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Daviteq Electro-chemical Oxygen Sensor
LoRaWAN
Sigfox
Sub-GHz
NB-IoT

Daviteq Electro-chemical Oxygen Sensor

O2

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Replaced SKU

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1. Introduction

Overview

Daviteq Ex d Electro-chemical Oxygen sensor module is a Oxygen measuring module that utilizes the Seri-4 electrochemical sensor with a high sensitivity to low concentrations of detected gas, high selectivity, and a stable baseline. It has an ultra-low noise amplifier to amplify the nano-ampere current signal from the sensor and delivers the stable and high-resolution output to the reading devices such as Sub-GHz transmitter, Sigfox transmitter, LoRaWAN transmitter, RS485 output transmitter, etc.


Typical Applications: Gas, toxic gas detecting, air quality monitoring for the facility, building, pump station, HVAC...


* For some applications with high humidity ambient all the time, the sensor can come with a heater to control the humidity within the working range of the sensor.


Specification

Sensor technology: Seri-4 electrochemical Oxygen sensor. Please check this link for the specifications of each gas type.


Cross Sensitivity Data

What is cross-sensitivity?

The electrochemical sensor is normally affected by other gas. It meant the sensor not only measure the target gas but also the other gases. If there is a concentration of other gas, it would also cause the change in sensor output with a factor listed in the below table.


Please check the cross-sensitivity data of each gas type in this link.

2. Principle of Operation

The very low current from the gas sensor is amplified by a special amplifier circuit to deliver stable and high resolution.


The special mechanism provides noise filtering so that it can deliver a very stable output. The ADC chip can provide a resolution from 16-bit to 24-bit.


The circuit will deliver the digital output to the reading device.


The Exd Electro-chemical Oxygen sensor module will deliver 02 values:

  • Oxygen concentration, in %.

  • An optional temperature of the circuit board, in ℃.

3. Calibration

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

4. Application Notes

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5. Installation Notes

Notes: Avoid the place with humidity higher than 90% RH all the time (a short time in 2-3 days is acceptable)* if the Sensor is intended to install outdoors, please use a rain guard to protect the sensor from rain and direct sunlight. Please contact us to buy this accessory.


  • Place the sensor in the area to monitor the target gas concentration. Please always check the gas molecular weight vs. the air.


Note for Outdoor installation: For outdoor installation, please use the Rain guard to protect the sensor from raindrops or snowflakes. Please find below the steps of installation of a typical Rain-guard.


Step 1. Prepare the rain guard


Step 2. Insert the rain guard into the sensor filter and tighten the locking screws

  • Insert the Rain guard from the bottom up to the position with the red color mark;

  • Tighten the 02 screws on both sides of the Rain guard to secure it in place.


6. Troubleshooting

1. The measured value is not within the expected value

  • The sensor is drifted over time: Re-calibrate the sensor

  • The sensor was in a high humidity environment (> 90% RH) for more than 03 days continuously: Place the sensor in low humidity for its recovery. It may take up to 30 days to recover. If the sensor cannot recover after 30 days, please replace the new sensor module.


2. The measured value is always zero or near zero

  • The sensor module was removed: Please check the sensor;

  • The sensor is at the end of its life: Replace the sensor module.


3- HW_Error = 1

  • Loosed connection of sensor module and wireless transmitter: Check the internal wiring.

  • The measuring module got a problem: Please consult the manufacturer for a warranty or replacement.

7. Maintenance