Abstract:

A tray to hold 12 ISCO bottles was previously designed without considering the weight of water when they are filled. The plastic tray was redesigned using Fusion360’s sheet material modeling and static stress simulation tools.

Introduction:

I was recently made aware that the design of the ISCO bottle trays did not consider the weight of the filled bottles (thank you, Azad, for noticing that)! Stresses in bent sheet materials are quite difficult to calculate by hand, so the design process for the new tray, under load, is as follows:

1. Initial Design

2. Perform Computer Simulation

3. Check results for low safety factor and high stress points

4. Redo Steps 1-3 until I am confident in the design

Design:

Let’s go over the initial design:

The ISCO bottles are split into two trays of 12. The constraints are as follows:

– No longer than 740mm long

– No wider than 160mm

– Flat Pattern cannot exceed 2” x 4” dimensions of stock sheet plastic

– Must hold 12kg (about evenly distributed along bottom face)

– Material is .04” (~1mm) PETG

The problem is the open face of the tray. By holding the shape of the tray by two pieces of aluminum extrusion, nearly all the weight of the bottles stresses two sets of M3 screws. Even with washers, the plastic would likely stretch or tear. The design was modified to address these issues.

The new design uses a PETG sheet folded into an open-ended rectangular prism with an overlapping face. This overlapping face includes 6 holes for M3 machine screws and will also be glued together. Two kevlar straps for holding the device, connected on the bottom of the tray and tied with room at the top, are not shown. They will be located 1/3 of the length from either end.

The straps and glue are especially important because they aren’t included in the simulation. Because of them, I can assume that the safety factor for stresses on the sides and top of the tray can be lower than would be otherwise acceptable, since the straps will pull the device from the bottom face.

Simulation:

I performed a Static Stress simulation with a 120N distributed weight along the bottom face. The top face was constrained as “fixed”, and 6 ~M3 bolted connections were added on their respective holes, simulated as pinned connections, to hold the overlapping sides together.

While the result of the simulation appears catastrophic in the above picture, it is important to know that Fusion360 greatly exaggerates displacement by default! The actual displacement should be about 13mm which will look much better. The addition of kevlar straps will likely have a negligible effect on this bottom displacement. Notice in the image below that the max stress is located at the bottom corner of the tray. Because the Yield Strength for PETG plastic is about 47MPa, a stress of 64MPa as shown would lead to stretching and possibly tearing along the edge. However, distributing the weight via kevlar straps along the bottom will minimize the stress along the edges of the tray, greatly improving the safety factor shown in the subsequent image.

Safety factor is the ratio of applied stress over acceptable stress. A SF of 1 or lower means that under static conditions, the component will start to exhibit failure. This will improve greatly by the addition of glue in between the bolted flanges in addition to adding kevlar straps. However, if the tray was accidentally carried from the top face it would potentially break.

Iteration:

To account for additional stresses and accidental carrying methods, I decided to add an additional flange on the bottom that is the same length as the side flange, as shown below.

The results of the new simulation are much more promising:

With the new flanges, the maximum deflection is slightly reduced and the safety factor is well above what is necessary. Because straps will be used rather than holding the tray from the top, the actual safety factor is likely much higher, but the tray is unlikely to break if accidentally picked up by the plastic.

Conclusion:

The new bottle tray was simulated with promising results. Larger safety factors and smaller displacements show that it is less likely to break under greater load, and the addition of straps to support the weight from the bottom will reduce the stress in the sheet plastic and provide the user with a more optimal carrying method.

Abstract:

Much has happened with the samplers in the past two weeks! There are two categories of updates: those relevant to the Zurich Sampler and those relevant to the new versions of samplers. In short, the frames have been assembled for the new samplers and the PCBs are being soldered up; the new pumps arrived; the final pieces of the Zurich Sampler are coming together and it will be ready to ship very soon.

