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michka:research:microbial_fuel_cells:cardiff5 [2016-09-19 09:12] – michkamichka:research:microbial_fuel_cells:cardiff5 [2016-09-21 09:56] (current) – michka
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 =====MFCitizen @Cardiff===== =====MFCitizen @Cardiff=====
  
-**September 7th-9th, 2016** +**[[http://www.zprod.org|Paul Granjon]] & [[http://michkamelo.wordpress.com|Michka Mélo]], September 7th-9th, 2016** 
-**[[http://www.zprod.org|Paul Granjon]] & [[http://michkamelo.wordpress.com|Michka Mélo]]**+ 
 +{{:michka:research:microbial_fuel_cells:20160909_184144.jpg?500|}}
  
 ====Abstract==== ====Abstract====
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 We decided to focus this second session on the topic of microbial fuel cells (MFCs), a renewable electricity production device in which electrogenic bacteria digest organic matter in an anaerobic environment. We decided to focus this second session on the topic of microbial fuel cells (MFCs), a renewable electricity production device in which electrogenic bacteria digest organic matter in an anaerobic environment.
  
-Even though the MFCs power output are very low (typically 100 W/m3 [REF], or [[http://www.instructables.com/id/How-to-Make-a-Microbial-Fuel-Cell-MFC-Using-Mud/?ALLSTEPS]|50-100 microwatts]] for the 0.5L container of the Mudwatt kit, they are very interesting because: +Even though the MFCs power output are very low (typically [[http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.469.1949&rep=rep1&type=pdf|100 W/m3]], or [[http://www.instructables.com/id/How-to-Make-a-Microbial-Fuel-Cell-MFC-Using-Mud/?ALLSTEPS]|50-100 microwatts]] for the 0.5L container of the Mudwatt kit, they are very interesting because: 
    * they may be made 100 % from municipal solid waste and renewable resources (mud, organic waste) which can be found everywhere ;    * they may be made 100 % from municipal solid waste and renewable resources (mud, organic waste) which can be found everywhere ;
    * their operation can be quite continuous, compared to other renewables.    * their operation can be quite continuous, compared to other renewables.
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 After few prototyping sessions and a first workshop led on the topic, we got actual voltage and current measurements - even though they were quite low to be of any use. Exploring possible designs for a Power Harnessing System (PHS) harnessing and storing these little amounts of energy to make them usable seemed quite necessary. We also needed to build more accurate and more easy-to-use performance tracking system (PTS) than a simple manual multimeter, to monitor efficiently the voltage and current output of the MFCs. After few prototyping sessions and a first workshop led on the topic, we got actual voltage and current measurements - even though they were quite low to be of any use. Exploring possible designs for a Power Harnessing System (PHS) harnessing and storing these little amounts of energy to make them usable seemed quite necessary. We also needed to build more accurate and more easy-to-use performance tracking system (PTS) than a simple manual multimeter, to monitor efficiently the voltage and current output of the MFCs.
  
-We therefore decided to start from a solid ground, the Mudwatt kit. Paul ordered three of them, which we set up. The one filled up with a smelly damped clay slush from the beach took-of on the second day of the Hackathon, and its LED was blinking twice every second on the end of the fifth day. The other two had not started yet at the end of our 3-days hackathon, but Mudwatt says they take at least 3-7 days to start [REF].+We therefore decided to start from a solid ground, the Mudwatt kit. Paul ordered three of them, which we set up. The one filled up with a smelly damped clay slush from the beach took-of on the second day of the Hackathon, and its LED was blinking twice every second on the end of the fifth day. The other two had not started yet at the end of our 3-days hackathon, but [[https://s3.amazonaws.com/cdn.teachersource.com/downloads/lesson_pdf/MudWatt_Educational_and_Instructional_Booklet.pdf|Mudwatt says they take at least 3-7 days to start]].
  
 While the Mudwatts were starting up, we worked on the PHS design and fabrication. We explored many different ways detailed below, including small surface-mount DC/DC boost converters, as well as through-hole-components-based circuits such as Joule Thiefs. Some further work needs to be done there to find the appropriate solution for the MFCs. While the Mudwatts were starting up, we worked on the PHS design and fabrication. We explored many different ways detailed below, including small surface-mount DC/DC boost converters, as well as through-hole-components-based circuits such as Joule Thiefs. Some further work needs to be done there to find the appropriate solution for the MFCs.
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 We decided to focus this second session on the topic of microbial fuel cells (MFCs), a renewable electricity production device in which electrogenic bacteria digest organic matter in an anaerobic environment. We decided to focus this second session on the topic of microbial fuel cells (MFCs), a renewable electricity production device in which electrogenic bacteria digest organic matter in an anaerobic environment.
  
