Dew Point And Vapor Pressure

Volume 137, No. 5, Friday, July 24, 2015

As an unshakable idealist, I always imagine that revelation comes in the form of an epiphany, something climactic. But based on experience it almost never does. One such moment, however, occurred for me some five or so years ago. After the initial receipt of new knowledge there was a period of assimilation, owning the new knowledge, reflecting on why I never knew this before, and why something so clear and obvious was not common knowledge.

Throughout my cheesemaking career I have witnessed an ongoing need for aging room spaces that work. Aging room spaces were not uncommon but the conversation around these rooms and their function always seemed to be about how to make them operate better. It didn’t seem to matter if they were high humidity or lower humidity, on some level they were all sub-optimum.

The control philosophy for those old rooms was based around the use of a horse hair mechanical humidistat and verification with a sling psychrometer, both items that are today almost curiosities. Early rooms used refrigerant coils. The refrigeration contractor of the day was not an average contractor. He enjoyed working on our projects.

All I remember is the rooms were frequently too dry, the air movement seemed high and not uniform in the room, and the great innovation of the day was the addition of reheat coils which we used in our drying rooms as a source of free heat. Little did I know that most of the free heat was wasted, forcing our drying rooms to operate in temperature mode when they should have been operating in latent mode.

For years I felt that I understood a little about how to make an aging room work with a focus on temperature differential across the coil, minimizing it for rooms with high humidity, maximizing it for the removal of moisture. Air movement requirements were never clear. Low air movement aided in the removal of moisture, higher air movement for smear cheese, lower air movement for mold ripened.

To this day there is a lack of definitive information readily available for cheese makers and manufacturers. It is always interesting to hear that cheese makers believe they will have a wetter room if they reduce air movement. This is not correct; slower air moving over a coil will remove more moisture

Working with cheese makers, charcuturers and others who want ideal specialized environments, it was always a frustration to not be able to answer some key questions about how to create the ideal environment. I include myself in the cheese maker category because there was nothing in common text. We often deferred to suppliers to solve our problems, problems that we as professionals should understand.

You would think, after all the effort that has gone into the production of aged cheese, some with clean rinds, some with mold, some with bacteria, that a matrix of empirical learnings would have been created, something we could tattoo indelibly into our database of solutions. We just are not there yet.

An aging room that works is the sum of many parts but not the least is the control philosophy. After 30 years of working with a control philosophy, simple thermostat to drive temperature up or down, a custom coil, a humidistat to activate either heater or humidifier, this was roughly the process. The end result was a marginally controlled room. A series of systems that often ended up hunting, one system responding to another, the rooms worked sort of, each system being triggered at the point the value was veering out of control.

Then on a job I am introduced to an oracle, someone who speaks on the topic with an authority never heard before. I heard, “You cannot control humidity using a humidistat”, and I feel an emotion occurring that I don’t like.

You cannot control humidity using a humidistat? It had always been the device that provided me with the key piece of information: too high add heat to make the cooling/drying part of the room function to work and keep the room in the sweet spot. Too low start the humidifier and add vapor back, frequently this system was compensating for the side effects of the cooling system. The heat was added to compensate for the side effect of cooling when the room was being dehumidified.

Cooling coils are a blend of sensible and latent cooling, sensible is cooling of the air, latent is the removal of water from the air. When using coils it is often difficult to separate the two functions.

Humidity is one of those things that few understand well. Even less so when we talk about relative humidity, a value which compares the actual humidity in air compared to how much it could carry at saturation. Relative humidity is a value that you can have at two completely different temperatures and in each instance the amount of moisture in the air is very different at each value. 90% relative humidity at 90 degrees has more water suspended in the air than 90% relative humidity at 40 degrees.

As craftspeople aging food in controlled environments, this is something we should understand. It should be a fundamental block of knowledge, not a black art. Physics and microbiology should lead us down a well illuminated pathway, not the dark rabbit hole so many find themselves in.

Our products are comfortable when we have the correct temperature/dry bulb and correct RH/relative humidity, e.g. 53 F and 89.51% RH, change the temperature to 52 F and the RH% jumps to 94. When you consider that most thermostats controlling temperature have a typical differential of 2 F min., change that temperature to 51 F and the RH% jumps to 96.38. That is a swing of 6.87% for a two degree change in temperature. To reach this conclusion I needed to know one piece of information.

