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 ASTM specs call for biodiesel to be less than 500ppm. Crystal clear, as in not cloudy, biodiesel can contain as much as 1500ppm. As a result when blending it is possible to mix clear biodiesel with clear diesel and get a cloudy blend. The Carbide Manometer is a tool for measuring water content of biodiesel and WVO down to a few ppm making it a valuable resource for anyone wanting to make high quality biodiesel.
Imakebiodiesel, a regular at infopop, has come up with an alternative to the Sandy Brae water test kit for determining the water content of biodiesel. His Carbide Manometer uses Calcium Carbide, the fuel used in antique miners lamps. When exposed to water, calcium carbide chemically reacts to make acetylene. He puts a measured weight of biodiesel or oil in a water bottle, adds an excess of calcium carbide, mixing it after he has placed the lid on his water bottle. The lid of the water bottle has a tube connected to a manometer. A manometer measures pressure using water in a U-shaped tube. The more water in the oil, the higher the pressure. The pressure is read directly on the manometer as millimeters of water and converted to % water content using a graph. Imakebiodiesel states the Carbide Manometer has an accuracy of about 10%, making it the most accurate DIY tool available for measuring the water content of biodiesel.
Building the Manometer
To make a carbide manometer you will need a plastic drinks cup with an airtight screw on lid. I bought mine in a local supermarket for under 2 euro. You will also need 3 meters of clear 5mm pvc hose. Bore a suitable hole in the lid and push the hose through an inch or so. Seal with a hot glue gun or epoxy resin. Arrange the hose as shown in the picture using 5mm cable clips. I fixed mine to a white board but you can use any light colored surface or wall.
Mix a little water with food coloring and a couple drops of liquid dish washing detergent. Fill the U bend using a syringe. Tap the hose to remove any air gaps. Your carbide manometer is ready for use. before you can use it you need to get some Calcium Carbide. I bought 2.75lbs on Ebay for about 15 euro. This is enough for hundreds of tests.
Using the Manometer
 Calcium Carbide can burn without an ignition source if left open in humid air. It will absorb water from the air and give off acetylene. Store in an airtight container. Do not leave the container open for longer than necessary. For more information on Calcium Carbide, review the MSDS.
To use the carbide manometer, place the plastic cup on a digital kitchen scales and pour in precisely 100 grams of biodiesel. Using a hammer break up
a small stone of calcium carbide and crush it into powder. Fill a plastic drinks bottle cap with the powder and gently place it in the
cup of biodiesel. Without delay screw on the cap and mark the level of the right hand side of the manometer.
Hold the cup by the cap to avoid heating the biodiesel with your hand and shake gently for 5 minutes or until there is no measurable movement for a minute. Now measure the increase in the level of the manometer in millimeters.
With my manometer a measurement of 150-180 mm indicates that my biodiesel is dry to ASTM specifications but to be truly accurate you should calibrate your manometer as described in a previous post. Your plastic cup may be a different size and give a completely different reading to mine.
Calibrating the Manometer
 To calibrate the carbide manometer, start by making reference sample. Bring 2.5 liters of biodiesel up to 125 degrees C and keep it there for 2 hours. Steam will stop rising from the surface after an hour but continue for two full hours to be sure the biodiesel is completely dry.
Make the 2000 ppm reference by measuring out one liter into a clean dry container and adding 2ml of distilled water.
Make the 500 ppm reference by measuring out one liter into a clean dry container and adding a half a ml of water.
Save the remaining 500ml as a "dry" reference.
When making the references, use the most accurate methods available to measure out the water and the biodiesel. A graduated cylinder and syringe should give you at least a 10% accuracy.
When performing the calibration, make sure all the tests are performed with the reference samples at the same temperature. Room temperature (68°F) is best.
Test 100g of each reference solution and record the movement on the manometer, including the "dry" reference. The dry reference may not read exactly "zero" mm of water. Record the manometer reading for all three references.
When I tested my sample I found:
2000ppm = 622mm,
500ppm = 183mm,
0ppm = 9mm.