-Even though the MFCs power output are very low (typically 100 W/m3 [REF], or [[http://www.instructables.com/id/How-to-Make-a-Microbial-Fuel-Cell-MFC-Using-Mud/?ALLSTEPS]|50-100 microwatts]] for the 0.5L container of the Mudwatt kit, they are very interesting because: +Even though the MFCs power output are very low (typically [[http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.469.1949&rep=rep1&type=pdf|100 W/m3]], or [[http://www.instructables.com/id/How-to-Make-a-Microbial-Fuel-Cell-MFC-Using-Mud/?ALLSTEPS]|50-100 microwatts]] for the 0.5L container of the Mudwatt kit, they are very interesting because: 
    * they may be made 100 % from municipal solid waste and renewable resources (mud, organic waste) which can be found everywhere ;    * they may be made 100 % from municipal solid waste and renewable resources (mud, organic waste) which can be found everywhere ;
    * their operation can be quite continuous, compared to other renewables.    * their operation can be quite continuous, compared to other renewables.
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 ===1.1 Getting mud for the Mudwatt=== ===1.1 Getting mud for the Mudwatt===
 +
 +{{:michka:research:microbial_fuel_cells:20160907_090610.jpg?500|}}
  
 We started the working session by walk to the seaside in Barry (Wales, UK), to collect: We started the working session by walk to the seaside in Barry (Wales, UK), to collect:
    * a kind of clay slush    * a kind of clay slush
 +{{:michka:research:microbial_fuel_cells:dscf8150.jpg?300|}}
 +{{:michka:research:microbial_fuel_cells:dscf8151.jpg?300|}}
    * a more sandy slush    * a more sandy slush
 +{{:michka:research:microbial_fuel_cells:dscf8153.jpg?300|}}
 +{{:michka:research:microbial_fuel_cells:dscf8154.jpg?300|}}
 +
 … to feed the Mudwatts that Paul got previously. … to feed the Mudwatts that Paul got previously.
  
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 ===1.2 Filling the Mudwatt=== ===1.2 Filling the Mudwatt===
  
-We filled the Mudwatts first thing when we arrived at the workshop, following Mudwatt’s manual.+We filled the Mudwatts first thing when we arrived at the workshop, following [[https://s3.amazonaws.com/cdn.teachersource.com/downloads/lesson_pdf/MudWatt_Educational_and_Instructional_Booklet.pdf|Mudwatt’s manual]]. 
 + 
 +{{:michka:research:microbial_fuel_cells:dscf8156.jpg?300|}} 
 +{{:michka:research:microbial_fuel_cells:dscf8155.jpg?300|}}
  
 We labeled them as follows: We labeled them as follows:
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    * the sand-slush one was called **S** (for **“Sable”**)    * the sand-slush one was called **S** (for **“Sable”**)
    * the garden soil one was called **J** (for **“Jardin”**)    * the garden soil one was called **J** (for **“Jardin”**)
 +
 +{{:michka:research:microbial_fuel_cells:20160907_104135.jpg?500|}}
  
 ===1.3 First performance measurements=== ===1.3 First performance measurements===
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 ===1.4 First LED blinks !=== ===1.4 First LED blinks !===
  
-Mudwatt’s manual [REF] tells us that the MFC may take 3 to 7 day to start-up sufficiently for the LED to blink.+[[https://s3.amazonaws.com/cdn.teachersource.com/downloads/lesson_pdf/MudWatt_Educational_and_Instructional_Booklet.pdf|Mudwatt’s manual]] tells us that the MFC may take 3 to 7 day to start-up sufficiently for the LED to blink.
  
