In our last update, I wrote about the automated percolation tester and the issues that we were trying to solve. So how did we automate the percolation test? Let me first warn you that this update is going to be somewhat technical, but I’ll do my best to make it easy to understand. I’m hoping that this gives you a bit of an idea of what my day to day work often looks like. If you want more of the technicalities, I’d be happy to send you the 70-page design manual that I’m wrapping up this week.
Overall, the problem is quite simple. We need to do 2 things:
1. Measure the depth of water in the test hole.
2. Fill the hole with water to the prescribed depth depending on the test phase
For this update, I’m just going to focus on the depth sensing (#1), which was the main source of issues throughout this project.
There are many ways to measure the depth of a column of water. The most common method is pressure. If you can measure the pressure at the bottom of the water and compare that to the ambient air pressure, you can determine the depth of the water. Early on in the project, we selected a submersible pressure sensor, easily be sourced on Amazon, that only cost about $35, a very low price for a decent pressure transducer. Early tests were quite positive, so we went ahead and ordered all of the sensors that we needed.
As we continued tests, we realized that the sensor wasn’t quite as consistent as we first thought. The sensor tended to drift over time and had noticeable variation from one test to the next. Nothing huge, but it was cause for concern for a device that would be used for multiple years with minimal maintenance. Even high-quality pressure sensors need annual calibration, which we knew wouldn’t happen once we delivered these to the offices. So, we decided to add a secondary depth measuring system to constantly recalibrate the pressure sensor.
The secondary system needed to be completely separate from the pressure sensor, provide consistent data that wouldn’t drift, and be very cheap. We decided pretty quickly that we wanted to use level switches, which provide a simple on or off signal based on water depth. Think of the ball float in a toilet – when it reaches a set height, it turns off the water flow. In our case, it sends a signal to our control system indicating that the depth just reached the height of that switch. With a number of these switches at set heights, we can get accurate depths as those switches trigger on and off that can be used to double check the values that we’re getting from the pressure sensor.
We decided to complicate things even more by using entirely electrical level switches. This was done to remove any moving parts like a float ball that could get gummed up over time and stop working. The concept is simple: we run one wire to the bottom of the hole that provides a 12V power source, essentially electrifying the water. We then have 6 additional wires with exposed ends held at the desired heights. If they detect voltage, then that means that they are submerged in the electrified water. If no voltage is detected, then they must not be in the water. Sounds simple, right?
This is the part of the story where I explain part of the reason this project took almost a year and a half longer than we initially anticipated. These level switches caused us issue after issue. Our first design worked great initially, but after a few tests some of the level switches started malfunctioning. At first, it was the level switches farthest from the voltage source that weren’t triggering high when submerged. After we solved that, they started not triggering low after the water level drained below them. We designed iteration after iteration: calibrating the voltage dividers on the PCB, changing wire types due to corrosion and issues, changing level switch mounting schemes to minimize water film sending false electrical signals, and even making changes to 3D print layer direction to prevent capillary-flow up the level switch assembly. We changed the base design for better printing quality and stability, added a stabilizing ring to keep the sensor upright in the test hole, and made a lot of other almost invisible improvements that work to ease assembly and maintenance of this component.
I could go into much more detail about all of the design considerations, but that would take a lot of time, and I doubt anyone would read it. But I am quite proud of the final design of this depth sensor assembly. In total, the full assembly can be made for about $45 (or about $55 with the current price of the pressure sensor), and I think it is a much more reliable system than just a pressure transducer at 5 times that price. At the end of the day, our goal was to make a reliable system that would lead to better data, which in turn would result in better septic systems for our ministry partners. These devices have already been used to size 4 septic systems over the past few months – I’m excited to seem them being used on EMI project sites around the world.
Arensen Update
As mentioned in our last update, there are a lot of transitions going on at the EMI Uganda office. We said goodbye to the Gruen family, who are moving back to Texas this week. Tom has been our lead civil engineer for the last 6 years. And we’re saying goodbye to the Greene family, who are also returning to the US soon. Phil has been the Uganda office executive director for the last 8 years; they are going to be greatly missed.
On a brighter note, we had the official handover to our new director team last week. Valerie Rubombora is stepping into the role of executive director. She has been serving as our Architecture team lead for the past few years. She started at EMI as an intern in 2014 and was part of the first cohort that went through EMI’s fellowship program. We are super excited to have her step into this role and lead our office in this new season. Adam Gordon is also stepping up as our Technical Director – he’s been our Engineering team lead for the past few years. The month of June is the transition month as people shift from their current roles to their new ones; please be praying for this process!
Switching to more personal news, the last couple of months have been quite busy for us with doctors appointments, gaining documentation and preparing for our upcoming Home Assignment. Hannah got sick with an amoeba (a stomach parasite) and malaria in May which kept her in bed for several days. Thankfully it was a fairly mild case of malaria and she is feeling much better now. Jamie has also been having some stomach issues on and off, and we are still concerned about the in-toeing of her feet. We look forward to having more reliable medical care for her during our upcoming time in the States!
We leave for home assignment in a few weeks and there is a lot of preparation for our visit to the US. This means preparing our house for others to stay in it, figuring out what to pack, etc. We finally had our appointment at the US embassy last week for Sage – she is now officially a US citizen! We are now waiting for her passport to arrive before we fly out in 2 weeks. We also just received Jamie’s dependent’s pass (visa) which will allow her to leave the country. Hannah is loving being at home with the girls and has still been able to speak into projects occasionally, which brings her lots of joy.
We leave Uganda on June 24 and arrive in Virginia on June 25. Here’s a brief overview of our travel plans during our home assignment:
Virginia: June 25- 28
Minnesota: June 28-July 18
Ohio: July 18-20
Corning, New York: July 20 – August 2
Virginia: August 2 – August 30
Corning, New York: August 30 – September 24
Pray With Us
We are so grateful for your prayers and support! Please join us in prayer and praise for the following:
Prayer requests:
- Pray for us as we say goodbye to dear friends who are moving back to the US.
- Pray for Sage’s passport to arrive before we leave.
- Pray for the EMI Uganda office during this time of transition.
- Pray for our home assignment as we visit churches and supporters and share about the work we’re doing here. Pray for the girls as they are toted around to different places, that we would help them adjust to new people and places as we travel.
- Pray for our health.
Praises:
- Praise God that we had a successful embassy appointment to get Sage’s US citizenship! Also, praise God for Jamie’s dependent’s pass getting approved without any issues.
- Praise God that the UG Directorship role has been filled and that both the Global Board and the Uganda Board of Directors came to an agreement. We thank God for Valerie who is taking on that role.
- Praise God that Hannah recovered from illness quickly and that no one else (especially the girls) got malaria.
Stay Connected
Each year, EMI’s people complete over 100 development projects which serve the poor and proclaim the Gospel of Jesus Christ. To keep these design services affordable for Christian ministries, professionals like the Arensen’s on EMI’s staff are responsible for raising their own salaries. We are so grateful for God’s provision through the generosity of donors. Would you prayerfully consider partnering with the great things God is doing through the Arensen’s ministry at EMI?