Commit 65eebcd5 authored by Zach Fredin's avatar Zach Fredin
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parent ae1d7226
# Negative Pressure Covid Box
This to-be-better-named project is an isolation box that prevents aerosols from spreading beyond the patient's immediate area. It allows caregivers to use currently-prohibited respiratory aides, such as CPAP, BiPAP, and nebulizers. This rough CAD render shows the general idea:
This to-be-better-named project is an isolation box that prevents aerosols from spreading beyond the patient's immediate area. It allows caregivers to use currently-prohibited respiratory aides, such as CPAP, BiPAP, and nebulizers. The first physical prototype uses a modified version of our [Covid Isolation Box](, now equipped with a fancy cupola (images include protective film for clarity; in use, the box is transparent):
The box covers the patient from the torso up so they can be covered while prone or sitting up. The circular port at the back is connected to a HEPA filter and a blower inlet, while the cutout at the front has a flexible skirt to provide a reasonable seal.
## prototype details
This prototype is intended to validate simulation results and represents the first complete iterative spiral around the negative pressure box concept. Importantly, this model will answer a fundamental question about real-world airflow requirements; our initial simulations and expert design consultations (below) revealed a range of airflow specifications that spanned two orders of magnitude (~3 CFM - ~240 CFM). The fan used in this model splits the difference, topping out at ~90 CFM with substantial speed adjustment capability.
## status
Early development/exploration.
The design differs dramatically from other concepts in a few ways:
- the box itself is an innovative folded and latched design, which is lightweight, easy to ship and store, low cost, and disposable.
- the fan and filter assembly are integrated with the box to save cost and space.
- sophisticated electronic controls are used to monitor and control the system.
The upper section that holds the fan and electronics is cut and creased on our Zund using off-cuts from the larger Covid Isolation Box enclosure, then snapped together as shown here:
A [Bosch 6055C HEPA cabin air filter]( is adhered using a generous silicone bead to the top of the Covid Isolation Box, which now has a matching rectangular cutout to accommodate the filter:
The cupola then fits neatly around the filter:
During operation, the vacuum created by the fan securely holds the cupula assembly in place. Optionally, this piece can be taped or glued in place. Note that the critical leak path which leads around the HEPA filter is carefully sealed using silicone; other ingress paths, such as the snap assembly holes, caused negligible losses in efficiency and do not increase risk.
The holes next to the fan cutout will support a circuit board which will include a 24 VDC power jack, a status LED, a differential pressure sensor with extension tube and snubber, a buck converter, a USB debug/logging port, an on/off switch, and a SAMD11 microcontroller:
The differential pressure sensor will measure the vacuum inside the cupola, using the extension tube and snubber to reduce turbulence effects from the fan. This measurement will be used to validate simulation results at different fan speeds, and to quantify the effects of different shroud configurations around the patient's torso and caregiver access port. If necessary, this circuitry could remain in place to provide an alert if airflow conditions are no longer sufficient to evacuate aerosols from the interior of the box; ideally, the eventual version will simplify this portion to a simple fixed speed circuit to reduce cost and complexity.
Next steps:
- finish electronics design, construction, and programming.
- update simulation with detailed box model, fan flow rate, and filter.
- smoke wand, anemometer, and differential pressure testing to validate simulation results.
- iterate as needed.
## need
As discussed in an [issue I posted]( in mid-April, covid-19 has dramatically curtailed the support toolkit available to respiratory specialists. To recap:
......@@ -19,17 +52,6 @@ In short, a system that makes BiPAP, CPAP, and nebulizers viable again has great
- less need for PPE during non-physical patient interactions
- improved caregiver safety due to reduced aerosol contamination risk
## plans
While it is tempting to simply bolt a blower onto our [Covid Isolation Box](, one must remember the ubiquitous warning sticker present on most fume hoods:
In order to scavenge hazardous vapors, fume hoods need laminar flow and adequate velocity; the aforementioned sticker reminds users that raising the sash above a marked point disrupts airflow enough to prevent the system from functioning properly. Similarly, we must understand the airflow in our system and design the exhaust mechanism accordingly if we want it to protect caregivers. As such, we are starting with simulation using Dassault's [Simulia platform](, building on their work simulating sneezing, coughing, and breathing for our mask and face shield design efforts. The simulation results will feed into [blower sizing and selection](, at which point we will build our first physical prototype.