 Paul was performing a quick open-circuit voltage measurement on the Mudwatts in the middle of the afternoon of day 2, when he noticed that the open-circuit voltage across the B Mudwatt had jumped from several dozens mV to 0.4 V. Paul was performing a quick open-circuit voltage measurement on the Mudwatts in the middle of the afternoon of day 2, when he noticed that the open-circuit voltage across the B Mudwatt had jumped from several dozens mV to 0.4 V.
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 He decided to plug the Hackerboard with its red LED on the B Mudwatt, to see what happened… and it blinked ! One time per second ! He decided to plug the Hackerboard with its red LED on the B Mudwatt, to see what happened… and it blinked ! One time per second !
  
-The B Mudwatt, made from Barry’s bay clay-slush, was ready to the power the LED a little bit over 24 hours after setup !+The B Mudwatt, made from Barry’s bay clay-slush, was ready to the power the LED around 30 hours after setup !
  
 ===1.5 Performance measurements=== ===1.5 Performance measurements===
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 We decided to use our freshly prototyped PTS (the INA219 current sensor on an Arduino Uno, see section 3 for details) on the B Mudwatt to see how much power was coming from out there. We decided to use our freshly prototyped PTS (the INA219 current sensor on an Arduino Uno, see section 3 for details) on the B Mudwatt to see how much power was coming from out there.
 +
 +{{:michka:research:microbial_fuel_cells:20160909_100612.jpg?500|}}
  
 Right after connexion, the load voltage was 1.03 V, and the current measured was around 10 mA, but decreasing very quickly. Right after connexion, the load voltage was 1.03 V, and the current measured was around 10 mA, but decreasing very quickly.
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 Mudwatt’s PHS - also known as its Hackerboard - is super simple: it consists of a single five-leads chip, connected to one or two small capacitor, and the load, which can be an LED or a clock. Mudwatt’s PHS - also known as its Hackerboard - is super simple: it consists of a single five-leads chip, connected to one or two small capacitor, and the load, which can be an LED or a clock.
  
-Info on the five-leads chip was hard to get. An old version of Mudwatt’s manual [REF] was refering to a now dead forum of Keegotech [REF]. +Info on the five-leads chip was hard to get. An old version of [[https://s3.amazonaws.com/cdn.teachersource.com/downloads/lesson_pdf/MudWatt_Educational_and_Instructional_Booklet.pdf|Mudwatt’s manual]] was refering to [[http://keegotech.com/forum|a now dead forum of Keegotech]]. 
  
-Nowhere in the manual, nor in the old forum thread scavenged from Discs archives [REF] could we find the reference of the component. There was just a reference to a “charge-pumping” activity.+Nowhere in the manual, nor in the old forum thread scavenged [[https://disqus.com/home/discussion/keegotechnologies/keegotech_forum_73/|from Discs archives]] could we find the reference of the component. There was just a reference to a “charge-pumping” activity.
  
-By analysing thoroughly the situation, Paul noticed that the component had the same packaging and number of leads of a charge-pump I had seen in a paper from Meehan and Gao [REF], the Seiko 882-Z.+By analysing thoroughly the situation, Paul noticed that the component had the same packaging and number of leads of a charge-pump I had seen in [[https://cdn.hackaday.io/files/10801462109984/Energy%20Harvesting%20With%20Microbial%20Fuel%20Cell%20and%20Power%20Management%20System.pdf|a paper from Meehan and Gao]], the [[https://cdn.hackaday.io/files/10801462109984/S-882Z.pdf|Seiko 882-Z]].
  
