Applied Psychrometrics – the Psych Chart Dry Bulb Temperature Axis (Plus a Bit about Dry Bulb Thermometers)

This post looks at dry-bulb temperature and how it is measured.  Then, it looks at how you plot a dry-bulb temperature on a psych chart.  The links below will jump you to the various topics in the post.

What Is Dry Bulb Temperature?

Dry bulb temperature is a way we measure sensible energy;  i.e. energy we can feel, and is something most people are very familiar with, even if they are not in the HVAC field.  The symbol tdb or tdrybulb is often used to represent it.  It is a direct measure of the kinetic energy (energy of motion) of a substance.  If something is heated, the dry bulb temperature increases and if something is cooled, the dry bulb temperature decreases. 

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How is Dry Bulb Temperature Measured?

Dry bulb temperature is measured pretty much as the name implies;  by a thermometer exposed to dry air.  Now days, we often measure dry bulb temperature with electronic instrumentation.  But back in the olden days we used a liquid in glass thermometer. In the really olden days, as in when the thermometer was invented, it was air and water in glass and based on the thermoscope invented by Galileo.   That device was subject to error due to variations in air pressure and the design evolved to a sealed liquid in glass thermometer

Gabriel Fahrenheit was the first person to use mercury in glass, which, when combined with better glass making techniques, made the instrument much more accurate.  But it turns out that mercury is fairly hazardous to handle, so they are less common these days, especially out in the field.

While I have a number of electronic temperature measuring devices, like my data loggers …

Hobo External Temp RH - Front

… and my 4-in-1 …

4 in 1 02

… I also own a number of lab grade liquid in glass thermometers. 


What I like about them is that they are based on a fundamental principle,  And, they don’t need batteries.  So, they are good way for me to check some of my other instruments and can be used to provide a “third opinion” if I am wondering which instrument is right.

The longer thermometers are generally expanded ranges with half degree graduations. In the picture above, from top to bottom:

  • H-B Instrument 6/0124 Durac Plus Precision Liquid-In-Glass Partial Immersion Thermometer, Organic Fill with White Back Glass, 30 to 124°F, 460mm Length, 76mm Immersion, 2°F Accuracy, 0.5°F graduations.
  • H-B Instrument 6/0214 Durac Plus Precision Liquid-In-Glass Partial Immersion Thermometer, Organic Fill with White Back Glass, 30 to 214°F, 610mm Length, 76mm Immersion, 2°F Accuracy, 0.5°F graduations.
  • Pair of Fisher Scientific Mercury thermometers, 0-300°F, 30mm Length,76 mm Immersion Length, 2°F Accuracy and Graduations. Given that these are Mercury thermometers, I don’t take them out on the road with me anymore.

If you look closely at the left side of the thermometers, you will notice they all have a line on them that is typically several inches up from the tip.  If you are going to measure something besides air, you need to immerse the thermometer in what ever it is you are measuring up to to that point to get an accurate measurement.  Otherwise, conduction and other factors might influence the reading.

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Thermometer Accessories

The thermometer well I discussed in my previous post on thermal lags is a very common thermometer accessory, but there are a few others that are not as common which are shown in the picture above.

The gray PVC conduit is my home-made storage case for the shorter thermometers towards the front.  The thermometers come in a plastic tube that is inside the bubble wrap.  The bubble wrap is a slip fit inside the PVC conduit and the plastic caps have sponge stuffed in the ends and are a friction fit on the conduit.  I travel regularly with them and by far and wide, it is more likely one will be broken during use than during transport, which is one of the down sides of using them.

The silver colored device on the top thermometer is a magnifier that slides up and down the thermometer and makes it easier to read the them.


They all come with certificates of traceability which tie back to a NIST traceable thermometer. 

DSCN6635 - End 2014-04-06

This can be an important thing to have if you have to certify readings.  You may even need to have you’re thermometer calibrated and provided with its own multipoint calibration certificate as illustrated in this sample from the H-B Instruments web site.

H-B Instruments Sample Calibration Certificate

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The Galileo Thermometer

My least portable but most colorful and fun thermometer hangs over my desk in the form of a Galileo thermometer.

DSCN6637- End 2014-04-06

The colored bulbs have different densities and thus, float up or down as a function of the temperature of the fluid they are in.  At the time I took the photo, the 72°F bulb was still floating and the 68°F bulb had sunk, so the office was someplace between 68°F and 72°F. This video clip shows a time lapse of the response of a smaller Galileo thermometer I had when I heated it up with a hair dryer if you have never seen one work and are curious.

A Galileo Thermometer in Action

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Plotting Dry Bulb Temperature on a Psych Chart

The dry bulb temperature axis on a psych chart is the horizontal axis across the bottom of the chart, as illustrated below.


Constant dry bulb temperature lines are nearly vertical lines that extend from the bottom of the chart to the saturation curve.  As you move from left to right across the chart, the temperature and energy increase.

Since a psych chart is a multi-dimensional graph, its true value shows up when you add a second coordinate and plot a point.  If you have any two coordinates, for instance dry bulb temperature and relative humidity or dry bulb temperature and wet bulb temperature, then, once the point they represent is plotted, all of the other psychrometric properties for the point can be read from the chart.

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In the next post, we will look at wet bulb temperature and then use wet bulb temperature and dry bulb temperature to plot a point on the chart, allowing us to read other useful parameters like dew point temperature, enthalpy, specific humidity, and relative humidity.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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