Understanding the Temperature-Humidity Curve
By Keith Beveridge
Have you ever wondered why water condenses on your cold beverage on a hot summer afternoon? Condensation is when a vapor or gas turns into a liquid. The warmer air has moisture in it, so when the warm air comes in contact with the cold surface, it condenses on the surface. This is the same phenomena as morning dew on the grass, or your glasses fogging up when you leave a cold building.
Evaporation is the opposite of condensation–it is when a liquid turns into a gas. Water in the Earth’s environment is evaporating into the atmosphere constantly. We see evidence of this in clouds, fog or precipitation and we sometimes “feel” the humidity on our skin on a humid summer day. We also notice the lack of humidity on a cold winter day.
Science Lesson
So let’s talk about the science. Humidity and, correspondingly, Relatively Humidity fall under the same general category of “Thermodynamics.” Livescience.com defines Thermodynamics as “The branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter.”
This leads us to the myth that colder air holds less water than warmer air, but this is not true. The chart below shows that cold air does in fact contain less water than warm air but it is Thermodynamics that causes this and not some special property of “air.” Water evaporates constantly and condenses all around us. This is why you get net condensation on your cold drink on a hot day. The outside surface of the drink is colder than the surrounding air causing more moisture to condense faster than it evaporates.
So based upon Thermodynamics, temperature and air pressure impact the amount of moisture in the air. The maximum amount of water vapor at a given temperature is called the saturation vapor density. Similarly, the dew point is a temperature at which a given amount of moisture in the air hits the saturation vapor density. Relative humidity is the percentage of actual vapor density in the air versus the saturation vapor density at that temperature. A simple calculation of relative humidity is the actual vapor density divided by saturation vapor density X 100 = Relative Humidity percentage. Shown below is a chart that illustrates Relative Humidity.
So what does this tell us? If you look at 25° C and 50% Relative Humidity curve, if you go to the vapor density axis, you will see that vapor density would be 15 g/m3 . If you look at the 100% Relative Humidity Curve, you will see that the same amount of moisture is present at approximately 16° C. This illustrates what we discussed earlier—there is less moisture in the air at colder temperatures.
What This Means for Shop Owners
1. You need to check for condensation on windshields and fixed auto glass parts. Condensation can be prevalent if you have preloaded your van the night before and the outside air is warmer than the inside of your installation vehicle. If condensation is present, you will need to remediate the bond line per your auto glass adhesive manufacturer’s instructions.
2. Most one-component auto glass adhesives use the ambient moisture in the atmosphere to cure. The more moisture and the higher the ambient temperature, the faster the adhesive will cure. Quicker cure means faster drive-away times.
3. You need both the temperature and humidity to calculate minimum drive-away times. You use the information supplied by your auto glass adhesive manufacturer’s recommendations.
4. To comply with the AGRSS Standard, you also need to log the relevant information for #3 above on your job sheet. This is simple as it is the ambient temperature, relative humidity and drive-away time recommended to your customer. Your auto glass adhesive manufacturer should be able to assist you with your compliance to these requirements.
So how do you comply? It is easy. All you need is a hygrometer and your auto glass adhesive manufacturer’s minimum drive-away chart.
Let’s say the hygrometer indicates the temperature is 54.7° F with a relative humidity of 48%. The next step is to review the applicable drive-away chart of your auto glass adhesive manufacturer. If your drive-away chart has multiple times indicated, you will need to calculate the applicable drive-away time.
So based upon the hygrometer data shown above, you would first look at the temperature column of 50°F to 59.9°F. Next, you would look at the 40% to 59.99% relative humidity row. Where the column and the row intersect, this cell indicates the applicable drive away time. In this example, the drive away time is five hours.
Knowing the ambient air temperature and the relative humidity is crucial along with your auto glass adhesive manufacturer’s drive away chart is crucial in informing your customer on the appropriate drive away time.
