First Testing and Designing

A project log for CPAP Water Reservoir Monitor - Sleep Apnea

What happens when the humidifier for your CPAP runs dry in the night? It isn't pleasant. Let's fix this.

Scott ClandininScott Clandinin 07/24/2017 at 05:010 Comments

I ordered a 5kg load sensor and HX711 amplifier board that came with it. This amplifier takes in the wheatstone bridge input and outputs the scaled value through a serial pin. I used the Q2HX711 Arduino library to do the work of interfacing with this amplifier.

I won't be doing any scaling to proper weights with this as I am basing the calculations off of percentage of water remaining rather than comparing to any preset weight values. This way I won't need to worry about zeroing the scale or drifting results.

Designing the Scale

I will be making this out of wood as it needs to be fairly rigid to hold the weight of the CPAP machine. Below are rough sketches of what I am planning for the scale.

The load bar is set up as shown above so that a moment of force will be exerted on one side of the bar to create strain which can be related to weight.

There was some experimenting and trail and error before looking at the datasheet for the load sensor and realizing that the screw holes were for metric screws rather than imperial screws. It seems obvious now, but prior to this I had not considered imperial and metric screws incompatible.