juliennm
Posts: 1
Joined: Tue Jun 20, 2017 8:59 am

RPi, MCP3008 and Pressure Transmitter

Tue Jun 20, 2017 9:31 am

Hi everybody,

I am using my Raspberry Pi 3 Model P with a MCP3008 to read the pressure (Chilled Water) for an HVAC application from a Pressure Transmitter Wika A-10.

Transmitter Specification:
- Power Supply: 8 - 30 V
- Pressure Range: 0 - 16 bar
- Signal 0 - 5 V.

http://www.wika.us/upload/DS_PE_A_10_en_us_16479.pdf

In fact, the pressure is always inferior than 10 bar, so my signal is always inferior than 3.3 V.

I connected my transmitter as below:

Image
Schematics

However the reading is not really accurate. Is it because of the 3.3V from Raspberry Pi which is not accurate enough? Do you have any alternative to propose?

Thank you very much,

Best regards,

Julien

User avatar
joan
Posts: 14265
Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: RPi, MCP3008 and Pressure Transmitter

Tue Jun 20, 2017 3:17 pm

I believe the Pi's 3V3 level is regulated from the Pi's 5V (nominal) power supply. It should not be a poor voltage reference.

A photo of your set-up may be useful to those who understand these things.

User avatar
mikronauts
Posts: 2722
Joined: Sat Jan 05, 2013 7:28 pm
Contact: Website

Re: RPi, MCP3008 and Pressure Transmitter

Tue Jun 20, 2017 5:26 pm

As Joan said, the Pi's 3v3 is a reasonable voltage reference, however it can fluctuate under load, with the error being the exact 3v3 instantaneous voltage at the time the sample is taken.

What kind of accuracy do you need?

Even with a perfect 3v3 reference (which does not exist) you can only get 3.3/1024 = 3.22mV resolution.

Switching to an MCP3208 will give you 12 bits of resolution, basically 0.81mV/bit

Personally, I'd use an MCP3208 with a precision 4.096V reference (say 0.05%) for a nice easy 1mV/bit
http://Mikronauts.com - home of EZasPi, RoboPi, Pi Rtc Dio and Pi Jumper @Mikronauts on Twitter
Advanced Robotics, I/O expansion and prototyping boards for the Raspberry Pi

Return to “Automation, sensing and robotics”