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To detect liquids, Solinst Interface Meters use an infra-red beam and detector. When the probe enters a liquid the beam is refracted away from the detector which activates an audible tone and light. If the liquid is a non-conductive oil/product the signals are steady. If the liquid is water, the conductivity of the water completes a conductivity circuit. This overrides the infra-red circuit, and the tone and light are intermittent.
Both sensors use exactly the same zero point, giving accuracy as good as 1/200 ft. or 1.0 mm. The circuits are powered by 2 standard 9V batteries which are housed in easy-access drawers on the faceplate of the reel.
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To measure the thickness of a product layer, lower the probe into the well till the signals activate. If there is an oil/product layer on the top of the water, the light and tone will be steady, indicating an air/product interface.
Read the depth off the permanently marked tape. Then lower the probe further into the water, where the signals become intermittent, then pull back up and take a reading at the product/water interface.
The thickness of the product layer is then determined by subtracting the first reading from the second.
If there is only water in the well and no product, there will only be intermittent (water) signals.
The presence or absence of dense (sinking) non-aqueous layers (DNAPL) is determined by continuing to lower the probe to the bottom of the well.
If the steady tone and light return, this indicates a non-conductive liquid. Measure the depth and continue lowering the probe till the probe touches bottom and the tape goes slack. Record that depth. The thickness of the DNAPL layer is determined by subtracting the first reading from the second.
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