Converting Specific Humidity to Relative Humidity

Quote of the Day

Keep away from people who try to belittle your ambitions. Small people always do that, but the really great people make you feel that you too, can become great.

— Mark Twain. As always, he said it just right.

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Introduction

I often have to interpret odd test requirements. In a test specification based on GR-487, a humidity test is called out where we need to have a fixed specific humidity (i.e. 24 grams of water vapor per per kilogram of air). For a given specific humidity, the relative humidity will vary with temperature. Since my test gear can control relative humidity, I need to derive a relationship between relative humidity and the specific humidity, which I show in Figure 1.

This was a quick Friday afternoon calculation. I was a bit surprised that I could not find a table that contained this information – I pulled out Mathcad and solved the problem on my own.

Here is my Mathcad work if you are interested.

Background

GR-487 Test Requirement

Here is an excerpt from GR-487 that calls out the specific humidity requirement.

At temperatures above 32° F (90°F), the relative humidity may be limited to that corresponding to a specific humidity of 0.024 kg of water per kg of dry air. During the period of descending temperatures – i.e. 32° C (90°F) to 4.4° C (40° F) – the relative humidity shall be 80-95%.

There is a small error in this statement. Strictly speaking, the requirement is specified in terms of kg of water vapor versus kg of dry air. Technically, this term is known as the mixing fraction, but it is very close in value to the specific humidity. Both terms are described below.

Definitions

mixing fraction (symbol w) AKA Humidity Ratio (HR)

Ratio of the mass of water vapor to the mass of dry air. It is often expressed in terms of gram of water vapor per kg of dry air. Symbolically it is defined as , where m_v is the mass of water vapor, and m_d is the mass of dry air (Source).

Dew Temperature (symbol tdew)

The dew temperature is the temperature at which dew forms and is a measure of atmospheric moisture. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation (Source).

Relative Humidity (symbol RH)

Relative humidity is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. Relative humidity depends on temperature and the pressure of the system of interest (Source).

Absolute Humidity (symbol AH)

Absolute humidity is the total mass of water vapor present in a given volume of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C (Source).

Specific Humidity (symbol SH)

Specific humidity is the ratio of water vapor mass (m_v) to the air parcel's total (i.e., including dry) mass (m_a) and is sometimes referred to as the humidity ratio. Specific humidity is approximately equal to the "mixing ratio", which is defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel. We can express the specific humidity as .

Analysis

Relative Humidity Given Dew Point and Temperature

Figure 2 shows how I computed the relative humidity given the air temperature and dew point. For references, see the documents in Appendix A or click on the links in Figure 2 (brown color).

I perform a verification of this routine in Appendix B.

Calculation of Specific Humidity (SH) Given RH and Temperature (T) and Pressure (P)

Figure 3 shows how I calculated the specific humidity given the air temperature and relative humidity given the model in this paper. Here is a numerical example that I used to check my result. Further checks are done in Appendix C.

In Appendix D, I compare the the formula from Figure 3 to a standard psychrometric curve that relates the humidity ratio (HR) = SH/(1-SH) to relative humidity and temperature. The agreement is good.

Calculation of RH Given T and SH

In Figure 3, I have a relationship between SH vs RH, T, and P. In Figure 1, I need to invert this relationship to obtain RH vs SH, T, and P. I perform this inversion numerically (i.e. root function) in Figure 4 to generated Figure 1.

Conclusion

Our testing of an assembly was held up while we discussed the required RH required. I view this as another example of the endless number of unit conversions that I end up doing.

Appendix A: Key Reference Material

I am going to use Appendix A to store some useful reference material.

• The Use of Dew-Point Temperature in Humidity Calculations
• Humidity Conversion Calculations
• Good Lecture on the Topic
• NASA Technical Note on Humidity from Dew Point

Appendix B: Reference Material For Checking Results

Figure 5 shows a table of dew points that I found on the web. I will duplicate that table using my routine for computing relative humidity given temperature and dew point.

Figure 6 shows the same type of table generated by my Mathcad routine. The agreement at high temperatures (>20 °C) is excellent – less so at low temperatures. Since my GR-487 work is at high temperature, my results will be accurate.

Figure 7 shows the Mathcad code that generated Figure 6.

Appendix C: More Reference Material For Checking Results

Figure 8 shows a simple comparison of a table of specific humidity values generated using my SH formula for 100% humidity at various temperatures. A comparable formula from the web is also shown.

Figure 9 shows the Mathcad code that generated the results of Figure 8.

Appendix D: Psychrometric Curve vs Fig 3 Formula.

Figure 10 shows a comparison between the formula shown in Figure 3 and a standard psychrometric chart.