Relative Humidity is a derivative number, you have to know two things in order to calculate Relative Humidity, 1. Temperature, 2. Dew Point. or Wet Bulb. Relative humidity can be calculated from Temperature/Dry Bulb (the same thing) and Dew Point or Wet Bulb. However Dew Point is more useful to this author. Both Dew Point and Wet Bulb are part of the psychrometric chart along with Dry Bulb, all occurring on straight lines. Only Relative Humidity has a curved line.

Developing equipment for measuring the wet bulb temperature occurred early in the development of psychrometrics, a fabric tube covering the bulb of a thermometer recorded the effect of evaporative cooling, depressing the temperature shown on the thermometer.
Dew Point is another story. Dew Point is the point at which moisture will condense out of the air, to demonstrate this placing an object which is below the Dew Point in a space will see it become covered with condensation.

How on earth do you make a piece of equipment that will measure Dew Point? There are two common methods/instruments today, chilled mirrors or capacitance sensors. A chilled mirror is a surface that can be heated and cooled, as it cools condensation will form, a light beam will be refracted and picked up by a sensor, the instrument notes the temperature at which this occurs.

Another method is a capacitance sensor, a polymer which will provide an electrical output at different combinations of temperature and humidity, from this data the Dew Point can be calculated. Chilled mirrors are expensive but very accurate enabling measurements to tenths of a degree. Capacitance sensors cost less and accuracy to plus or minus 2 – 3 degrees F. Capacitance sensors are calibrated against a Chilled mirror standard. A couple of years ago in Dallas, TX at the ASHRAE conference there was a manufacturer demonstrating a Chilled Mirror Hygrometer, price $50,000.00. To be fair this was a laboratory only item for calibration purposes. Lesser chilled mirrors retail for around $3,000 or $4,000.

So why is Dew Point more interesting than Wet Bulb as the other half of the Relative Humidity equation? Vapor Pressure is the barometric pressure in the room, the reason a piece of cheese loses or gains moisture is due to the differential vapor pressure between the cheese and the air around it. When the vapor pressure inside a cheese is greater than the vapor pressure around it, it dries out. The greater the difference, the faster it dries out.

The way to change the vapor pressure in an aging room is to change the Dew Point. If you carefully control the temperature in an aging or drying space and control the Dew Point at the same time you will have the desired humidity. Gaining an appreciation of Vapor Pressure and how to control it has been a revelation. Moving the focus away from relative humidity and temperature and controlling temperature and dew point has produced better results. Add in fully modulating control and you no longer have a series of events happening out of sync, each with their own differential.

So back to the point… Was the reason we never spoke about Dew Point because it was not practical to measure? Today it is practical and can be done with relatively inexpensive sensors. These sensors will have similar or greater accuracy than a Wet Bulb sensor, are available and reasonably priced.
Dew Point and Vapor Pressure travel together. Dew Point and Dry Bulb are Relative Humidity.

To learn more about how this knowledge can be applied in your aging room, visit our booth at ACS in Rhode Island next week.


Neville McNaughton, president of Cheez Sorce, St. Louis, MO, has many years of experience manufacturing dairy products in both New Zealand and US. He has been a judge at several cheese competitions. Neville will be writing a regular column in Cheese Reporter and will take any questions regarding cheese manufacture. You can reach him at His column is graciously sponsored by Hypred USA

Other Neville McNaughton Columns
Cheese Makers Get Organized: ACS Is Your Best Networking Opportunity of the Year
Going Beyond Legal: Grandfathering:
In Defense Of Cheese And Salt
Captain of Your Vat
Designing A Farmstead Cheese Operation
Judges Recognize High Art in Cheesemaking
Cutting the Curd
One Day Analysis - Beyond Being Legal
The Shape of Cheese
Adding the Extra Column
Pasta Filata Cheese
Very Hard Cheese
Hard Cheese with Eyes
Gouda, Edam And Other Washed Curd Cheese

Other Cheese Reporter Guest Columnists
Visit Dan Strongin
Visit John Umhoefer


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