How the percolation rate is calculated.

What the test measures

A percolation test measures how fast water soaks out of a test hole into the soil. The result is a rate in minutes per inch: how long the water level takes to fall one inch. A bigger number means slower soil.

One row of the table

Each reading becomes one row of the hole's table:

• TIME — when the reading was taken.
• DEPTH — the depth to the water, in inches below the ground.
• DROP — how far the level fell since the previous reading.
• INTERVAL — the minutes since the previous reading.
• RATE — the interval divided by the drop.

RATE (min/in) = INTERVAL (min) ÷ DROP (in)

The first row has no drop, interval or rate: there is nothing before it to measure against.

The final rate and the bottom 3 inches

A hole's final percolation rate is its slowest interval — with one exception. Readings taken with 3 inches of water or less left in the hole are not counted. The DPH Technical Standards end the test at 2 to 3 inches remaining, because fines settle at the bottom of the hole and slow the last inches down.

The water left is worked out from the hole depth and the reading. With the Percometer it needs no hole depth at all: the sensor face sits a fixed 25 inches above the bottom of the hole, so the water left is 25 inches minus the reading.

If every reading falls in the bottom 3 inches, the slowest of all stands, and the software asks for the hole depth to be checked.

A worked example

A 24-inch hole, read every 10 minutes:

The last reading's interval is the slowest of all — 13.33 min/in — but it was taken with only 2.60 inches of water left, so it is not counted. The slowest interval that counts is 10.53 min/in, which falls in the DPH class 10.1 – 20.0. That class is what the drawing prints as the final percolation rate.

Illustrative values. The 11:05 row stays in the table; it just does not set the final rate.

Refills and top-ups

A hole that drains fast is often topped up and the test carries on. That is normal, and everything stays in the one table for that hole.

• In Percolation Manager, a jump back of 1 inch or more against the run of the test is a refill. Its row keeps its time and depth, and its DROP cell reads REFILL.
• The refill row has no interval or rate, so it can never become the final rate. The next reading measures from the refill, never across it.
• The Percometer itself flags any rise of 0.5 inch or more as “water rose”, and leaves that interval without a rate.

When the hole runs dry

The device keeps reading on schedule whether or not water is left. Once the hole is dry, the level stops moving and the rest of the run is the sensor looking at the same surface.

Percolation Manager finds the earliest reading after which the level never moves more than 0.05 inch again, across at least three readings, and leaves the readings after it off the drawing. A genuinely slow soil still moves: at 300 minutes per inch, three five-minute readings fall about 0.05 inch. Nothing is trimmed silently — the cutoff is shown, it can be changed, and “Use all” puts every reading back.

The design percolation rate

A lot usually has more than one percolation hole. Its design percolation rate is the slowest final rate of its holes, reported as its DPH class. That rate sets the percolation factor (PF) in the lot's minimum leaching system spread:

MLSS = HF × FF × PF

Values are compared with the DPH tables to one decimal place, and a class's upper bound is inclusive — 10.0 is in the 10.0-and-faster class, 10.1 is in the next.

When a value needs correcting

Field data is not always clean, and engineering judgement comes first. Every value the software works out — final rates, and every TIME, DEPTH, DROP, INTERVAL and RATE cell — can be typed over. The typed value is printed exactly as typed and marked on screen, the cells around it are not recalculated, and the software's own value can be restored at any time.