Portable Hive Scale


The goal is to build an accurate, electronic bee hive scale for under $50 that allows anyone to weight 4 hives per minute – up to 250 lbs each – without materially disturbing the colony.


Problem

In my first year as a beekeeper, I had 2 out of 3 hives swarm. I think. I experienced a Tulip Poplar nectar flow. I think. I saw bees gather nectar – some days more than other days. I think. I say, I think, because I am led to believe that these things happened and I saw evidence that they did occur but I cannot be sure. And if they did occur, I cannot tell you if it was more or less than previous occurrences. But if I could have weighed the hive once or twice a day, I would have known for sure:

  • I would know the population of the runaway swarm …estimated at 3500 bees per pound.
  • I would know the mass of nectar (and pollen) gathered during the day and of water evaporated at night. One pound equals roughly 1.04 US pints. (Note to physics nitpickers: I know the difference between force, mass, and volume. STP is assumed here.)
  • I would know the number of bees foraging by monitoring the loss of weight in bees leaving in the morning.
  • I would know the rate of growth of daily nectar collection as a nectar flow began.
  • I could compare my hives with the hives of others and with my own hives in previous years.

Lord Kelvin said, “To measure is to know.” If I could weigh a hive, I would know a lot more than I do now…


Prior Art

Hive on a feed scale

Probably the most common way to weigh hives today is to set them on old iron grain or feed scales bought at antique shops or online at eBay or Craigslist. There is currently a NASA-sponsored nation-wide research project (http://honeybeenet.gsfc.nasa.gov) that asks volunteer beekeepers to take daily measurements of their hives on feed scales. The data is used to estimate when nectar flows begin in order to answer how changing climate effects honey bees.

There are several problems with the feed scale approach:

  • There are not enough antique scales on Craigslist or eBay for every beekeeper that wants one.
  • Scales are expensive. Based on my eBay and Craigslist searches, a workable scale is at least $200 or more not counting shipping or transportation.
  • Accuracy. These cast iron scales sit outside all day and rust. I have to wonder if that effects accuracy and uniformity in some way.
  • Setting a hive on a scale rules out the use of a screened bottom board. I believe this increases temperature and humidity in the hive and thwarts the bee’s natural hygienic behaviors.

There have been other approaches to weighing hives – all seem to require two people or a customized vehicle.

Patent drawing for hive scale

There is one patent that caught my eye. US Patent US4047584 (Click here to view the patent) filed in 1977 by Daly suggested that a split tongue with a strain gauge be inserted into one side of the hive to measure half the hive’s weight. That expired patent and my discovery of inexpensive electronic luggage scales inspired my idea for an electronic pry scale.


Pry Scale Design Concept

In the last year or so, I have seen electronic luggage scales being offered for $25 or less. You attach your luggage to these hand-held devices and lift them with the luggage attached. The scale beeps or stops changing value and then you read off the weight. Their maximum range is about 125 lbs – less than a lot of bee hives. I wondered if I could use this as a starting point for a bee hive scale.

One clever observation of the patent described above is that it simply weights one edge of the hive – if the weight is distributed symmetrically about the centroid or center of the hive, then the force required to lift one edge of the hive will be half of the total weight of the hive. That makes our little luggage scale capable of weighing a hive up to 250 lb!

Luggage Scale

Luggage Scale

Confused? Imagine I weigh 200 lbs. If I stand upright with my weight distributed equally on both feet, then each foot applies a force downward of 100 lbs. You could estimate my total weight by simply weighing the force under one foot and multiplying by two. Tilting the hive up on one edge accomplishes the same thing – assuming that the hive’s internal mass is distributed symmetrically around the center of the hive and that I am not tipping the hive more than 1 or 2 degrees – a pretty fair assumption that we can test.

I imagined a scissors-like tool to pry up one edge of the hive. But instead of the working end closing (cutting), it would be prying open a narrow gap. The trick is to measure the force required to lift one side of the hive. Once it is lifted or tipped up one side, that force required to keep it from falling would equal half the weight of the hive.

