Extension Pole Calculator — AddGlobe GFM 2.0 High Volume Sampler

The GFM 2.0 doesn't "sniff" a leak rate. It does two simple things at once: it measures the total flow passing through it (using a venturi - a smooth narrowing in the pipe; gas speeds up in the throat and its pressure drops by an amount that reveals the flow), and it measures what fraction of that flow is methane (the sensor). Multiply the two:

Leak rate (CFM) = total flow (CFM) × methane fraction

That multiplication is only honest if every bit of the leaking gas passes through the meter. Miss 20 % of the gas at the cone and your answer is 20 % low - and nothing on the screen will warn you. The entire rest of this page exists to answer one question: will all the gas actually make it to the meter at your pole length?

Diluting the leak with ambient air is fine - the math above still works (more flow, lower concentration, same product). Losing gas is what ruins a measurement.

Static pressure (SP) is just how hard the gas is being pushed or pulled compared to the air around it. We measure it in inches of water column ("WC): connect a clear U-shaped tube half full of water between the duct and the open air - the suction pulls the water up the duct side. Lift it one inch, and that's 1 "WC.

It's a tiny unit on purpose, because fans like the GFM 2.0's are gentle: 1 "WC ≈ 0.036 psi - atmosphere is about 407 "WC. Sucking a drink through a straw is roughly 20-50 "WC. Our whole fan, flat out against a blocked inlet, makes about 1 "WC (the placeholder spec). Every foot of pipe, every elbow, and every foot of buoyant methane spends a slice of that single inch.

Two numbers on this page speak this unit: System SP (what the duct demands) and Fan SP available (what the fan has left at that flow). When demand exceeds supply, flow falls until they balance - that's the next lesson.

A fan is not a flow rate. The label "12 CFM" is its free delivery - what it moves with nothing attached, at zero pressure. Block its inlet completely and it moves 0 CFM at its shutoff pressure (~1 "WC). Between those endpoints it slides along its fan curve: the more pressure it must develop, the less flow it delivers.

The duct fights back with a system curve: pushing flow through pipe costs pressure, and the cost grows with the square of the flow (double the flow ≈ 4× the pressure). More pole = a steeper curve, and the buoyancy penalty lifts the whole curve up before any gas even moves.

The fan runs where the curves cross - the operating point. Lengthen the pole and the crossing slides left: same fan, less flow. The "max usable pole" verdict above is literally the longest pole whose curve still crosses the fan's at (or above) the flow your measurement needs. This is why a fan that "can pull 12" delivers far less through 30 ft of duct - and why the live chart above is the most honest picture on the page.

Three things spend the fan's pressure budget (the segment table above itemizes them):

Wall friction - gas rubbing along the pipe. Grows with length, with the square of velocity, and explodes in small or rough pipe: the 4 ft corrugated hose often costs more than 20 ft of smooth pole. Fittings - every elbow, entry, and diameter change makes the gas turn or squeeze, shedding energy as turbulence. The two 90° elbows at the tip are paid at the tip's highest velocity. Buoyancy - the odd one out, and often the biggest.

Methane is ~45 % lighter than air. A 30 ft pole full of it is like a long, skinny balloon trying to rise while the fan drags it down. The cost (~0.19 "WC for pure methane at 30 ft of rise) is charged before a single CFM flows - it depends only on height and how rich the gas in the duct is. That's why the "Stream CH₄" stat matters: with an air gap the leak gets diluted with entrained air, the column gets heavier (closer to air), and the penalty shrinks. A fan whose shutoff pressure is below the buoyancy penalty can't move any gas down the pole, no matter how small the leak.

Sealed (snug cone or bag, zero gap): physics does the capturing for you. The gas has exactly one exit - through the meter. Even if the leak is bigger than the fan can pull (the "forced" regime), the reading is still the full leak; the only question is whether suction holds the cone on by itself or you hold it. Best fit, always, when the vent allows it.

Air gap: now the gas has a choice of exits. To win, the fan must pull extra air inward through every opening fast enough that no methane drifts out against it - the capture velocity (the tool demands 150 ft/min by default; more in wind). That extra air is pure overhead: a 5 in² gap eats ~5 CFM of a 12 CFM fan before the leak itself is even counted. The entrained air also dilutes the stream (lesson 4's silver lining).

Rule of thumb: every square inch of gap you close is worth about 1 CFM of fan capacity back. A strap or tape is the cheapest upgrade this system can get.

1. Know the leak rate you're checking. This tool models a known or assumed leak rate - the methane the GFM 2.0 is there to measure (the value a calibrated bag would show if one were used). You normally do not bag the vent: the high-volume sampler IS the measurement. Bagging is only a fallback when a reading approaches the sampler's trusted ceiling (~70% of the advertised max - about 12 CFM gapped, ~22.95 CFM sealed for the GFM 2.0), where high-volume readings get unreliable.
2. Look at the vent outlet. Pipe size? Straight cut, rain flapper, or hood? A flapper deflects the jet sideways and makes a gap fit harder - plan to seal below or around it.
3. Choose your fit - aim for sealed. If the cone or bag can sit snug, every other problem shrinks (lesson 5). Pack straps and tape.
4. Enter the scenario here (leak, fit, gap honesty, weather) and read the verdict: max usable pole, and whether you'll be fan-dominated (cone holds itself) or forced (hold the seal - the reading is still valid).
5. Watch the cone when you're on the vent. Seats itself = suction is winning. Tries to lift = forced flow; hold it firmly and carry on.
6. Verify capture before you log it: change the fan speed and watch the computed leak rate. If the reading stays put, capture is complete. If it rises with more flow, you were losing gas - improve the seal and repeat. (Industry-standard Hi-Flow practice, and the single best habit in this list.)
7. Record temperature and pressure from your weather meter alongside the reading - they set the gas density behind every number here.

Sanity anchor: the leak rate you set out to measure (step 1), the verdict here (step 4), and the flow-change check (step 6) should all tell one consistent story. When any two disagree, the measurement isn't done yet.