A hygrometer was built in order to measure the humidity in the gas systems. This is particularly important for the MicroMegas Detectors, which are very sensitive to humidity and can be damaged if operated in high humidity situations.

The hygrometer uses a BME280 sensor, this sensor measures humidity, temperature and pressure. As a result, we are able to simultaneously collect more data from from the gas system than what was originally desired. This sensor is read out over I2C. In our setup, this is done through a raspberry pi.

The housing for the sensor is critical. Plastic should not be used since it contributes to increasing the humidity. Ideally a metal should be used, the best option is steel, brass would be a lower quality second option. To seal the enclosure, for example after drilling a hole to pass through wires, do not use silicone. Silicone will off-gas and this will damage the detector. Use of a very good quality and stable glue is required in this situation, we have been recommended to use Araldite specifically the long setting one but the short setting one should also be fine.

We predict too have several sensors running simultaneously, at least one at the line entrance and one at the exit of the gas system. This minimum would allow to see how much humidity is going in to the detectors and how much is coming out. The desired situation is to have a sensor at the entrance and exit of every detector, allowing this monitoring to be done per detector. In this situation if we have a setup with N detectors in a single gas line, we would need N+1 humidity sensors.

Due to time constraints a quick and flexible solution to the readout was put in place. As a result, each sensor is paired with a single Raspberry Pi. One of the raspberry pis is connected to the network but also creates a local internal wireless network with its onboard WiFi. The other raspberry pis connect to this wireless network and consequently will have network access through WiFi and report the sensor reading to the first raspberry pi. The first raspberry pi is running the EmonCMS software in order for us to do some monitoring of the sensor data. At the moment no automatic actions are taken, such as turning off the HV if the humidity is too high, so no DCS-like functionality is available.

Raspberry Pi setup

Five Raspberry Pi 4 Ver 1.2 with 4GB of ram were purchased for this setup. In addition with enclosures, power supplies and some other needed accessories.

For details on setting up and configuring the Raspberry Pis, please see Raspberry Pi Setup.

Hygrometer Configuration

The device bl4s-rpi-01 has the EMonCMS software running on it. This device also has a BME280 sensor connected to it in order to measure the ambient conditions. This device must be configured with the WiFi Access Point Flavour and the EMonCMS Flavour (in this order) from the Raspberry Pi Setup as well as the BL4S BME280 Flavour.

The other devices (bl4s-rpi-02, bl4s-rpi-03, bl4s-rpi-04, bl4s-rpi-05) are also connected to their own BME280 sensors and connect via WiFi to bl4s-rpi-01. Through this WiFi connection the acquired data is published to a MQTT broker running on bl4s-rpi-01. These devices only require to be installed with the BL4S BME280 Flavour from the Raspberry Pi Setup

In this way, the data becomes available to EMonCMS.

-- CristovaoDaCruzESilva - 2020-07-03

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Topic revision: r2 - 2020-07-03 - CristovaoDaCruzESilva
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