henryhunt
Posts: 17
Joined: Sat Aug 27, 2016 11:31 am

Possibility of reading Apogee SB-100 sensor with Raspberry Pi

Sat Mar 03, 2018 2:15 pm

I'm looking to upgrade the barometric pressure sensor on a weather station project I am working on (using a Raspberry Pi), and am looking for some advice on if, and how, a certain sensor I have picked out can be read by it.

The sensor I'm hoping to use is the SB-100 from Apogee Instruments (https://www.apogeeinstruments.com/barometric-pressure/)

However, I am not the most experienced in electronics, so looking at the manual has thrown up some questions about the ability to actually use this sensor with the Raspberry Pi.

The manual (https://www.apogeeinstruments.com/conte ... manual.pdf - from page 8 to 9) says that it produces an output from 0 to 5 volts, but the Raspberry Pi only accepts input up to 3.3v. Does this mean I will be unable to use the sensor here, or do I need extra circuitry to make the whole range available? The manual also says that I need to be able to measure millivolt signals (from 100 to 4800 millivolts). Is this too fine a value to measure, as I have read that it is?

Is this a sensor I can use with the Raspberry Pi? If so, how would I go about doing so?

Thank you,
Henry

User avatar
joan
Posts: 14668
Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: Possibility of reading Apogee SB-100 sensor with Raspberry Pi

Sat Mar 03, 2018 4:02 pm

I haven't looked at the provided links.

From your description the sensor returns an analogue voltage in the range 0-5V. You need to digitize this voltage to get the reading.

To digitize the reading you need an ADC (Analogue Digital Converter) between the sensor and the Pi.

A typical ADC is the MCP3008. The MCP3008 has 8 channels (each of which can be connected to a sensor) and converts the sensor voltage to 10 bits, a value between 0 (0V) and 1023 (5V), with linear scaling, i.e. 2.5V will be 512.

The MCP3008 connects to the Pi's SPI bus, other ADCs may connect to SPI, I2C, or serial link.

User avatar
scruss
Posts: 2818
Joined: Sat Jun 09, 2012 12:25 pm
Location: Toronto, ON
Contact: Website

Re: Possibility of reading Apogee SB-100 sensor with Raspberry Pi

Sat Mar 03, 2018 5:19 pm

Yup, as Joan said, 0-5 V analogue output and 5 V power. I haven't used this board, but the ADC Pi Plus looks like it would work. Note that the ADC Pi Plus can only read 3.75 samples per second, so you can't use it for high frequency work.

I see that Apogee make this sensor for Campbell Scientific dataloggers, and the companies are neighbours in Logan, UT. Quite a large chunk of my career was working with CS equipment, so it's likely to be a solid and reliable sensor, if not exactly fast.
‘Remember the Golden Rule of Selling: “Do not resort to violence.”’ — McGlashan.

henryhunt
Posts: 17
Joined: Sat Aug 27, 2016 11:31 am

Re: Possibility of reading Apogee SB-100 sensor with Raspberry Pi

Sun Mar 18, 2018 8:13 pm

Thank you for your replies (and apologies for my late one). So it seems it is just as easy to read as any other sensor then?

I am still concerned about the ability to read individual millivolt scale signals. I have read that this is difficult for a raspberry pi because millivolts are so small, and noise and an unclean power supply will affect the accuracy of readings at that scale. Is this true, or is a simple ADC with enough precision all I need to read this sensor?

User avatar
scruss
Posts: 2818
Joined: Sat Jun 09, 2012 12:25 pm
Location: Toronto, ON
Contact: Website

Re: Possibility of reading Apogee SB-100 sensor with Raspberry Pi

Sun Mar 18, 2018 11:59 pm

Yes, it should. If you're worried about a noisy ADC, see if you can find a differential 0-5 V ADC. This reduces the noise by separating the ground line from digital ground.

The main data logger that this sensor is used with only has a 13-bit ADC. Even if you're using an ADC with a 10-bit linear range over the full 0-5 V, you've got a resolution of roughly 0.1 kPa. Do you need better?
‘Remember the Golden Rule of Selling: “Do not resort to violence.”’ — McGlashan.

Return to “Automation, sensing and robotics”