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# FAQ00751 of Level Switches FAQ

FAQ No. FAQ00751

Question

Are there any precautions to observe when controlling pure water with 61F Floatless Level Controller?

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The conductivity or specific resistance is a factor in the selection of a 61F Controller. Measure the interelectrode resistance to select the proper model accurately. A 61F Controller can be used if the measured value is within the "interelectrode operate resistance" listed in the Specifications Table of the Datasheets (F030-E1-09).

When the resistance range is measured with an instrument such as a multimeter, the interelectrode resistance cannot be measured accurately because the electrode voltage is DC. Always use the 61F Controller's interelectrode voltage (AC) to measure the interelectrode resistance.

Measuring the Interelectrode Resistance

When the Controller is not operating properly even though it is wired properly, measure the interelectrode resistance. In this case, use a voltmeter and ammeter to measure the voltage drop, as shown in the following diagram.

The interelectrode resistance (liquid resistance between E1 and E3) can be calculated with the following equation.

R = V⁄I

R: Liquid resistance between the electrodes (kΩ)

V: Voltmeter's displayed value (V)

I: Ammeter's displayed value (mA)

Select an appropriate 61F Controller model based on the value of R.

The following list can be used as a guide to select a 61F Controller model from the conductivity.

Operate resistance:

0.5 to 2 μS (microsiemens) → 61F-UHS (0 to 1 MΩ)
2 to 5 μS → 61F-GP-NH3 (0 to 200 kΩ)
5 to 10 μS → 61F-GH (0 to 70 kΩ)
61F-GP-NH (0 to 40 kΩ)
61F-GPN-V50 (0 to 50 kΩ)
15 to 25 μS → 0 to 4 kΩ for standard general-purpose applications

Note:

1.With the 61F-HSL Ultra High-sensitivity model, a DC current flows through the electrodes. If the electrodes are usually submerged, galvanic corrosion will occur, so the Ultra High-sensitivity model cannot be used for control. Applications are restricted to upper limit alarms and leakage detection (e.g., errors, external tank overflow detection, etc.)

2.For pure water over 1 MΩ(18 MΩ model also available), use K7L Liquid Leakage Sensor or E7A Level Switch or E7M Level Switch.

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