Updates:

New Sampler Updates:

Frame:

The sample-bag OPEnSampler frame was assembled by Adnan and I the other week and it fits snug in the Pelican 80QT rolling cooler. It fills up quite a bit of the space as intended, and there is extra room on the sides (the thickness of the wheel wells) for ice packs. Before more are assembled I think I will reduce the width of the sampler by about 5mm to account for the wells being slightly convex.

PCBs:

The Main Control Boards arrived the other week along with all the components of the two samplers. They look great! Azad and Adnan are soldering the components to them and will likely finish in a couple weeks.

New Pumps:

The new pumps arrived and we will be testing them soon. It looks proming but we will need to change either the tubing or the tube fittings to work with the rest of the sampler tubing. These will be added to the new samplers Azad and Adnan are assembling.

Android App:

In other news, three students will be working on adding an Android app to receive updates from and control the sampler remotely as their senior capstone project for their majors in computer science. In the coming weeks they will be contributing to the blog by introducing themselves and eventually posting updates on their work on the app.

Zurich Sampler Updates:

Quick Disconnect Fittings:

I finished the design of the endcap on which the two quick disconnect fittings are mounted and it is currently printing. Designing the endcap was more of a challenge than I initially predicted due to the awkward nature of the barbed fittings and the Pelican’s small outlet port. Because the threaded outlet on the Pelican cooler is too small to fit two fittings side-by-side, the endcap was designed in two pieces that would clamp together with an o-ring and three M2 screws. The male fittings can be threaded through the front panel with an o-ring to mitigate airflow. The sampler tubing can be pushed through the main body of the endcap while it is disconnected from the front panel, and then attached to the barbed ends of the fittings. The main body is then screwed onto the Pelican’s outlet port and the front panel is attached to the main body via M3 screws and nuts, completing the assembly. Intake tubing is attached to the intake fitting via the female quick disconnect fitting.

Filter:

The optional filters were quite easy to integrate into the design. The filter mesh is 470 micron stainless steel from Grainger with open ends. The front of the intake is blocked with a simple 3D-printed plate and the other end is blocked with another plate with a compression fitting screwed into it. All components of this assembly are glued with “ABS paste”, a solution of dissolved ABS plastic filament in acetone. The paste acts to weld the plastic components together.

Operator Interface:

The new Operator Interface turned out quite well. It consists of a 12V barrel jack port for powering the device with either a battery or a voltage regulator, a power On/Off switch, an enable switch for changing sampler modes (field use vs lab testing), an interrupt override button for initiating sampling in the field and also for lab testing, and a USB type B socket for communicating to the Arduino with a laptop. The panel is 3D printed and slides into the 15mm extrusion.

Conclusion:

There’s a ton of development happening on the OPEnSampler and I’m excited to showcase the new design once the two new samplers are completely assembled. In the coming weeks we will ship out the OPEnSampler for the first time, I will update the gitHub page with the latest designs and bill of materials, the capstone students will introduce themselves on this blog, the new pump will be tested, and the new samplers will be fully assembled.

Abstract:

We observed our $1 solenoid valves were failing under low pressure conditions. After some investigation and testing, we found we could safely orient the valves in  to ensure the flush valve will fail first while the sample valves will form quite strong seals when closed. This solution allows us to continue to use the cheaper valves and keep our cost of materials down.

Introduction:

Cost is a limiting factor when choosing components for the OPEnSampler. There are many repeat components, including the 24 sample containers, 52 compression fittings, 25 feet of tubing, 32 aluminum brackets, 16-20 pieces of aluminum extrusion, and 26 solenoid valves. Increasing the cost of any of these by several dollars would increase the total cost of materials by $100 to $200, depending on the component. We chose cheap $1 solenoid valves from AliExpress with this factor in mind, knowing the next best alternative were adafruit’s $7 solenoid valves. It turns out, however, that our $1 solenoid valves are quite weak and fail under normal use conditions due to the water pressure from the pump. Luckily, the solution was quite simple.