 We got the confirmation from Johann during our Skype call that it was this component that they used, as they named it at the end of [[http://www.instructables.com/id/How-to-Make-a-Microbial-Fuel-Cell-MFC-Using-Mud/?ALLSTEPS|their Instructable]]. We got the confirmation from Johann during our Skype call that it was this component that they used, as they named it at the end of [[http://www.instructables.com/id/How-to-Make-a-Microbial-Fuel-Cell-MFC-Using-Mud/?ALLSTEPS|their Instructable]].
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 Several scientific papers detail PHS designs for single MFCs. Several scientific papers detail PHS designs for single MFCs.
- - Meehan and Gao [REF] designed a PHS based on [[https://cdn.hackaday.io/files/10801462109984/S-882Z.pdf|a Seiko 882-Z charge pump]], connected to a 2.2 F B0820-2R5335-R Cooper/Bussman supercapacitor and controlling MOSFET switches activating a ST Microelectronics L6920DB Boost Converter. + - [[https://cdn.hackaday.io/files/10801462109984/Energy%20Harvesting%20With%20Microbial%20Fuel%20Cell%20and%20Power%20Management%20System.pdf|Meehan and Gao]] designed a PHS based on [[https://cdn.hackaday.io/files/10801462109984/S-882Z.pdf|a Seiko 882-Z charge pump]], connected to a 2.2 F B0820-2R5335-R Cooper/Bussman supercapacitor and controlling MOSFET switches activating a ST Microelectronics L6920DB Boost Converter. 
- - Shantaram et al. [REF] designed a PHS based on a Maxwell 4F [@? TODO] supercap in series with the Max1797 DC/DC converter. + - [[https://www.researchgate.net/profile/Haluk_Beyenal/publication/7692918_Wireless_Sensors_Powered_by_Microbial_Fuel_Cells/links/004635316d9dccaf71000000.pdf|Shantaram et al.]] designed a PHS based on a Maxwell 4F [@? TODO] supercap in series with the Max1797 DC/DC converter. 
- - Thomas et al. [REF] designed a PHS based on a 1:20 < 1 V transformer, connected to an ultra-low-voltage LTC3108 step-up converter, connected to an output 680 uF capacitor and an storage 0.4 F supercapacitor. This PHS is the one that the Flower Power Pot project is trying to replicate at the moment.+ - [[https://hal-univ-rennes1.archives-ouvertes.fr/hal-00832354/file/post-print_j_power_sources_2013.pdf|Thomas et al.]] designed a PHS based on a 1:20 < 1 V transformer, connected to an ultra-low-voltage LTC3108 step-up converter, connected to an output 680 uF capacitor and an storage 0.4 F supercapacitor. This PHS is the one that the Flower Power Pot project is trying to replicate at the moment.
  
 Meehan and Gao’s design is very interesting, as it could be described as a “Mudwatt PHS on steroids”, starting as low-power, and powering stronger bursts thanks to the boost converter. Sadly, it also relies on the Seiko 882-Z discontinued charge pump. Meehan and Gao’s design is very interesting, as it could be described as a “Mudwatt PHS on steroids”, starting as low-power, and powering stronger bursts thanks to the boost converter. Sadly, it also relies on the Seiko 882-Z discontinued charge pump.
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 However, as we realized later on, their quite peculiar manganese-based MFC design with sacrificial electrode has a way higher output voltage, which allows to charge the supercapacitor without any charge pump. Our regular Mudwatt-like MFC design does not allow for such high voltages, and the capacitor charge will be stuck at the MFCs output, which is likely to be a few hundred mV at best. However, as we realized later on, their quite peculiar manganese-based MFC design with sacrificial electrode has a way higher output voltage, which allows to charge the supercapacitor without any charge pump. Our regular Mudwatt-like MFC design does not allow for such high voltages, and the capacitor charge will be stuck at the MFCs output, which is likely to be a few hundred mV at best.
  
-Thomas’ design is quite different than Meehan and Gao’s, using a small 1:20 coupled-inductor transformer instead of a charge pump. However, as we realized as Paul was cutting the PCB for the freshly ordered LTC3108 converter on his CNC, both recommended chips for the transformer [REF Flower Power] seemed not to be available anymore, or available only in large quantities (1000+).+Thomas’ design is quite different than Meehan and Gao’s, using a small 1:20 coupled-inductor transformer instead of a charge pump. However, as we realized as Paul was cutting the PCB for the freshly ordered LTC3108 converter on his CNC, [[https://hackaday.io/project/10801-power-flower-pot-plants-electricity-and-light|both recommended chips for the transformer]] seemed not to be available anymore, or available only in large quantities (1000+).
  
 Anyway, we: Anyway, we:
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 In addition to that, we: In addition to that, we:
-   * assessed the use of a Beam robotics Solar Engine design [REF] as a MFC PHS. +   * assessed the use of a [[http://www.beam-online.com/Robots/Circuits/1381.html|Beam robotics Solar Engine 1381 design]] as a MFC PHS; 
-   * explored in length the use of a Joule Thief circuit as a MFC PHS+   * explored in length the use of a Joule Thief circuit as a MFC PHS.
  