![initial simulation setup](img/Screen_Shot_2020-04-21_at_9.58.33_AM.png)
Due to the inherent uncertainty in sealing around the perimeter of the box, it is likely that we will need to implement some kind of active blower control to maintain sufficient aerosol scavenging. Since the exhaust will pass through a safety-critical filter, we should also monitor filter performance online and alert caregivers when replacement is needed. In both cases, we will likely instrument the box with low-cost PCB-mounted differential pressure sensors such as [these models]( from Amphenol (~$30 each single-lot from Digi-Key and in stock). Again, simulation results should point us to the needed pressure differential range we need to maintain to produce good flow conditions, eliminating the need for airflow sensors.
## design consultation
Gordon Sharp (chairman of Aircuity / visual artist / fume hood design veteran) and Willie Baker, MD (BMC) provided many useful comments they permitted me to reproduce here:
......@@ -54,7 +76,7 @@ Gordon Sharp (chairman of Aircuity / visual artist / fume hood design veteran) a
> Gordon
> Zach,
> I’m no airflow expert, but I agree with everything Gordon’s mentioned below. We didn’t have a smoke wand when we were in the sim lab on Friday night, but used a fine streamer to measure directional flow (and provide gestalt regarding velocity) with various size and number of openings in 3 different raw ‘box’ designs and the “Sharp Unit” pulled air across all openings in all the designs.
> Look at the material in particularly the 2020CDC_IsolationDesign and OK DOH Design files, particularly materials pertaining to ventilated headboard which are featured extensively in this paper, one image being on page 55 as listed on bottom of PDF (pg 67/197), a portable unit being figure 25, page 80 (92/167). They were able to achieve adequate flow/filtration with this wide open design, 8 ft sq open area. If we narrowed down the open area even a little we could drastically diminish our flow requirements (they measured 16 air exchanges/h at 240 cfm).
......@@ -85,9 +107,9 @@ Gordon Sharp (chairman of Aircuity / visual artist / fume hood design veteran) a
> willie
> Zach:
> I have about 80 of these 12 VDC 140 mm fans and am happy to give you some if that would be easier and I can get them to you. These fans are best controlled by a pulse width modulated signal running at 20 to 25 kHz. I also have one more small analog board with a pot on it that can be used to generate this signal that you can have as well. Alternatively, you can of course generate these signals from various sources, even an Arduino programmed to do this.
> Having some indication of proper flow so that people know the unit is working would be good, perhaps using the pressure sensor to measure velocity pressure of the air flow. Measuring the pressure drop across the fan would be useful if the filter is intended to be used for a while so the gradual buildup of particles and blockage of the filter could be detected. This pressure drop is much larger than the velocity pressure so it will also be an easier means to get at least an indication of flow. If it will be thrown away more frequently then it will not be as much of a need there to measure filter loading. Additionally, if velocity pressure or flow is directly measured this will also be an indication of filter loading or blockage.
> Also be aware that one issue with small, high power inline fans is acoustic noise. To get the required flow and pressure they run at very high speeds in some case up to 15,000 rpm. The 140 mm fan that I use run at 7600 rpm at full speed with about 70 dBa of noise generation with a high tonal component. At full speed this frequency component is about 630 Hz. This is also an issue for my work with my kinetic sculptures so I am working to employ a variety of means to reduce this noise such as acoustic noise absorption with some 1” acoustic foam as well as plastic acoustic resonators that use passive noise reduction utilizing an old patented design of mine that I used with my airflow control valves. Perhaps we can reduce the speed if we don’t need the full flow of the fans as running at lower speeds will also significantly reduce the acoustic noise generation. With this noise in mind, putting these fan filter units close to the patient’s head instead of on the floor will aggravate this issue and create discomfort for the patient.
......@@ -111,17 +133,6 @@ Riley Kolus from BC helped consolidate design requirements from his discussions
- Access+visibility from the back and both sides for central line placement.
- Access+visibility from the right side for surgical airway procedures (may require two-handed access).
## BOM thoughts
Fan: [Sanyo San Ace 9GA0824P1S611](, $21.30 qty 1:
![San Ace](img/9GA0824P1S611_curve.png)
Differential pressure sensors: [Amphenol NPA-730B-05WD](, $27.96 qty 1, 5 inH2O range, PCB mount/SMT, 3 mm hose barb ports, I2C output
Filter: [Bosch HEPA cabin filter model 6055C](, common (many Toyota/Lexus models 2006-present), unknown specs
## other efforts
Harvard's GSD is working on a [similar concept]( a disposable folded box and a negative pressure system to reduce aerosol risk.
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