Explanation:

Solenoid valves open and close the flow of water with a linear actuator that pushes out or pulls in. In cheap normally-closed solenoids a spring holds the actuator out, pressing an o-ring against the face of the inlet as a seal. When current passes through the solenoid, the actuator pulls in against the spring and water is allowed to pass from inlet to outlet. The diagram below, taken from dyxminipumps.com, shows how our $1 valve’s inlet and outlet (openings A and B, respectively) are perpendicular to each other.

Using A as the inlet and B as the outlet, the valve will fail under quite low pressures. This is because the actuator is only blocking flow because of the force of the small spring; a small pressure from a pump only has to surpass the force of the spring to force the actuator backwards, opening flow from A to B. Orienting flow in the reverse direction, however, yields different results. Any pressure applied from opening B acts perpendicular to the motion of the actuator, neither adding nor subtracting from its closing force. In this orientation the valve will stay closed under very high pressures and a pressure buildup will cause other components of the water line failing first, such as the tubing popping out of fittings. This is quite undesirable in the normal application of these valves, such as in coffee machines, but we can take advantage of the failure behavior of the two orientations for the OPEnSampler.

By directing flow in the sample valves from B to A they will stay closed in the event of a pressure buildup, maintaining the integrity of the samples by preventing cross contamination. If the flush valve is oriented opposite, from A to B, it will be the weakest point in the water line and will fail first, effectively becoming a pressure relief valve for the system so that the water is released out the drain before fittings break. 

This was tested in the lab by connecting the outlet of the pump directly to each of the valve’s openings in turn and running until failure. The pump ran for two minutes without valve failure while B was connected as the inlet and A the outlet, but failed after a few seconds in the A-B orientation.

Conclusion:

The sample valves will be oriented with flow from B to A and the flush valve will be oriented A to B. By taking advantage of the failure conditions of each orientation we can continue to use the cheap $1 valves without worrying about their low-pressure rating. This not only reduces further design work and prototyping but also saves about $150 per unit compared to the $7/piece alternative.

Abstract

There are currently two pumps to choose from for the OPEnSampler: a low flow rate, unknown-precision peristaltic pump and a high flow rate, “low precision” pump. They have their pros and cons and this writeup will discuss the angles of attack for deciding which one is more appropriate for our system.

Specs:

The WMC Pump has the following specs:

• 12V DC 400mA
• 297 mL/min flow rate, among other lower flow rate options
• 5mm Viton Tubing, among other options
• Barbed Fittings

The Honlite Pump has the following specs:

• 12V DC 1.2A – 3.2A
• 1100 +- 8% mL/min flow rate
• Tygon Tubing, among other options
• Compression fittings for 1/4” OD tubing, among other options

Discussion

The WMC pump has been tested on our prototype system and has proven to be reliable and true to its specs. It can reliably pump 250mL/min of water at zero net suction head, which is about three times as fast as our previous dual-pump system. Despite such an improvement, the flow rate is still quite low for attaining representative samples of suspended sediments, such as fine-grain sands [source]. A flow rate of 530+ mL/min is required to reach the EPA’s recommended 60 cm/s minimum line velocity for such sampling where the flow of the analyte heavily relies on the mass and specific gravity of the particulates [source]. 

The Honlite pump from AliExpress has yet to be tested however the manufacturer supplies more specifications than the WMC pump. The defining characteristic of this pump is its 1100 mL/min flow rate at 12VDC, almost 4 times the rated flow rate of the WMC pump and twice the minimum recommended flow rate for sampling suspended solids. Some downsides are its high variance in flow rate of 8%, though further testing of the WMC pump could prove this is not an usual variance, and further testing of the Honlite pump could prove this variance is controllable. The pump head only has two rollers (three is standard) and so the rhythm of the pump could be noticeable, but will quite likely have a negligible effect on the sample quality.

