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Conductivity Sensor and Transmitter Calibrations

Calibration procedures and best practices

Conductivity Sensor and Transmitter Calibrations

Postby Unidec » Thu Aug 22, 2013 8:47 pm

Conductivity Sensor and Transmitter Calibrations

When calibrating conductivity transmitter circuits on industrial washers it is important to observe the following general rules:

1) No temperature compensation on WFI or Rinse Conductivity, especially if the WFI process temperature is fixed. We continue to see confusion with the instrumentation calibration processes that fail to mention that temperature compensation must be turned off on the Standard Meter if the Rinse Conductivity transmitter temperature compensation is turned off. The point here is that WFI contaminants are not known and therefore can not be compensated for using a linear compensation scheme. Do calibrate using standard solutions at the nominal process temperature so the reading error is minimized.

2) No first point (of a two-point or three-point) calibration should ever be at zero (0) if the process displayed value is greater-than or equal to zero only. ) If both positive and negative values are possible, zero may be used but for conductivity, WFI temperature, etc. in general are positive going only. For these, the first-point should be not less than to 1.5x the absolute magnitude of the tolerance. For example, on Rinse Conductivity, a 0-10 uS range with a ± 0.4 allowable tolerance should be calibrated not less than 0.6 uS or at 1uS ± 0.4uS (i.e. 0.6-1.4uS) for the first point. More importantly, on Wash Conductivity, a 0.0-2.0 mS range with a ± 0.4 mS allowable tolerance should be calibrated not less than 0.6 mS or at 1mS ± 0.4mS (i.e. 0.6-1.4mS) for the first point. In practice, we have observed some instrument techs in the industry taking a 0.0-2.0 mS Wash Conductivity measurement using WFI because it is “zero” but this can lead to a significant offset and span error. Instead, we recommend dosing WFI to 1.0 mS or use a fresh 1mS standard solution for the cal-measurement and cal-adjustment.

3) The second point of a two-point calibration should not be less-than the normal process operating range of conductivity and not less than 75% of the entire range of measurement. This prevents an error when the first-point (offset adjustment) and second-point (slope adjustment) are not at the extremes of the process range.

For example, when the transmitter is a 4-20mA type, we recommend using the following calibration set points to reduce slope and offset errors when calibrating a wash conductivity sensor with a scale range of 0 to 50 mS:

4.32 mA @1.0mS (2 % of scale)
12.0 mA @25.0mS (50 % of scale)
19.68 mA @ 49.00mS ( 98% of scale)

* calibration at zero mS not proper, does not allow for zero offset shift and leads to low conductivity failures.
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