What Are Conductivity Probe Cell Constants?


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Conductivity probe cell constants determine the precise volume of liquid between two electrodes inside a probe. The cell constant (K) multiplies the measured electrical current (EC) to determine the electrical conductivity of the solution. The units of a cell constant are 1 per centimeter (1/cm); the number is a reference to the distance between the electrode plates to the surface area of the plate inside the conductivity probe.  

Conductivity is a critical measurement of water quality, used in many industries and applications. For example, measuring conductivity is essential to evaluate water quality in drinking water and wastewater systems. 

There are different sensors for measuring conductivity depending on what sensor fits your application needs. But, before selecting a conductivity sensor, it is essential to understand what a conductivity probe cell constant is. 

What Is Conductivity?

Conductivity is the ability of a material, usually, a solution, to conduct an electrical current. However, in some circumstances, conductivity cannot correlate directly to the concentration of the solution. This is because ionic interactions in highly concentrated solutions can alter the conductivity’s linear relationship with the solution’s concentration.

Electrical conductivity measures the ability of a material to transmit an electrical current over a certain distance, typically measured in Siemens (S) per distance, milliSiemens (mS), or microSiemens (µS). 

Conductivity Probe Cell Constants

Cell constants (known as conductivity probes or electrodes) refer to the distance between the two plates inside the electrodes. A cell constant uses the units of per centimeter (for example, 1/cm), where the number refers to the distance between the electrode plates to the plate’s surface. 

If you are using a contacting conductivity sensor, conductivity cell geometry affects the reading, therefore, there are sensors with different geometry readings available. These differences are shown in the cell constant. 

Conductivity cell constants compensate for differences in conductivity cell geometry by multiplying the cell constant (K) with the conductivity reading, as seen below:

Specific conductivity = Measured conductivity (G) * Cell Constant (k)

The above equation is the key point of conductivity measurements, and the conductivity must always be expressed as per unit distance (S/cm). The cell constant is directly proportional to the distance between the two conductive plates and inversely proportional to the plate’s surface area. 

The cell constant of a conductivity sensor is usually specific to that sensor. For example, a cell constant of 1.0 will result in a conductivity value equal to the specific conductivity of the given solution. However, a cell constant of 1.0 is not always a fitting choice, therefore, cell constants are grouped by the following nominal values:

  • 0.01 cm
  • 0.02 cm
  • 0.1 cm
  • 1 cm
  • 10 cm

In solutions with very low conductivity, the measuring surfaces are placed closer together, so that the conductivity sensor can produce a signal. When path lengths between the plates are decreased, the cell constant is also reduced, and the analyzer will measure a lower resistance value. Small cell constants are typical to determine pure and ultra-pure water supplies. 

In solutions with high conductivity, the opposite happens; a longer path between the plates will result in a higher cell constant, and more accurate readings. Higher cell constants are commonly used in concentration control and phase separation of wastewater treatments. 

Below are the common solutions and their conductivity measurements:

  • Pure water: 0.55 µS/cm
  • Boiled water in power plants: 1.0 µS/cm
  • Surface city water: 50 µS/cm
  • Seawater: 53 µS/cm

Conductivity And Temperature

Conductivity is dependent on temperature, therefore when measuring the conductivity of a solution, it is essential also to measure the temperature of the solution. 

As temperature affects conductivity differently between solutions, the following formula is used:

Gt = Gtcal {1 + ɑ(t-tcal)}

Gt = The conductivity at any temperature. 

Gtcal = The conductivity when the temperature is calibrated.

ɑ = The temperature coefficient of the solution.

*You can use degrees Celsius or Fahrenheit, but you must keep the values the same throughout the formula when using a temperature sensor.

Conductivity Meters And Probes

Conductivity meters are the most accurate way to measure the conductivity of a solution. Most conductivity meters have a two-electrode cell usually made from platinum, yet some electrodes are made from titanium, gold-plated nickel, or graphite. 

Some conductivity probes use four-electrode cells. These probes use a reference voltage to compensate for any fouling or polarization of the electrode plates, resulting in higher accuracy. 

At Atlas Scientific, our conductivity probes work by providing you with stable and precise readings free of fringe effects over a broad conductivity range. 

It is important to note, that conductivity meters and cells should be calibrated using a calibration solution before use, however, as our conductivity probes don’t have any electrolyte that gets depleted, they only need to be calibrated during installation.

Yet, if the electrodes become polarized or fouled, you must clean the conductivity meter to renew the active surface of the conductivity cell and replace the conductivity cell if it continues to fail. 

Summing Up, Conductivity Probe Cell Constants

Conductivity probe cell constants determine the volume of liquid between the two electrodes inside the probe. A cell constant uses the units of per centimeter, where the number refers to the distance between the electrode plates to the plate’s surface. 

The cell constants compensate for differences in conductivity cell geometry by multiplying the cell constant with the conductivity value. Also, as temperature affects conductivity differently between solutions, you will need to measure the temperature of the solution to give you the conductivity at any temperature. 

If you have any questions regarding conductivity or what conductivity probe will best suit your needs, do not hesitate to contact our world-class team at Atlas Scientific.

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