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FAQs

Q.  How does temperature affect conductivity?

A.  Increasing fluid temperature always increases the conductivity significantly -- from 1 - 5 % per °C.  To compensate for temperature changes, conductivity measurements are commonly corrected to the value at a reference temperature, typically 25 °C. Using an integral temperature sensor, the microprocessor is able to correct the raw conductivity measurement appropriately.  A single temperature coefficient can be used with reasonable accuracy over a range of 30 - 40 °C.  Approximate ranges for linear temperature coefficients are:

  • Acids -- 1.0...1.6 % per °C
  • Bases -- 1.8...2.2 % per °C
  • Salts -- 1.8...3.0 % °C

Q.  Must I input the temperature coefficient value?

A.  No there is a default value entered during calibration in production. However, for improved accuracy the temperature coefficient parameter [T.Cmp] can be changed for the LDL family using the configuration software LRDevice.

  1. Set [T.Cmp] and [dAP] to 0 and download the change to the sensor.
  2. Measure conductivity (C1) at temperature (T1).
  3. Change the medium temperature and measure conductivity (C2) at new temperature (T2).
  4. [T.Cmp]  =  (C2 - C1)  ÷  C1 (T2 - T1)  x 100 
  5. Enter this new value for [T.Cmp] and download the change to the sensor.

Ex. If C1 = 500 µS/cm at T1 25 °C and C2 = 800  µS/cm at T2 45 °C,

then [T.Cmp]  =  (800 - 500) ÷  500 (45 - 25) x 100  = 3.

Q.  Do the conductivity sensors automatically compensate for temperature?

A.  Yes. The LDL family automatically measures medium temperature and compensates the conductivity measurement internally. Because temperature is measured internally, it is also available as a process value over IO-Link.

Q.  What is the cell constant and why doesn't ifm specify it?

A.  The cell constant is the ratio of the distance between the electrodes to the area of the electrodes and it is generally specified for direct contact (galvanic) measurement.  Most conductivity sensors on the market measure conductance, not specific conductivity and conductance x cell constant = conductivity.  ifm does not specify a cell constant because the sensor design does not use two probe-type electrodes.  The housing of the LDL100 forms one electrode and the sensor tip forms the other electrode.  Please refer to the Technology section for more details.

Q.  Why are only welding adapters listed with LDL100?  Why are there no triclamp fittings?

A.  Dead space between the triclamp fitting and probe of the LDL100 can cause air bubbles and turbulence.  Both of these can influence the measuring accuracy of the sensor.  Therefore, we only recommend the welding adapters be used with the LDL100.  

Q.  Are triclamp fittings suitable for use with the LDL200?

A.  Unlike the LDL100, the design of the LDL200 requires the fluid to flow through the measuring channel.  This minimizes the influence of turbulence. Note however, that LDL200 is not recommended in general for use in pipe sizes under 2", due to the size of the process connection and measuring tip.

Most commercially available triclamp tees have a neck that is too long for the measuring channel to reach the flow. Fluid must pass through the measuring channel for optimal results. Therefore, we do not recommend the use of triclamp fittings with the LDL200. If you believe the triclamp installation is sufficient based on your experience and you choose to use a triclamp tee, ensure the distance from the triclamp face to the inside wall is no more than 0.69" (17.5 mm).

Q.  How can I adjust the LDL readings to match a known standard or other sensor?

A.  The configuration menu for the LDL family includes the parameter "CGA" (calibration gain) that can be adjusted using the software tool LRDevice.  It is adjustable from 80...120 % of displayed value.

When adjusted to a standard solution, place the sensor in the solution and adjust CGA up or down to match.  To avoid temperature related errors, disable temperature compensation and adjust using the conductivity of the standard at the actual medium temperature.

When matching an existing sensor, both sensors should be installed in the pipe with temperature compensation turned off for both instruments.  Run the media at a high flow rate to avoid temperature influences.  Adjust the CGA up or down to match.

Standard conductivity / Measured conductivity x 100 = CGA

Q.  Does pipe size affect the accuracy of the LDL100?

A.  Yes.  You can gain significant improvement from our stated accuracy (10% of the measured value +/- 25 µS/cm). Adjust the "CGA" value using LR Device as shown in the table below.

Calibration Gain adjustment by pipe size
Installed pipe size Calibration gain (CGA)
1-1/2" 96%
2"  97%
3" 100%
> 3" / tank 107%

Q.  Can the LDL be installed in plastic pipes?

A.  The LDL200 can be installed in plastic pipes without adverse effect. For the LDL100, however, both electrodes must be in contact with the medium and therefore a metal adapter, such as E43313,  is required.  

Q.  Why does my process value over IO-Link seem to be so small?

A.  Notice from the IODD file, the process value is in standard SI units of S/m.  Convert as necessary to your desired units.  ifm's AOI converts the units to µS/cm.