LDL sensors: FAQs
Q. How does temperature affect conductivity?
A. Increasing fluid temperature always increases the conductivity significantly -- from 1 - 5 % per °C. To compnesate 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
The temperature coefficient parameter "T.Cmp" can be changed for the LDL familiy using the configuration software LRDevice.
Q. Must I use the temperature output for temperature compensation?
A. No. The LDL family automatically measures medium temperature and compensates the conductivity measurement internally.
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 LDL1 forms one electrode and the sensor tip forms the other electrode. Please refer to the Technology section for more details.
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 chamber. This minimizes the influence of turbulence. Note however, that LDL200 is not recommended in general for use in pipe sizes under 2", since the measuring chamber does not reach the flow in the pipe.
Most commercially available triclamp tees have a neck that is too long for the measuring chamber to reach the flow. If you wish to use a triclamp tee, ensure the height of the tee is no more than 0.625" (16 mm) from the top of the pipe wall.
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" (calibation 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 exisiting 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.