An Appropriate Technology Aspirated Psychrometer

copyright 1999 Donald L. Baker

Here it is in the field, mounted to the instrument mast of a pickup camper that was converted into a weather station. The square plate on the back end of the mast boom hides a pyroheliometer for measuring solar flux. That's a three-cup anemometer on the front end of the boom. The box below is the psychrometer, set with the access door facing north. The round, white object on the roof behind the mast is a rain gauge. All the dataloggers for taking weather data and soil temperature and water content are inside. A set of cables runs out a vent on the bottom of this side to a trench where the temperature and water content sensors were buried in a rice field near Wynn, Arkansas.

This does psychrometer suffer from one design error, which can be corrected in a future version. Instead of making the sides fully vented, pink insulation was used. So there is some heat buildup in the box between measurements. But if the fan motor, rated for 14 cubic feet per minute operates at no less than 10 cfm, then it exchanges all the air in the box in about 1.2 minutes. A daylight field test shows that the difference between wet and dry thermistor temperatures settles after about 30 seconds. So the fan motor was turned on about 60 seconds before the measurement.

In the picture on the right, air enters the box through the double set of screened soffit vents in the access door. The vents are set back-to-back so that no heat radiation can enter the box directly. They also keep out insects. The air then enters the main psychrometer assembly through an STP SAF-905 auto air filter on the left. It passes through 3-inch, schedule 40 pvc pipe into a 90-degree elbow, then to a 3-inch to 1-1/2 -inch reducer before the 1-1/2-inch schedule 40 pvc venturi tube. This arrangement is intended to make sure than clean air speeds up and passes over the dry and wet thermistors without unnecessary turbulence. The air passes first over the dry thermistor to get the air temperature, and then over the wet thermistor.

A common one-quart polyethylene kitchen container holds the supply of de-ionized water below the venturi tube. A wick made of washed and clean cotton clothesline rope passes throught a plastic tygon tube from the water supply to a pipe nipple in the bottom of the venturi tube. The tygon tube helps keep the wick from becoming contaminated and the supply from evaporating. The polypropylene core of the clothesline has been removed so that it can pulled completely over the wet thermistor and tied on with sewing thread. As air is pulled over this wick, it cools by evaporation to a temperature lower than the incoming air, determined by the relative humidity of the air. It is possible to use tables or equations to calculate humidity from the dry and wet thermistor readings.

The air is pulled through the pvc tubing by a brushless 12VDC fan, (NMB Technologies model 2410ML-04W-B10P00, from Newark in Tulsa), situated in the top of the vertical exhaust tube on the right. At $11.70, the fan was the most expensive part of the construction. A set of double screens in the bottom of the exhaust keep insects from entering it.

The thermistors are very small Keystone Carbon Company bead thermistors with teflon-coated leads, model RL0503-5820-97-MS, 10kW at 25 degrees Centigrade, which cost $2.06 each. If they are calibrated in a bridge circuit over a 0 to 50 degree centigrade range, a fifth-order correction polynomial can produce measurements better than 0.005 degrees C.

Now let's look at construction details to see how tough this unit is to build with common materials from places like Lowe's and Wal-Mart. The picture below shows the access door with the outside soffit vent. It is simple plywood with two hinges and 1x2 inch braces. Please bear in mind that I'm only as good a carpenter as I need to be. Many could do better-looking work.

The next picture shows the bottom of the box. On the left, a hatch makes it possible to remove the air filter if it gets dirty. The hatch is closed with a hasp, and lock, if necessary. On the right a simple hook and eye latch secures the door.

The next picture, on the left, shows the back of the box with two muffler clamps to attach it to the weather instrument mast. The next picture on the right shows how the air filter is attached (horizontal photo below). A short length of 3-inch pvc tube is connected to a 90-degree elbow coupling with sheet metal screws. The coupling forms a stopping shoulder for the air filter, into which the short tube slides. A metal hanger, screwed inside the short tube, holds a brazed nut for a thumb screw. The thumb screw (see horizontal photo below) passes through a plywood disk and a 3-inch test cap that just happens to fit on the end of the air filter to seal it.

The right of the horizontal photo above also shows how the pvc assembly is attached to the box. The elbow fits into a wooden cradle with a half-moon cut out. It is secured to the cradle with eye-screws in the elbow and the cradle and common plastic cable ties.

