The Humidity-Measurement Laboratory Plan for College Students

1. Introduction

A humidity-measurement laboratory plan was developed within our 'Earth Science Experiment’ course; which consists of 15 3-hour lab experiments. This course covers fundamental experiments in geology, meteorology, oceanography and astronomy.

Earth Science Experiment is a compulsory course for a junior-high school science teacher certificate because a licensed science teacher teaching junior-high school weather requires an understanding of humidity measurement. We develop a laboratory plan for measuring humidity for deepening students' understanding of humidity.

2. Background of this study

Students of the 'digital generation' tend to consider digital measurements to be more precise. For example, a bar thermometer is accurate to within 1/2 - 1/5 degree versus 1/100 degree for a digital device. Also, students of the 'game generation' basically do not read manuals. The manual of a digital hygrometer indicates that the measurement accuracy is about ± 5%. Under such circumstances, it was important to intentionally offer a situation that requires one to think about a measurement's accuracy and uncertainty. Therefore, the humidity-measurement lab was aimed to let them consider differences in operators, equipment and measuring method.

The 'Meteorological Observation Guidelines' prepared by Japan Meteorological Agency, states that the accuracy of a portable psychrometer is within ± 2% versus an average 10% variation within students’ experiments. There are at least a couple of reasons for this: a) it is caused by not using an instrument with the certificate; and b) it is not possible to calibrate the instrument (thermometer) beforehand. However, we consider that the difficulty of measuring humidity as the main cause of their variation; and hence, students should consider such difficulties.

IPCC AR5 described that the water cycle is expected to intensify in warmer climates because warm air can hold about 7% more water vapor for each degree Celsius of warming. Also, IPCC AR5 showed increases in surface and lower atmospheric water vapor based on the satellite observation since the 1970s. These explanations may be mistaken to mean that ‘the amount of water vapor has increased by 7% due to global warming’. These observations only refer to satellite measurements instead of the much-longer ground-based measurements.

When we look at temperature, relative humidity, and vapor pressure changes at the Yokohama Local Meteorological Observatory since 1931, we noticed that the rise in temperature is remarkable; albeit the vapor pressure had fluctuated year by year and the relative humidity tended to decrease. This could be due to the temperature rise and the definition of ‘relative humidity’. And hence, it’s isn’t easy to confirm that the increase in water vapor is due to global warming via direct observation. Humidity experiments are considered valid to cultivate climate literacy which requires the ability to judge, based on scientific evidence.

3. Methods of humidity measurement and results

The main purposes of this experiment is to understand humidity indicators and its measurement method and to acquire the measurement skills. The following methods were used: relative humidity (%), dew-point temperature (Celsius), and volumetric humidity (mg/L). The following tools were used: an August psychrometer, a sling psychrometer, a Lambrecht dew point hygrometer, a water vapor detection tube, and a Vernier’s Relative Humidity sensor. In the fall semester of 2017, thirteen students divided into six groups within which experiments were conducted. The results shown below are average values of the six groups.

Approximately 50 g of silica gel was placed in a syringe (50 mL) and a silicon stopper with a hole was placed as its lid. Then air was pushed with a syringe of a syringe (100 mL). The syringe’s mass was measure per liter of air pushed in; and hence, the volumetric humidity of air was estimated to be 12.9 mg/L based on the amount (10 L) of the pressed air and the increased mass.

The air was measured to be 24 C with 75% relative humidity via a August psychrometer on the wall; and based on these values, volumetric humidity was computed to be 16.3 mg/L. A sling psychrometer determined the air to be 24.1 C with a 69.4% relative humidity; and based on these values, volumetric humidity was computed to be 15.2 mg/L. The relative humidity measured by swing-type psychrometer under the wind was low even in the laboratory.

The Lambrecht Dew-Point Hygrometer and Ether showed the dew-point to be 13.46 C with the volumetric humidity of 11.7 mg/L; which was the lowest value of their measurements. This is perhaps due to the difficulty for students to determine the timing of dew condensation. Another possible reason is due to structural problems of the Lambrecht Dew-Point hygrometer such as the temperature difference between its metal surface and the bar thermometer.

The Kitagawa-type gas-detection tube applies the fact that water vapor reacts with magnesium perchlorate to form an alkaline double salt which changed the indicator color. This tube detected 22.0 mg/L of water vapor; corrected per 24 C to show a volumetric humidity of 17.4 mg/L which was the highest value of their measurements. The measurement read the boundary of the discolored layer, but the boundary becomes unclear due to the influence of the interfering gas. And that is the reason why this method gave them a higher reading value.

The students shall share the measurement results per group after the experiment and write a Lab Report. A humidity estimate per measurement inside the earth-science laboratory; and an analysis of the spatial distribution and the representativeness of humidity of different environments (i.e., laboratory, building and outdoor) are also required.

4. Conclusion

It’s theoretically possible to show the true value of humidity via a calibrated instrument. However, the actual value may be different per measurement method; and we’re not sure that the water-vapor distribution within the laboratory is uniform. Students tend to deepen their understanding of humidity and its measurement by measuring humidity directly and indirectly. We hope that students understand humidity’s difficult nature through our experiments.

The purposes of the proposed humidity-measurement laboratory plan for college students are: the understanding of humidity definitions and methods, acquiring measurement skill, and considering measurement error and uncertainty.