Pry Scale Prototype

Pry Scale Prototype

The design I came up uses the luggage scale to pull a center tongue upward, away from two flanking stationary tongues. To make it all a little more compact, a pulley is used to redirect forces 90 degrees. I call it a pry scale because it measures the force required to pry two faces apart. Since the force required for lifting one side of a hive 1/8 inch is not materially different from lifting it ½ in or 1 inch, the movable tongue need not move very far at all. That presumes that the transmission of force, from the prying tongue to the luggage scale, does not stretch. That is why we use 1/16″ coated steel cable with crimp connectors.


Parts List & Cost

Quantity Description Unit Cost ($) Total Cost ($)
3 6” steel mending plate (bar) to be used for station and movable tongues 1.58 4.74
1 2-1/2” utility nylon pulley. The pulley came mounted on a hanger. I removed this by drilling out the rivet holding the axle on. 2.74 2.74
2 2.5 x 1.5 corner brace. Used as pillow blocks to hold the pulley. 3.77 7.45
2 ft. 1/16” uncoated wire cable 0.31 0.61
2 1/16” ferrules and stop for connecting cable 1.48 2.96
2 4” extra heavy tee hinge used to anchor middle wooden finger 2.47 4.94
2 1.5” corner brace used to anchor pry handle 2.98 5.96
1 3/4” x 5” x 16” piece of scrap plywood wood plus a handle 0 0
1 nylon spacer, 1” long, 0.5” OD, .257” ID, cut in half to get 2 1/2” pieces, spacers on either side of pulley 0.31 0.31
1 Clevis pin, 1/4” x 1” long, axle for pulley 1.51 1.51
1 Electronic luggage scale 16 16
Total: $47.22

Assembly

Pry Scale Parts

Pry Scale Parts

1. Assemble all the parts in the table shown above.

Not listed are bolts, washers, and screws. I used mostly 1/4″ x 1″ rounded headed screws. You need washers to prevent the bolts from sinking into the wood and getting loose. Some of the parts are shown at right. Click on the image for a larger image. Moving clockwise, the parts shown are

  • Electronic luggage scale
  • 1-1/2″ corner brace (one of two needed)
  • tee hinge (one of two needed)
  • pulley as purchased. You need to break off the black hanger and drill out the axle.
  • steel mending plate shown in the middle of the picture
  • pulley assembly at top consisting of:
    • two (2) 2-1/2″ corner braces shown prior to bending of bottom flanges
    • 1/4″ clevis pin axle
    • one 1″ nylon spacer with .257” ID, cut in half to get 2 1/2” pieces

      Sawing the Paddle

      Sawing the Paddle

    • one nylon pulley

2. Drill and cut the wooden paddle and lever

Finished paddle, tongue, & lever

Finished paddle, tongue, & lever

Saw out the outline of the paddle in 3/4″ plywood, oak, maple, hickory, or other tough wood. Make a lever that the user will use to pull the luggage scale with. Then drill the holes. Cut out the center tongue as shown at right. I used a band saw for the two long cuts down the tongue then used a jigsaw to join the two cuts. Sand and finish with a tough marine varnish.

A note on drilling the holes: the mending plates on the bottom share some of the same holes as the corner braces on the top. I could see no way around it. You just need to be precise in your drilling.

3. Assemble the parts

The parts screw together quickly. Note that the two smaller corner braces hold the luggage scale to the lever. In order to allow the luggage scale to move freely, I had to saw some plastic off of the side of the scale near the attaching screws. You may need a few slightly longer screws to hold the pulley assembly to the paddle and mending plates on the bottom.

Bottom of Pry Scale

Bottom of Pry Scale

4. Attach the lifting cable

The cable is 1/16″ wire rope attaches the lifting tongue to the luggage scale with ferrules on both ends. The ferrule is used as a stop on the lifting tongue.