The next picture shows the thermistors in the venturi tube. An oval has been cut out of the front of the tube for access, and screwed to a piece of aluminum from a soft drink can. The black dot on the left of oval hole, inside the tube, is the dry thermistor. You can see the cotton wick on the right of the oval hole as it comes through the bottom of the tube and goes over the wet thermistor.

The next photo shows the thermistor cable on the back of the venturi tube. It is secured with small plastic cable ties, barely visible in the picture above, through holes drilled in the tube. In this picture the oval hatch with the aluminum can metal is just visible over the hole. One can also see the two sheet metal screws used to secure the venturi tube between the reducer couplings. Since this assembly is not exposed to mechanical stresses, only a few screws are needed to hold it together.

The aluminum flashing visible in the top of the picture above is used to both to cover the oval hatch and to make a grounded electronic shield around the thermistors. In the picture below, it is secured with a hose clamp. The pipe/tubing nipple for the cotton wick is also clearly visible.

The picture at the left shows the fan installed in the 3-inch pvc exhaust tube. The corners of the fan mounting ears have been cut off, and the fan has been screwed to a 1/8-inch plexiglass plate. The plate in turn had been screwed to the end of the exhaust tube. The end of the tube had been inletted with a rotary tool to hold the remaining mounting ears of the fan, so that the plexiglass plate can sit flush on the tube. The fan cable passes through the exhaust tube wall, with rubber and pvc electrical tape wrapped around it on both sides of the hole to keep it in place.

The bottom end of the exhaust tube passes through the box, where a butt-splice coupling slips over the end and is secured with sheet metal screws. This coupling holds a modified test cap on the end of the exhaust tube. The test cap has been cut open with a hot knife, and two layers of screen wire cover the hole. They are secured in the test cap with hot-melt adhesive, which is also very useful in sealing electrical connections under heat-shrink tubing.

As one can see, this can all be constructed with ordinary shop and electronic hand tools. The physical skills are within the grasp of junior high students, especially if presented as kit instructions, with drawings and measurements. Well-motivated high school students, given the opportunity, are capable of grasping the necessary theory to calibrate and run this kind of instrument. A cooperative effort between the University of Arkansas and the State's secondary schools can produce many more instruments like this using appropriate technology, and eventually connect them, as is being done now in Georgia, into an Internet-based weather system. Doing this now in the public secondary schools, just because we can, will raise the average understanding of math and science, which will, in turn, allow educators to raise the bar.

The entire National Academy of Sciences, with all its learned men and papers, can never have as much effect in motivating students as a set well-presented and accessible science experiments. For those of us old enough to remember "Watch Mr. Wizard", Don Herbert taught that eternal lesson.

Acknowledgements

I would like to thank and acknowledge Dr. H. Don Scott, of the Agronomy Department of the University of Arkansas, Fayetteville, for the opportunity to work on the summer project that allowed this instrument to be built.
The project involved placing temperature and water content sensors in a trench in a rice field near Wynn, Arkansas, and backing them up with weather data. Only three weather instruments were available, a rain gauge, an anemometer and a pyroheliometer. All were in poor repair, having been left in the field for an extended period, and needed fixing. The anemometer ball bearings, for example, had rusted. It was interesting to find that they could be restored by dropping them into a test tube of 3-in-1 machine oil and placing the test tube in an ultrasonic cleaner, until the oil turned red. After about three test tubes of oil, they worked pretty good.

After investigating the possibility of building a shack for the data recorders, it seemed that the best choice was to buy and used pickup camper and refurbish it. It was simple enough to transport it to the rice field, lower it down on a dry-block foundation and tie it down with mobile home strap and anchors. Other improvements were made as time allowed, including the addition of the psychrometer. Click here for another picture of the camper

The camper provided a place out of the rain, sun and mosquitoes to sit down, hook a laptop computer up to the data loggers and download the measurements. All it needed was an air conditioner and a mud room. The camper itself wasn't high technology, but it was fun to set up. And Dr. Scott allowed the latitude to experiment that a limited budget required.

This page designed by

Donald L. Baker

2000 West Maine Street

Fayetteville AR 72701 (Previous Address)

 

Ó 1999 Donald L. Baker