature in the dew point calculator and read the %RH in the designated window

Psychrometric chart

A number of different charts are available to determine %RH. Identify the relevant data required on the chart’s two axes and find the required result at their cross-point.

Example: Locate the dry bulb temperature in the left column and the difference between the wet bulb and dry bulb temperatures in the column header row. The %RH will be found in the intersection between the column and the row.

IX Diagram

The IX Diagram is a special form of psychrometric chart. Based on wet and dry bulb measurements, %RH can be determined from an IX Diagram, see separate IX-section.

There are multi-function instruments that can measure dry and wet bulb temperature, wet drop-down (depression), relative humidity, dew point and substrate temperature in one go.

As stated earlier, dew point is the temperature where the RH is 100%, so any moisture in the air are about to start forming condensation. We are interested in determining the dew point in order to check that the temperature of any surface being involved in our work is minimum 3^oC above the dew point. This will ensure that all surfaces being worked on will not suffer from condensation. If such condensation should occur, the quality of any surface preparation would deteriorate and the paint is likely to suffer from weaknesses or complete failure.

Dew point is not measured, it is calculated with basis in wet and dry bulb measurements. There are several tools which can be used for this calculation:

Sling hydrometer

Sling hydrometer of the Bacharach type (round, black plastic body) has a built in sliding scale where the dew point can be determined, using the wet and dry bulb measurements:

1. After measuring the wet and dry bulb temperatures, slide the body back into the tube (aligning the groove with the arrow) until the wet and dry temperature readings are opposite each other on the top two scales
2. The %RH is shown on the scale below the arrow
3. To find the dew point, note the Wet bulb temperature. Find this figure on the Dry bulb scale. Dew point is the equivalent value on the Wet bulb scale.

Dew Point Calculator

1. Use sling / whirling hygrometer to measure dry bulb and wet bulb temperature
2. Follow the instruction printed on the face of the circular plastic calculator

ISO 8502-4

Use Annex A Table for determination of dew-point in this standard:

1. Locate the% RH in the left column (the column extends over two pages)
2. Locate the air temperature in the column header row (the header extends over several pages)
3. The dew point will be found at the intersection between the %RH and air temperature

Psychrometric chart

A number of different charts are available to determine dew point. Identify the relevant data required on the chart’s two axes and find the required result at their cross-point.

Example: Locate the dry bulb temperature in the left column and the difference between the wet bulb and dry bulb temperatures in the column header row. The dew point temperature will be found in the intersection between the column and the row.

IX Diagram

The IX Diagram is a special form of psychrometric chart. Based on wet and dry bulb measurements, dew point can be determined from an IX Diagram, see separate IX-section.

There are multi-function instruments that can measure dry and wet bulb temperature, wet drop-down (depression), relative humidity, dew point and substrate temperature in one go.

The IX Diagram is a special form of psychrometric chart. Based on wet and dry bulb measurements from a sling hydrometer, dew point, %RH and moisture content can be determined from an IX Diagram.

The IX Diagram for air and water vapour mixtures was first published by Richard Mollier, Dresden, in 1927. The diagram has since been extended and updated, and can today be found in a few slightly different versions. The principle for using an IX Diagram is largely the same for all versions:

1. Find the Dry bulb temperature on the left edge scale and draw a horizontal line
2. Find the wet bulb temperature on the diagonal scale and draw a line at 90 degree angle. (NOTE: If the wet bulb scale is split and continues at a different angle, draw the line at 90 degrees to the first part of the scale)

3. Start from cross between the two drawn lines and draw a third line vertically down to 100% RH curved scale
4. From the crossing point on the 100% RH curve, draw a horizontal line back to the dry bulb scale and this cross point will show the Dew Point

Examples of IX-Diagrams

• Within the time of a work shift, ambient measurement should be taken at least before the job starts, at the middle of a job and after the job
• Additionally, more measurements must be taken if weather conditions change

The substrate temperature must be at least 3ºC above the dew point.

Commonly, the specification require Relative Humidity should be less than 85%.