 ===2.3 Cutting a PCB for the LTC3108 DC/DC converter (Thomas et al. PHS)=== ===2.3 Cutting a PCB for the LTC3108 DC/DC converter (Thomas et al. PHS)===
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      * sometimes, templates come up in large .zip files with components from a manufacturer, such as a Linear-Technology.zip. Then, you have to check if your component is in it.      * sometimes, templates come up in large .zip files with components from a manufacturer, such as a Linear-Technology.zip. Then, you have to check if your component is in it.
    * open the template in Eagle, and check that it is the right one.    * open the template in Eagle, and check that it is the right one.
 +{{:michka:research:microbial_fuel_cells:20160907_122902.jpg?500|}}
      * if you are using, as we did, a fiber-glass support with a copper layer on top, keep in mind that were you cut, you are removing the copper, and therefore the electrical connexion.      * if you are using, as we did, a fiber-glass support with a copper layer on top, keep in mind that were you cut, you are removing the copper, and therefore the electrical connexion.
    * export the G-code to Eagle’s plug-in connecting it to the CNC    * export the G-code to Eagle’s plug-in connecting it to the CNC
 +{{:michka:research:microbial_fuel_cells:20160907_122918.jpg?500|}}
      * beware that the G-code is defined in the right unit - metric or imperial system.      * beware that the G-code is defined in the right unit - metric or imperial system.
    * make the CNC run    * make the CNC run
 +{{:michka:research:microbial_fuel_cells:dscf8160.jpg?500|}}
      * this part of the process is quite technical, as the dimensions there are challenging Paul’s CNC sensitivity. Paul had to try out several times, and play with :      * this part of the process is quite technical, as the dimensions there are challenging Paul’s CNC sensitivity. Paul had to try out several times, and play with :
        * the number of passes of the CNC on each cut        * the number of passes of the CNC on each cut
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 ===2.4 Building Shantaram et al.’s PHS=== ===2.4 Building Shantaram et al.’s PHS===
 +
 +{{:michka:research:microbial_fuel_cells:dscf8174.jpg?500|}}
  
 This was basically the same process as stated above, with a digression in updating Eagle to read the library files for the Maxim1797 layout, and getting compatibility issues with the CNC plug-in. This was basically the same process as stated above, with a digression in updating Eagle to read the library files for the Maxim1797 layout, and getting compatibility issues with the CNC plug-in.
  
 Last but not least, soldering the small surface-mount Maxim1797 and its tiny leads to the chip was quite a challenge, which Paul has adressed with years of hand-made-machines crafting experience. He drops a spot of superglue below the chip to paste it to the PCB, and then solders each lead with a tiny soldering iron tip. Last but not least, soldering the small surface-mount Maxim1797 and its tiny leads to the chip was quite a challenge, which Paul has adressed with years of hand-made-machines crafting experience. He drops a spot of superglue below the chip to paste it to the PCB, and then solders each lead with a tiny soldering iron tip.
 +
 +{{:michka:research:microbial_fuel_cells:20160909_134347.jpg?300|}}
 +{{:michka:research:microbial_fuel_cells:20160909_162801.jpg?300|}}
 +{{:michka:research:microbial_fuel_cells:20160909_162823.jpg?300|}}
 +{{:michka:research:microbial_fuel_cells:dscf8175.jpg?300|}}
  
 This being said, god bless through holes components ! This being said, god bless through holes components !
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 ===2.5 Beam robotics “Solar engine” PHS=== ===2.5 Beam robotics “Solar engine” PHS===
 +
 +{{:michka:research:microbial_fuel_cells:dscf8163.jpg?500|}}
  
 When discussing about a lower-tech charge pump circuit, Paul mentionned the Beam robotics circuits, which are often made of basic through-hole off-the-shelf components, and are powered by renewables. When discussing about a lower-tech charge pump circuit, Paul mentionned the Beam robotics circuits, which are often made of basic through-hole off-the-shelf components, and are powered by renewables.
  
-Paul identified a circuit which may be interesting for our application : [[http://beam-online.com/Robots/Circuits/1381.html|the Solar Engine 1381]].+Paul identified a circuit which may be interesting for our application : [[http://beam-online.com/Robots/Circuits/1381.html|the Beam Robotics Solar Engine 1381]].
  
