Reading values from a NPC-100 Wheatstone bridge pressure sensor

[Ron_Blain]

A few things you may wish to try and or consider. Your HX 711 module should look like this. Also this is the data sheet for your sensor. Using an Arduino rather than mess with the Excitation Voltage of the HX 711 module If I measure the 5 Volt pin of my Uno or Mega boards I get a very stable 4.97 volts when the boards are externally powered using a 9.0 volt supply. Obviously if you are relying on USB to power your board your mileage will vary.

Your sensot relies on an excitation voltage of 1 to 10 VDC (Calibrated at 6.0 VDC). Your sensor sensitivity is uV/V/mmHg and your range is -30 to 300 mm Hg (-0.58 to 5.8 PSIG). That tells you pretty much what your output should be for a given excitation voltage at a given pressure. Likely here, the most important thing is your excitation voltage must be stable. I would use your Arduino board 5.0 volt out to supply your HX 711 and your sensor excitation.

While this example code and samples applies to load cells you may want to at least look at the code with a focus on the Calibration code and the Scale Output code. The main difference between what you are looking to do and the linked code is the units of measure.

Anyway this is just an optioon if you continue to have problems. Also Your board ground pins should be tied to your HX 711 ground and your excitation negative of your bridge. All a common bond.

Ron

[Wawa]

The HX711 ground><Nano ground wire needs investigation.
My Nano definitely has the USB socket grounded to the Nano ground pin.
Check all continuity with a DMM.

@Ron.
Powering the sensor from 5volt is unwise.
You loose ratiometric relationship between the A/D of the HX711 if you do, leading to instability.
Leo..

[Oliverhowden]

The HX711 ground><Nano ground wire needs investigation.
My Nano definitely has the USB socket grounded to the Nano ground pin.
Check all continuity with a DMM.

I swapped to using a different MEGA2560, so the issue above is fixed.
I have ordered a new HX711 (The proper SparkFun breakout).
I'll post back with any changes - the 0.7v E- just doesn't seem right, and nothing I can determine the source of so far.
Cheers,
Oliver

[Ron_Blain]

The HX711 ground><Nano ground wire needs investigation.
My Nano definitely has the USB socket grounded to the Nano ground pin.
Check all continuity with a DMM.

@Ron.
Powering the sensor from 5volt is unwise.
You loose ratiometric relationship between the A/D of the HX711 if you do, leading to instability.
Leo..

Thanks Leo, so your saying use the HX711 module excitation correct?

Ron

[Wawa]

Yes, E+ excitation is regulated (powered from the potentially dirty 5volt supply),
and the HX711 uses the same E+ excitation voltage for it's internal 24-bit A/D.
Ratiometric, so it the 4.25volt increases, sensor output increases, and A/D sensitivity decrease with the same amount. Keeping A/D result the same.
Leo..

[Ron_Blain]

Yes, E+ excitation is regulated (powered from the potentially dirty 5volt supply),
and the HX711 uses the same E+ excitation voltage for it's internal 24-bit A/D.
Ratiometric, so it the 4.25volt increases, sensor output increases, and A/D sensitivity decrease with the same amount. Keeping A/D result the same.
Leo..

OK, makes good sense. Never gave it a thought from that angle.

Thanks
Ron