Modernization of a Bendix fan-driven Psychrometer
- From: "Chuck Olson" <chuckolson01@xxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 2 Mar 2009 00:50:35 -0800
First of all I apologize for misspelling the Key Word in the title of my previous post of 2/24/09 - - so much for guiding anyone to recent thinking about a Fan-Aspirated Psychrometer.
I see a number of these excellent instruments appearing on Ebay at various times, so maybe the steps I took to modernize certain aspects of their construction would be of interest to new and old owners alike. The unit I bought had a terrible vibration from an unbalanced, noisy motor driving the fan blades and of course the backlight behind the thermometers was a standard 6.3-volt bayonet-base light bulb, the ubiquitous GE #47, drawing 50 mA, with its intensity controlled by a 40-ohm linear dimmer control with switch. Designed in the mid 1950's, it used what was readily available in those days, and successfully so, based on the likely number still going strong, showing what liars many current-day hygrometers are. I know - - I have a half dozen of them, from German-made 130mm dial indicators to liquid crystal multi-displays of temperature, humidity, barometer, pressure history, and time. The electric fan driven psychrometer is definitely a step-up from a sling psychrometer, since you can watch the wet bulb temperature drop and stabilize in comfort and you control what air is actually sampled. It's portable, well designed and assembled as a quality instrument.
I changed four things - - the fan, the backlight, the fan mounting-panel and the dimmer control while keeping the 4.5-volt power source unchanged. This cut down operating current from 680 mA to about 70 mA, and greatly extended battery life. I also added a small improvement - - a variable-length post to lift the front so seated viewing at a desk can be optimized. The thermometers have built-in cylindrical magnifiers that require the viewer's eyes to be at a certain level in front of the display to make the mercury columns easily seen and relatable to scale values, so I decided to aim the display rather than require positioning of the viewer's eyes.
I found a somewhat rare 5-volt brushless fan, similar to what is used in computers these days, that fits perfectly in the existing space in the instrument - - a Sunon GM0535PEV2-8, 35mm by 35mm by 6mm fan for $11.71 at Newark Electric, Stock # 96M8146. This fan is thinner than most, which leaves the largest possible opening for exit air out the rectangular openings near the fan. I mounted it on 3/16" hex posts threaded 4-40 and stacked up to a 1.279" column measured from the new 2.276" by 1.523" rectangular panel behind the fan. Looking at the thermometers, the fan center must be located 0.875" from the left edge of the panel, which requires its mounting posts to be 0.848" from the right edge, and I put 1" high electrical contact posts 0.543"away from the right edge of the panel to press on the main contact strips for fan power. The new panel was made from 1/16" copperclad epoxy-fiberglass and the connection posts to the motor were isolated by engraving a groove in the copper around the connection posts and soldered wires with a Dremel Moto-Tool.
For the backlight I found a pair of white LEDs out of a lot of 15 pieces that lit equally bright when in a direct parallel connection, and they required a 120-ohm resistor in series with the dimmer control to limit the current at its highest setting to about 5 mA, for excellent brightness. I also changed the dimmer to 3500 ohms, which was the closest I found to the 1000 ohms needed for a good brightness range, and which also included a power switch. I sharply bent the LED leads and soldered them together compactly so they would fit in the lamp well, each facing away from the other. I made sure the heat of soldering didn't exceed the permissible temperature of wires into the LEDs by gripping the wires with pliers on the LED side of the soldering operation. Another copperclad board was required to mount the LEDs and current-limiting resistor, and that was supported with a couple of right-angle brackets screwed to the metal piece the original lamp was clipped onto so that the LEDs on long leads would stick 1/4" up out of the case into the lamp cavity and illuminate the length of the thermometer housing.
The last item was the external post to lift the front for viewing. I screwed it to a 1/8" thick plastic panel stuck to the bottom charts with double-sided foam tape rather than drill and tap through the charts. The length of the post depends on the distance you need for optimum visual focus on the thermometer marking when comfortable at your desk. I made mine variable with telescoping sleeves and an internal 8-32 thread that allowed 0.6" of height range, but a fixed height post would probably be just as useful since objects can be placed under the foot of the post to adapt it to different distances. My vision required the unit to be supported at a 31-deg angle, with a post 2.1" long located 3.65" from the bottom back edge. The polycarbonate panel I used was 2.5" by 3.5" secured by three 1" squares of foam tape - - one under the post 1/2" from the edge, and two in the back corners - - seemed strong enough.
I enjoy the unit's quiet, brightly lit operation while I'm comfortably seated at my desk, and anticipate over 200 hours of operation from a $4 set of alkaline D-cells. If anyone cares to resume production of this excellent instrument, these changes should give it new life as a modern product.
.
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