Daviteq Electro-chemical Oxygen Sensor

The Daviteq Ex d Electro-chemical Oxygen Sensor must be connected to a reading device, normally it is a wireless transmitter like Sub-GHz, Sigfox, or LoRaWAN, or a wired transmitter with Modbus output, 4∼20mA output, 0∼10V output.

In the reading device, the following parameter is configured in advance:

• Sensor + amplifier sensitivity (mV/ %): it is the voltage output of the amplifier circuit = Sensor current output (nA/ %) x R_gain

For example, with an NH3 gas sensor, the default value of the Sensor current output is 110nA/ % and R_gain = 100 kΩ.

Therefore, the default NH₃ Sensor + amplifier sensitivity = 11 mV/ %.

Depending on the sensor type and R_gain value, the sensor sensitivity must be calculated and pre-configured into the reading device.

Why do we need to calibrate the gas sensor? There are some reasons:

• The sensor current output of a sensor is different from the other sensor. It is not the same value for all sensors after manufacturing.

• The sensor current output of a sensor will be changed over time. The O2 sensor current output will be reduced by about 4% of the signal per 3 years in clean air at 25℃ temperature. For the air with a high concentration of target gases, the sensor's sensitivity will be reduced quickly.

• The R_gain of the circuit also has a 0.1% or 0.05% tolerance;

• Therefore, users need to calibrate the sensor before use or in a pre-defined interval time (3 years for example).

How to calibrate the Electro-chemical Oxygen sensor?

Instructions to attach the calibration cap onto the sensor module to get Zero or Span values.

NOTE: THE CALIBRATION CAN ONLY BE DONE IN THE SAFE ZONE!!!

Please follow the steps for Instructions to attach the calibration cap to the sensor module to get Zero or Span values:

Step 1. Remove the Filter and prepare the calibration cap

Step 2. Attach the calibration cap to the sensor head

Step 3. Installed the Regulator to the Gas cylinder

Step 4. Attach the tube to the regulator

Please select the flow regulator with a flow rate of 2.5 LPM or 5.0 LPM.

With the 2-point calibration method, the user can define the A and B factors. Please find below the steps of calibration.

Step 1. Get the Span value.

- Power ON the device;

- Place the device in a clean-air environment (the target value is nearly zero) at a temperature from 20∼30℃, in at least 60 minutes.

- After 60 minutes, force the device to send data, read and record the Raw_value, so now you got the Span_value = Raw_value value.

Recommendation: Record many Raw_ values ​​at least 10 minutes apart (10 values). Span value is the average of the recorded Raw values.

Step 2. Get the Zero value

Note: Keep the sensor Power ON all the time

- Use the standard gas cylinder of Nitrogen 99.99% to supply the gas to the sensor;

- Use the calibration cap as above pictures to attach to the sensor and connect the tubing to the gas cylinder;

- Open the valve on the Cylinder slowly and make sure the gas has reached the sensor. The flow regulator should be 2.5 LPM or 5.0 LPM.

Notes: The tube length is short as possible to reduce the gas loss.

- Press a timer to start counting the time;

- After 2 minutes, force the device to send data once every minute, and stop forcing at 5 minutes;

- The lowest Raw_value is the Zero value.

Note: Just get one value for Zero.

Note: The Raw_value values can be positive or negative;

- After that, immediately turn OFF the valve to save the gas;

- Remove the calibration cap from the sensor;

Note: Always keep the sensor Power ON all the time.

Step 3. Calculate the new A and B

The calculation of new A, B value based on basic linear formula:

y = A * x + B

Where:

A, B is calibration coefficients.

x is the sensor process value (example gas level in %) read on reading device such as on application server/network server, on offline tool. The process value is the RAW_VALUE in the payload

y is the correct value. y is the value of standard gas/standard condition

Which condition of Zero value: y₀ = A * x₀ + B

Which condition of Span value: yₛ = A * xₛ+ B

From the two formulas, the calculation of A, B as below

A = (y₀ - yₛ) / (x₀ - xₛ)

B = (yₛ * x₀ - y₀ * xₛ) / (x₀ - xₛ)

Example of A, B calculation for LoraWAN Oxygen sensor:

* With condition of clean-air environment at a temperature from 20∼30℃, there is Oxygen of 20.96% (yₛ = 20.96); while the Oxygen level on reading device (RAW_VALUE in the payload) is 19.00 (xₛ = 19.00)

* When the sensor is connected to standard gas cylinder having Nitrogen of 99.99% and Oxygen of 0.00% (y₀ = 0.00); while the NH₃ level on reading device (RAW_VALUE in the payload) is 0.55 (x₀ = 0.55)

The calculation of A, B for the Oxygen sensor:          

A = (y₀ - yₛ) / (x₀ - xₛ) = (0 - 20.96) / (0.55 - 19.00) = 1.13604336

B = (yₛ * x₀ - y₀ * xₛ) / (x₀ - xₛ) = (20.96 * 0.55 - 0 * 19) / (0.55 - 19) = -0.624823848

The factory default A = 1 and default B = 0

The RAW_VALUE in the payload is used for calibration

Step 4. Configure the new A and B into the device

- User can use the off-line tool or downlink to write the values of A and B;

- User can put the sensor in fresh-air to re-check the Oxygen value