Rick Shory

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Building a one-meter vegetation sampling frame for the P3 protocol

You can build a vegetation quadrat sampling frame from PVC pipe. These plans and measurements are for a “collapsible” version which packs up to take less space. After these instructions, I give variants; the non-collapsible version, and the “lightweight” collapsible version.

The assembled frame encloses an area one meter square. For transporting the frame and threading it through vegetation, the frame comes apart into four sides, or bars. The bars are identical, so the assembly instructions are for one bar.

Bars one meter long may not seem like much in open space, but hiking with them through brush, hauling them around in a crowded rig, or putting them through airport checked baggage can be awkward. The collapsible bars fold up to about one third the length. They work on the same principle as the shockcorded “poles” of dome tents.


Materials for one bar are:  Three sections of PVC pipe, two straight couplings, one 90 degree elbow, and a length of elastic shock cord.  Here is the parts list for the four bars which make up a complete sampling frame.

All this PVC is “nominal” half inch.  They call it half inch even though the inside diameter of the fittings (and the outside diameter of the pipe) is actually about 7/8 inch (21 mm).

“Schedule 40” PVC, half inch (nominal), 90 degree elbows. Number needed: 4

“Schedule 40” PVC, half inch (nominal), straight couplings. Number needed: 8.

“Schedule 40” PVC, half inch (nominal) pipe, ten-foot sections. Number needed: 2. This kind of pipe usually comes in ten-foot lengths. You will need two of those to cut the 12 thirteen-inch sections you need.

Drill and 5/16 inch bit. If you are constructing a frame while on the road, many hardware stores will drill the holes for you.

Elastic shock cord, 1/4 inch (7 mm) diameter. Quantity needed: 12 feet.

Black electrical tape (or black paint), one roll, for marking the black calibration bands on the bars.

Drill a hole in the back of each 90 degree elbow, as shown.

Cut 12 PVC sections thirteen inches long.  This length allows for the amount which will slip into the connectors, and makes the finished frame fit almost exactly one square meter.

You can cut the PVC pipe any number of ways.  If you are on the road, hacksaw blades are cheap and effective.  You can wrap one end of the blade with tape to make it easier to grip.

Drill a hole in the side of four of the PVC sections (one for each bar). If you are having the holes drilled at the time you buy the pipe, drill them in each end of the two ten-foot sections.  Then you can cut the four needed 13-inch sections off of those ends.

Drill the hole about 1-1/4 inch (33 mm) from one end.  This distance is not critical.  It must be at least as far as the length which will slip into an elbow, 7/8 inch (22 mm), but it can be as much as 2 or 3 inches.

Push one end of the shockcord through the side hole.  Easiest, from outside in, then down the inside of the pipe section.

When you have the shockcord through the pipe section and can get enough to work with, thread components together as shown:

  • pipe section with side hole
  • one straight coupling
  • a pipe section with no side hole
  • a second straight coupling
  • another pipe section with no side hole
  • a 90 degree elbow

Thread the shockcord out the hole in the back of the elbow.

Tie a knot close to one end.  Pull that end snug to the hole.  Bring the PVC pieces together so the shockcord is just about the same length through them.  Then, pull the shockcord out about a foot so it is stretched.

Tie a knot close to the second hole.

You can test it at this time to see if the tension feels right. You should be able to pull the PVC sections apart easily and fold up the bar. But when straightened out, the shockcord should hold the pieces tight enough together they don’t slip apart by accident. There is no need to glue the PVC sections together.

When you are satisfied with the tension, you can tighten up the knot and cut the shockcord off to about an inch (few cm).

Electrical tape is a convenient way of marking the black bands.  Less messy than paint, plus it holds the ends of the shockcord from fraying.  If you use the wrapping pattern shown, all the cut ends of the shockcord will be covered.

Wrap tape to mark alternate tenth-meter bands.  Simply skip up and over any couplings you come to.

Note that tape is free of junctures, so segments can come apart

Make the other three bars the same way.  It will be easier to get the shockcord threaded through the bars if you do not cut it first.  When you are done, you will have a length of cord left over.  You can tie this into a ring and use it like a big rubber band to hold the folded up frame all together in one bundle.

To assemble the frame at a survey site, you push the “empty” end of a bar into the open side of the elbow on another bar.  This usually holds together for the minor movements of adjusting the frame position on the ground, but comes apart easily to go around stems and brush.


Variant: Non-collapsible frame:

All this PVC is the same “nominal half inch” as in the design above.

Materials needed:

  • “Schedule 40” PVC, half inch (nominal) pipe, ten-foot sections. Number needed: 2;
  • “Schedule 40” PVC, half inch (nominal), 90 degree elbows. Number needed: 4.
  • PVC glue
  • Black electrical tape (or black paint), one roll, for marking calibration bands.

Cut four sections of pipe, each 39-1/4 inches (about one meter) long. If you are being precise, make them 99.8 cm to allow for length added by the elbows, but this is usually negligible. You will be able to get three pieces out of a ten-foot length of pipe, but you will need another ten-foot section for the fourth bar.

Glue one elbow onto one end of each bar. It is necessary to glue them or the frame falls apart too easily in use, and the elbows get lost.

Wrap with black tape, or paint bands, to mark the tenths of meters.


Variant: Lightweight collapsible frame:

This is the same as the regular collapsible frame except you use (nominal) half inch CPVC pipe and fittings. Note, this is different than “schedule 40”. This CPVC type of pipe is usually yellowish in color, and much thinner and lighter. Even though both these kinds of pipe are called “half inch” the CPVC is about half the size of the “Schedule 40”.  For nominal half-inch CPVC, the inside diameter of the fittings (and the outside diameter of the pipe) is about 5/8 inch (16 mm)

You can use thinner shock cord, or even flat elastic (from a fabric store) in a pinch.

Materials:

CPVC half inch (nominal), 90 degree elbows. Number needed: 4.

CPVC half inch (nominal), straight couplings. Number needed: 8.

CPVC half inch (nominal) pipe, ten foot lengths. Number needed: 2. This kind of pipe usually comes in ten-foot lengths. You will need two of those to cut the 12 thirteen-inch sections you need.

Shock cord, 3/16 inch (4 mm) diameter, or flat elastic 3/4 inch (18 mm) wide. Quantity needed: 12 feet.

Also:

  • Drill and 3/16 inch bit. If you are constructing a frame while on the road, many hardware stores will drill the holes for you.
  • Black electrical tape, for marking the black calibration bands on the bars.

Construction is the same as the collapsible frame above except that holes are 3/16 inch, and must be at least 1/2 inch (14 mm) from the end of a bar section.

This design is a welcome saving in weight on long hikes.  However, the frame is “floppier”, and more fragile.  It is hard to avoid cracking the straight couplings.  For general use, I recommend a frame made of half inch (nominal) “Schedule 40” PVC.

Left: Lightweight frame, about 1.5 pounds (670 g)

Right: Standard frame, about 2.9 pounds (1.3 kg)

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