 This very smart and simple circuit, which I took quite a bit of time to fully understand (lacking some basic transistor dynamics understanding), works quite well, but is limited by the storage capacitor voltage, which depends on the solar cell voltage. This very smart and simple circuit, which I took quite a bit of time to fully understand (lacking some basic transistor dynamics understanding), works quite well, but is limited by the storage capacitor voltage, which depends on the solar cell voltage.
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 Anyway, we prototyped and tested the circuit (replacing the motor by a LED), and it worked perfectly well. A 2200 uF proved to be way to small to really see the circuit effect on a LED. We replaced it by a 10 F supercapacitor, and it worked great, lighting the LED for several minutes after cutting having charged the supercapacitor and shut down the power supply. Anyway, we prototyped and tested the circuit (replacing the motor by a LED), and it worked perfectly well. A 2200 uF proved to be way to small to really see the circuit effect on a LED. We replaced it by a 10 F supercapacitor, and it worked great, lighting the LED for several minutes after cutting having charged the supercapacitor and shut down the power supply.
 +
 +{{:michka:research:microbial_fuel_cells:dscf8162.jpg?500|}}
  
 ===2.6 Through-hole-components-based charge pump=== ===2.6 Through-hole-components-based charge pump===
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 As we realized that none of our circuits will be completed during our Hackathon for various reasons - missing components, high voltage operation, I tried to go for a through-hole-components-based charge pump. As we realized that none of our circuits will be completed during our Hackathon for various reasons - missing components, high voltage operation, I tried to go for a through-hole-components-based charge pump.
  
-The Wikipedia page of charge pumps [REF] refered to a quite simple circuit which could be made of simple components.+The [[https://en.wikipedia.org/wiki/Charge_pump|Wikipedia page on charge pumps]] refered to a quite simple circuit which could be made of simple components.
  
 We were skeptical regarding the ability to design a charge pump operating at low voltages and low currents with basic through-hole components, but wanted to give it a try anyway. We were skeptical regarding the ability to design a charge pump operating at low voltages and low currents with basic through-hole components, but wanted to give it a try anyway.
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 ====2.7 Joule Thief PHS==== ====2.7 Joule Thief PHS====
 +
 +{{:michka:research:microbial_fuel_cells:dscf8171.jpg?500|}}
  
 ===2.7.1 Literature review=== ===2.7.1 Literature review===
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 ==2.7.2 First prototype and next steps== ==2.7.2 First prototype and next steps==
 +
 +{{:michka:research:microbial_fuel_cells:dscf8168.jpg?500|}}
  
 While searching all the information mentionned above, we wanted to do a quick test with a basic JT design. While searching all the information mentionned above, we wanted to do a quick test with a basic JT design.
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 ====3. Performance Tracking System==== ====3. Performance Tracking System====
 +
 +{{:michka:research:microbial_fuel_cells:20160909_100612.jpg?500|}}
  
 ===3.1 Why INA219 current sensor ?=== ===3.1 Why INA219 current sensor ?===
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 ===4.1 Graphite electrode=== ===4.1 Graphite electrode===
 +
 +{{:michka:research:microbial_fuel_cells:dscf8165.jpg?500|}}
  
 Paul built a graphite electrode of out graphite rods we bought at The Works, an Arts & Crafts shop. Paul built a graphite electrode of out graphite rods we bought at The Works, an Arts & Crafts shop.
 +
 +{{:michka:research:microbial_fuel_cells:20160908_182623.jpg?500|}}
  
 He cut down the rods in smaller chunks, drilled them, and used an old bicycle stainless steel cable to assemble them like a collar. He cut down the rods in smaller chunks, drilled them, and used an old bicycle stainless steel cable to assemble them like a collar.
 +
 +{{:michka:research:microbial_fuel_cells:20160908_182640.jpg?300|}}
 +{{:michka:research:microbial_fuel_cells:20160908_182710.jpg?300|}}
  
 Then, he tightened them up with a second bit of bicycle staineless steel cable (a choice of material advised by Johann, from the Flower Power pot project). Then, he tightened them up with a second bit of bicycle staineless steel cable (a choice of material advised by Johann, from the Flower Power pot project).
 +
 +{{:michka:research:microbial_fuel_cells:dscf8164.jpg?500|}}
  
 A quick multimeter test showed that the overal resistance of the assembly was about 3 ohms. A quick multimeter test showed that the overal resistance of the assembly was about 3 ohms.
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