Testing calcium in water with ion sensors

What Calcium is and How to Measure it in Water

Calcium is an alkaline-earth metal that is an important component of the cell walls of aquatic plants, and of the bones and shells of aquatic animals.

Calcium ions find their way into water as it flows over rocks that contain calcium, such as limestone and gypsum. In freshwater, the concentration of calcium ions is generally in the range of 0 – 100 mg/L.

Aquaread Calcium Sensor

The Calcium sensor is an Ion Selective Electrode (ISE). It tests for calcium in water by measuring the levels of charged calcium ions.


Key Features

  • Solid state ISE sensor.
  • Calcium Measurement range 0 – 2,000mg/l.
  • Calcium Resolution: two auto range scales: 0.00 – 99.9mg/l, 100.0 – 1,999.9mg/l.
  • Calcium Accuracy: +/- 10% of reading.
  • Simple to install on a wide range of Aquaprobes.
  • Sensor life of around 6 months depending on usage.
  • Long storage shelf life.

Our Calcium ion sensors can be fitted on many of our probes, below are some examples.

To see all of the probes that feature calcium please visit the Products section



Advanced portable multi-parameter Aquaprobe.



The Aquasonde-2000 has 2 spare ports. They can house either 1x ISE sensor and 1x Optical sensor. Or it can house 2x ISE sensor.



Advanced portable multi-parameter Aquaprobe.

Solid State Sensor

This is a solid state sensor in that it is not gel-filled. The advantage of this kind of sensor over those offered by other manufacturers is that the sensor will not deteriorate over time so long as it's dry.

A new, dry sensor can be stored for years without degradation. Gel-filled sensors, on the other hand, have a finite life from the point of manufacture.

Calibration and Temperature

Successful calibration of an ISE is fairly complex. On its initial use, a three-point calibration is required. Generally, this is at 10ppm and 100ppm of the sensor's specific calibration solution, with the third point intended to characterise the effect of temperature on that specific sensor. The third point is a second calibration at 10ppm however, this calibration point must be at least 10 degrees colder than the initial point. This is because the measured concentration varies with temperature as well as the actual concentration of the ion.

The good news is that once this calibration is complete, subsequent calibrations do not require the third point to be re-done making future calibrations much simpler to perform.


An ideal ISE sensor would only allow the ion in question to be passed through the membrane. However, in practise all ion selective electrodes suffer from interference from ions which are similar in nature to the target ion. For this reason, ISE Electrodes are not recommended for use in brackish or salt water due to the high level of interfering ions.

Simple installation

Installing the calcium sensor into an Aquaprobe is a very simple process. Simply unscrew the blanking plug from an appropriate aux socket, apply some silicon grease to the thread of the sensor (grease provided) and screw in the sensor. After installation full calibration is required.