A DIY paint viscometer

Figure 1 showing my viscometer bench testing setup.  The DVM is showing the motor's back EMF when operating in air.

I bought an HVLP hand-held sprayer for finishing some audio projects I've been working on, and became pretty tired of using the cup viscometer that came with the sprayer.  The cans of water-based paint and varnish I've been getting all need to be thinned in order for the sprayer to deliver a good splatter-free coating.  I don't want to thin-down the entire can -- that's not going to work due to the added volume of water -- and when thinning smaller batches I usually find it necessary to perform the add-water/mix-thoroughly/test several times to get it right.  This adds up to what I think is an unnecessary amount of mess, even before putting on a lick of paint.

So I started thinking about a paint viscometer that isn't so onerous to use and clean.  What I came up with is a variation on a so-called disk viscometer.  This type of device measures the drag a spinning disk experiences when immersed in the liquid-to-be-measured.  Online searching told me that the usual approach is to use a small motor to turn the disk at a set RPM and measure the current necessary to maintain that.  A more-viscous liquid will impose more drag on the disk so the current is approximately proportional to the viscosity.  Calibration is done with liquids of known viscosity at a specific temperature.

I knew I could manage this since I've got a benchtop lathe, mill and 3D printer but knew it would take some time to come up with something that was compact enough (and robust enough) to use for mixing-up paint.  The motor would need to have a tachometer-type system that would output a voltage proportional to the RPMs -- which would then be used in a control loop to set a constant RPM value.  Then I'd need to measure the current flow through the motor.  All that also started looking like an Arduino-style controller to convert the current into viscosity and then generate some sort of output to help guide the paint dilution procedure.

Looking for something simpler, I thought about driving the motor with a constant current.  I knew that the torque a DC motor outputs is proportional to the current flowing through it, so a motor + viscometer disk that is driven with a constant current will spin up until the viscous drag imposed by the liquid on the disk balances the torque produced by the constant current.

This removed the feedback loop needed to maintain a constant RPM value, but didn't remove the need for MEASURING the RPMs.  However, some additional online searching found a discussion regarding a viscometer that used the same constant-current drive scheme I wanted to use.  One of the comments regarding that design suggested that the motor's back EMF could possibly be used as a measure of the motor RPMs.  Now THAT removed the need for a tachometer attachment, basically reducing the viscometer to a constant-current source and some comparators to turn the motor's back EMF into an indication of the paint viscosity.

Here's the circuit design I came up with:

The opamp + transistor on the left form a constant-current source, by forcing a constant voltage across the sense resistor R1.  The motor's back EMF is connected to the three remaining opamps in the quad package, operated as comparators.  The resistor divider network plus the two trimmer resistors are used to set the trip points of the comparators.  Using three glycerine/water mixtures to cover the "acceptable" paint viscosity range gave me the expected variation in the motor's back EMF for the high-to-low viscosity range.  BTW I haven't taken this to the final stage of calculating what the resistor values need to be, but that should be pretty trivial.

The 78L05 is used as a cheap +5 voltage reference to provide a stable current drive to the motor.  

Since the motor back EMF will drop as the RPMs drop, the BEMF is inversely proportional to the paint viscosity.  Due to internal friction and resistive losses in the motor I don't expect it to have a linear relationship but with proper calibration that shouldn't matter.  Calibration over a wider viscosity range plus a polynomial curve fit could turn this idea into a fully-fledged viscometer but I'm after a quick-and-dirty(ish) solution to my painting problem.

I designed a two-sided printed circuit board using the EasyEDA web-hosted tool and a few weeks later I had 5 small PCBs to play with.  It can be seen in the top photo of this blog post, held in the left-hand board vise.

Here's a screen shot of the PCB layout (top side only):

My bench testing showed that I could expect about a 350mV variation in the motor's back EMF for the low/high viscosity range.  That may not sound like much but it will be enough for the purpose.

One thing I haven't mentioned yet is using the spinning disk to mix the paint thinner by shunting the sense resistor with a smaller value, thus increasing the motor current and RPMs.  This would eliminate a messy stirring stick.  And the feature also can be used to clean the disk after the paint has been properly thinned.  Just immerse it in clean water and set the disk RPMs to "high".

The original "disk", as shown in the above photo, was 5cm long -- more of a cylinder than a disk -- and the RPMs in water were so low that I needed to decrease its size.  I eventually went to a 20mm diameter, 5mm thick disk that was printed on my 3D printer.  I had thought that I would need to _increase_ the effective drag coefficient of the disk and had designed some with scalloped perimeters to increase the disk's effective surface area but that turned out to be unnecessary.

My approach differs from the more widely-used approach in that the RPMs are allowed to vary with the viscosity.  This could lead to higher-order nonlinearities due to turbulence and therefore a deviation from the classical Newtonian laminar flow model that commercial viscometers ensure by using low RPMs when testing viscosity.  Again, I'm taking care of all that by restricting my ambitions to testing paint and using a practical calibration approach to set the high/low limits of paint viscosity.