One method of fixing the inconsistency of any pump used is to add a flow rate meter in series with the pump line. This flow rate meter can catch when the pump is struggling or increases its velocity significantly and adjust the PWM control of the pump driver chip, effectively increasing or decreasing the voltage supplied across the pump to account for the change in flow.

Conclusion:

Both pumps cost around $65 (the Honlite pump costs $30 with $35 shipping to the US) so reliability and flow rate are the most significant factors. Neither pump states its maximum suction head, though almost all peristaltic pumps seem to have a maximum suction head greater than 5m. This is why the Honlite pump was chosen for the Q4 2017 design, but several WMC pumps will be purchased as backups in the event we find the Honlite product to be faulty.

It is quite difficult to find a peristaltic pump with a sufficient flow rate for such a low cost and this is the primary source of skepticism for using the Honlite pump. How are they able to achieve such a large flow rate without increasing cost? The answer could be they are mass producing these pumps in a highly efficient system, or perhaps their pumps are not as reliable. Consistency in sample volume may not be a large concern, however an inconsistent sample volume is almost exclusively caused by a varying velocity of sampled water in the tubes. This varying velocity can certainly lead to varying turbidity and suspended solids measurements [source] [source], and so consistency is a huge factor in choosing a pump.

Intro

The EPA recommends a flow rate of 2 ft./s or greater to minimize the relative difference in velocities of suspended sediments. They also recommend a tubing diameter of 1/4 in. as well, but not for a particularly well documented reason. One unanswered question brought up in the design review meeting was the impact of tubing diameter on the quality of water samples. This short writeup discusses some of the considerations involved in the decision to use 1/4” OD Teflon tubing for future sampler designs.

Design Considerations

A paper published in 1985 collected some data on this subject [link to paper]. It discusses several trials where different concentrations of dissolved organic compounds were passed through tubing of varying diameters and materials at a known rate. The concentrations of the passed solutions were measured and recorded.

There are two obvious factors involved: the tubing absorption rate and friction. Absorption is based on contact time, tubing area, tubing material, and the analyte. Absorption is not a concern in suspended sediments but is critical when the analyte is dissolved carbons and gases. Contact time is based solely on flow rate. 

The friction coefficient is decided by the material and the force of friction will be proportional to the tubing diameter and flow rate. The force of friction will reduce the flow rate of the sample water, increasing contact time between the tubing wall and the sample water. Lower flow rates also reduce the accuracy of suspended sediment sampling where particle mass is a dominating factor that creates a differential velocity between the particulate sizes, shown by this study [link] mentioned to me by Dr. Babbar-Sebens.

The results of the experiment suggest the diameter of the tubing has a lesser effect than the material of the tubing on absorption rates for inner diameters between 1/4” and 1/2”, however increasing the tubing diameter decreases the absorption rate for the same material. This effect is unexplained in the paper and the relationship between tubing diameter and sample quality has very little research behind it. It is likely that the largest factor in choosing the tubing diameter is the maximum particulate size of suspended sediments, which requires a minimum diameter cross section throughout the entire hydraulic system including the solenoid valves and pump tubing. 

Both the study mentioned above and this other study [link] show that teflon tubing absorbs the lowest proportion of dissolved organics. The EPA in this paper [link] also suggest that a high velocity decreases the slime buildup against the inner surface. 

Conclusion

Because large suspended sediments (1mm+ diameter) are beyond the scope of the current sampler, Teflon tubing with a .17” ID and .25” OD will replace the current 3/16” ID Silicone tubing. Compression fittings will have to be used rather than barbed fittings. The change in diameter of the tubing will increase the velocity significantly and the teflon tubing will have much lower coefficient of friction, improving small particulate movement in sample water. Teflon tubing will be nearly impermeable to gases and will absorb extremely low amounts of dissolved compounds in the sample water, making it ideal for most of our intended applications, such as sampling for dissolved organics or volatile isotopes.