The Psychrometer Duel: Old School vs. New SchoolMatt Fajt
Why do we need to measure ambient conditions?
Ambient conditions are the prevailing conditions of air temperature, the moisture content of the air (relative humidity), and the temperature at which condensation will occur (dew point). Most coating specifications have set requirements for monitoring and documenting results for surface and air temperature, relative humidity and dew point. These conditions are to be measured and recorded in the specific areas where surface preparation and coating application will occur, then compared to the specified ranges and/or the coating manufacturer’s restrictions listed on the product data sheet.
While theoretically a surface temperature only slightly above the dew point temperature would preclude condensation, the 5°F safety factor accounts for instrument inaccuracies and changing or varying conditions.
You should not rely on prevailing conditions from a local weather service or from the internet as conditions at the project site and the specific work area can vary considerably. And surface temperature won’t be reported. Ambient conditions should be measured where the work will occur and recorded prior to start-up of operations and at 4-hour intervals thereafter, unless conditions appear to be changing. In this case, more frequent checks may be required.
Using Instruments for Assessing Prevailing Conditions
Whirling (Sling) Psychrometer: When discussing the measurement of ambient conditions using a whirling psychrometer (ASTM E337, Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures), you hear the terms wet bulb temperature and dry bulb temperature used on a regular basis, but how are these terms defined? Wet bulb temperature is an indication of the latent heat loss caused by water evaporating from a wetted sock or wick on the end of a bulb thermometer mounted in the psychrometer housing. While whirling the instrument away from your body in 20-30 second increments, the water evaporates from the wetted sock into the air, so there is a cooling effect on the thermometer causing a decrease in temperature. This process is repeated until two temperature readings from the wet bulb thermometer are with 0.5° of one another. The depression of the wet bulb thermometer from the dry bulb (air) thermometer is the calculated difference between the air temperature and the stable wet bulb temperature. For example, a dry-bulb temperature of 70°F and a wet-bulb temperature of 60°F nets a difference of 10°F. this is known as the wet-bulb depression.
Psychrometric tables are used to look-up the relative humidity and dew point temperature. First choose the table of interest (relative humidity or dew point temperature, then select the table corresponding to the prevailing barometric pressure for the geographical location that the project is in. Intersect the dry bulb (air temperature) with the difference between the dry and wet bulb temperatures, known as the depression of the wet bulb to determine the relative humidity or dew point temperature. A separate thermometer is used to measure the temperature of the surfaces to be prepared and/or coated. The temperatures and the relative humidity can then be compared to the requirements listed in the specification to determine conformance.
Digital Psychrometer: The use of a digital psychrometer for assessing prevailing ambient conditions and surface temperature is a much simpler process compared to the use of a whirling psychrometer, psychrometric charts and surface temperature thermometer. Most of the digital psychrometers will display the relative humidity, air temperature, surface temperature, dew point temperature and the difference (spread) between surface temperature and dew point temperature. Data are constantly updated and displayed simultaneously for easy recognition. This eliminates the need to use psychrometric tables to determine the relative humidity and dew point temperature, as well as any need for a separate surface temperature thermometer. The data can be auto-logged and uploaded to cloud-based software or downloaded to a device using USB or Blue Tooth®.
Which Method Wins the Duel?
Whirling psychrometers were first invented in the 1600’s (see image to right), and the US Weather Bureau Psychrometric Tables were first published in 1941. So, one may conclude that newer technology wins the duel. Not so fast! Digital psychrometers also have limitations and without user knowledge they too can produce erroneous data.
While having all the ambient conditions and the temperature of the surface readily displayed is a great benefit, there are important steps that must be followed when using these electronic instruments. It is very important that the digital psychrometer be allowed to ‘stabilize’ to the atmospheric conditions where the work is occurring. This could take anywhere from 20 to 30 minutes. That is, accurate readings are not possible immediately after departing an air-conditioned vehicle and walking onto the jobsite. Additionally, the humidity sensor used by most instrument manufacturers has a tendency to dry out during periods of inactivity, resulting in false, low humidity readings. To re-saturate the sensor, the manufacturers recommend placing the probe of the digital psychrometer in a re-sealable plastic bag or sealed container with a damp (not wet) cotton cloth for 24-hours. This will extend the life of the sensor and help ensure representative readings. And most instrument manufacturers recommend annual calibration.
Whirling psychrometers also have their limitations and the potential mis-use. These instruments cannot be used in freezing temperatures and proper use (thorough saturation of the wick with deionized water and reading the wet-bulb temperature after several 20-30 second increments of whirling until the wet bulb temperature stabilizes) is very important.
Despite the availability and apparent convenience of the digital psychrometers, many quality control and quality assurance personnel still rely on older “tried and true” technology. Both will work well when used properly.