Adhesives Buying Guide
Summarized below are a listing of the major auto glass adhesive manufacturers in the North American market and their products
Dinitrol
Fast Cure Auto Glass Adhesives – 60-minute drive away time or less:
• Dinitrol 9100: All in one modulus, cold applied, fast curing high viscosity adhesive with 30-minute Drive Away Time;
• Dinitrol D-9000: All in one modulus, cold-applied, fast curing high viscosity adhesive with 60-minute Drive Away Time.
Standard Cure Auto Glass Adhesives – Over 60-minute drive away time or less:
• Dinitrol D-501HV: All in one modulus, cold applied, fast curing high viscosity adhesive with 2 Hour Drive Away Time.
Dupont
Fast Cure Auto Glass Adhesives – 60-minute drive away time or less:
• Betaseal™ Xpress 30: All in one modulus, cold applied, fast curing high viscosity adhesive with 30-minute Minimum Drive Away Time;
• Betaseal™ Express+: All in one modulus, cold applied, fast curing high viscosity adhesive with 60-minute Minimum Drive Away Time;
Standard Cure Auto Glass Adhesives – Over 60-minute drive away time or less:
• Betaseal™ U-400HMNC: One component, cold-applied, high viscosity adhesive with 2-hour drive away.
Glass Auto Glass Adhesives:
• Betaseal™ U-418: Primerless to glass, one-component, cold applied, standard viscosity adhesive with 8-hour drive away at 70°F and 50% Relative Humidity;
• Betaseal™ U-428 Plus and U-838: Primerless to glass, one-component, cold applied, high viscosity adhesive with 7-hour drive away at 70°F and 50% Relative Humidity.
Sika
PowerCure Accelerated Adhesive Systems – 30-minute MDAT, 60 Minute OEM Level Cure:
• SikaTack® ELITE: Accelerated adhesive system with 30-minute drive away and 60-minute cure to Automotive OEM level. Enables fast, precise ADAS calibration.
High Performance, Fast Cure Adhesive Systems – 30-minute and 60 minute MDAT Products:
• SikaTack® Mach 30 XV: All in one modulus, cold-applied, fast curing high viscosity adhesive with 30-minute Minimum Drive Away Time;
• SikaTack® Mach 60: All in one modulus, cold-applied, fast curing high viscosity adhesive with 60-minute Minimum Drive Away Time;
• SikaTack® ASAP+: All in one modulus, hot applied, fast curing high viscosity adhesive with 30-minute Minimum Drive Away Time;
• SikaTack® Ultrafast II: All in one modulus, hot applied, fast curing high viscosity adhesive with 60-minute Minimum Drive Away Time.
Primeless to Glass Adhesive Systems
• Sikaflex® P2G: Primeless to glass, one component, cold-applied, high viscosity adhesive with 6-hour drive away at 70°F and 50% Relative Humidity;
• Sikaflex® P2 PLUS: Primeless to glass, one component, cold-applied, high viscosity adhesive with 3-hour drive away at 70°F and 50% Relative Humidity.
Standard Cure Adhesive Systems
• Sikaflex-220+: One component, cold-applied, high viscosity adhesive with 8-hour drive away at 72°F and 40% Relative Humidity.
SRP
Fast Cure Auto Glass Adhesives – 60-minute drive away time or less:
• SRP Velocity30: Universal modulus, cold-applied, fast curing high viscosity adhesive with 30-minute Drive Away Time.
• SRP Velocity60: Universal modulus, cold-applied, fast-curing high viscosity adhesive with 60-minute Drive Away Time.
Primerless to Glass Auto Glass Adhesives
• SRP VelocityProV+: Primerless to glass, one-component, cold-applied, high viscosity adhesive with three-hour Drive Away Time.
Keith Beveridge operates Beveridge Consulting which specializes in strategy development, marketing and operational review. He is a former member of the boards of directors of the Auto Glass Safety Council (AGSC) and the National Windshield Repair Association (now NWRD).
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