5. Test the Pry Scale

Test your completed pry scale on some reference weights.

So Is That It? It Just Works?

Think up an original idea – build it – it works. Easy eh? Not hardly. Thomas Edison said that “Success is 10 percent inspiration and 90 percent perspiration.” And this invention was no different. The prototype, as built and illustrated above had two problems:

  1. I could not lift a heavy hive with 2 hive bodies and a super with it.
  2. When tested, the results were not consistent and repeatable with sufficient accuracy.
Using the pry scale

Click for a larger image

The first problem was easy to solve. The first prototype had a short lever because that was the size of the piece of wood I had left over from cutting. The solution is to get a longer lever. Longer lever means greater leverage and more lifting force. But the problem turned out to be unrelated to weight anyway: the bottom board was stuck to the hive stand. Once it was unstuck (with help from a pry bar), the pry scale had no problem lifting it.

I discovered the second problem while making multiple measurements to weigh a heavy Dewalt DW735 benchtop planer. I got weight values all over the place. For 20 measurements of the heavy planer, I got a mean of 45.9 pounds, median of 46.8 pounds, and a standard deviation of 2.6! Not very good when you consider that we need to multiple all measurements by 2 to get the total weight.

To improve the accuracy of the scale, I did three things. Each one seeks to increase consistency in measurement:

  • A slotted shim stick is slid under the rear of the hive between the bottom board and the hive stand. It include a 1/8 inch slot to accept the tongues of the pry scale. This insures that the amount that the pry scale needs to lift is the same from hive to hive. It also helps the bottom board to drain out of the front of the hive.
  • A set screw is installed under the lever. This insures that the tongue is lifted the same amount in each measurement.
  • Finally, a bubble level is added to the surface of the scale to insure that the pry scale is held perfectly level for each measurement. The idea here is that if measuring can be made more uniform, the measurements will be more repeatable with less error.

The result? I make 20 measurements of my benchtop planer all over again. I removed the scale on each measurement, reset it to 0.0, watched that it was level, depressed the handle all the way to the set screw, and then waited for the value to settle until it would hold (stop changing). This time I got a mean value of 49.5 lbs, a median of 49.4 lbs., and a standard deviation of 0.72.

So how accurate is the scale? If we assume a normal distribution (which is reasonable since the mean value is close to the median value), then the scale should be within 1.4 lbs. of the correct weight 68% of the time. Given that it can weigh up to 250 lbs, that is less than a 1% error. At any rate, it is accurate to perform everything I set out to above.

A Challenge to Entrepreneurs

There is nothing stopping you from building these devices and selling them. Rather, I hope you do. And I hope you make a ton of money. If you do, you will end up helping a lot of beekeepers and even more bees. This is my original idea and it is not patented (as far as I know) and I have no intention to patent it. I only ask that you attribute the design to beehacker.com.

If you do build these or assemble kits and sell them, please let me know. I will provide links here for others to find you.

Next Steps

The hive scale is a beginning, not an end. Here are some ideas I have for using this scale:

  • Measure a nectar flow
  • Estimate how much food remains at winter time. This better for the hive than opening it up and chilling the brood and bees.
  • Create a standard profile (in my area) for hive weight throughout the year. This will allow me to determine if a hive is doing better or worse than normal.
  • Design a hive scale that is more accurate and able to log data every minute or so for a week. Do it all for under $100.
  • Design software for visualizing diurnal variations or annual trends.

Comments or Questions?

Click here to go to the Bee Scale stub. There you can post your questions and comments.

Update: June 22, 2011

Andrew Little of France has built two of these hive scales – one for himself and one for his local beekeeping club. He has included the following picture of his scale. His craftsmanship is not limited to hive scales – he also built a hive stand – check it out here.

Hive Scale Built by Andrew Little

Hive Scale Built by Andrew Little