On a A4 page of graph paper mark off a vertical scale from zero to 3000ppm and a horizontal scale from zero to 1000mm. Plot the results and draw a straight line through them with a ruler. With this graph you can translate any manometer reading into ppm instantly.
Adjusting for Temperature
Temperature has an effect on the readings. If you can't bring the temperature of your test to the same temperature every time, the adjustment is simple. Divide the manometer reading (mm) by the temperature of the reference test in Kelvin and multiply by the current test temperature, also in Kelvin. Kelvin is equal to the temperature in Centigrade plus 273.
Finding Calcium Carbide
Calcium Carbide is available on Ebay and locally from caver supply stores. A couple of pounds will be enough to last a long time. It comes in rocks. The smaller the rocks the more expensive it is.
Calcium Hydride is also an acceptable reactant. It is used for the Sandy Brae Tester and is more reactive than Calcium Carbide.
My third suggestion for improving the manometer is to use a solvent when testing WVO.
The Sandy Brea test unit calls for mixing a dry solvent with the WVO at a 1 part solvent to 3 parts wvo rate. In recent discussions with Sandy Brea Labratories it appears this is neccesary since the reactive reagent must be able to quickly contact all of the water present in the sample and thinning the sample aids in this process.
The fourth..and possibly final suggestion is that since the reaction may take in excess of 5 minutes to complete and is exothermic the air traped above the sample and some of the gas produced will experience expansion and skew the results slightly. As long as the temperature of the sample rises this will always result in an indication of more water in the sample than actually is present. And for most of our purposes that is OK.
But if accuracy is paramount it may be advisable to add a digital thermometer to the test vessel.
I always have a "bath" of room teperature water handy in which to set my Sandy Brea test unit. Snce it is machined aluminum I am sure that heat is transferred from my hand to the contents (even though I wear an insulative glove). By setting the aluminum test vessel in the bath of room temp water after an initial reading I find the reading a few minutes after is usually slightly lower. This may be a good dea when using CC in place of CH since the reaction may be a bit slower and it is wise to be certain that the reaction has completed before trusting accuracy of the result.
I do not think it is practical to add a digital thermometer to the SBL test unit. However it would be very nice to be able to simply adjust the results for the rise in temp inside the test vessel. And probe type digital thermoeters are relativly inexpensive.
Again...this may not be neccesary for anyone who is not obsessive about accuracy.
EDIT:
OK..four more suggestions which might make it more accurate.
1.I just added an inexpensive valve to the cap of my pressure vessel which allows me to shake the sample without getting any in tube leading from it.
2.I assembled it from PVC fittings which allow me to easily clean and dry it to be sure none of the previous sample remains. It is capable of holding over 60psi without leaking. So If I have multiple samples I can ready each sample shake and allow them to sit to complete the reaction..then hook them up to the manometer for a quick test of each by simply opening the valve.
3.I also just added a way to seal and zero out the pressure in the vessel and THEN add the CC.
4.Instead of making a U of my clear tubing I plumb the valve on the test vessel to the cap of a polycarbonate soda bottle with a bit of that tube. I then half filled the soda bottle with colored water and installed a second length of clear tube through the cap which extends all the way to the bottom of the bottle. I then run that length of tube strait secured to the wall. Behind the tube is a graduated scale which has a line where the water in the bottle must be to be "zeroed out". This avoids having to add colored water to the clear tube and allow smaller tube to be used..which allows greater accuracy due to its' smaller volume per inch.
more later...
Dana
-Is your carbide soft? the carblide I have (over 1000 lbs) is in rock form the largest being 1" it is extremely hard and takes a good wack with a hammer to break, the rocks are much softer if they have bean exposed to damp air, this also reduces the amout of gas that you get from the carbide/water reaction. I have been placing the stones in a 1 1/4 chunck of tube and smashing them with a piece of 1 1/4 round bar. the problem is not with dust created at this piont, but in subsaquent handeling, you don't want acetylene being produced in your lungs. Tom My carbide is hard.
But in chunks smaller than 3/4".
Since the unit is enclosed there is no place for errant bits to fly or dust to drift to.
And IF one just uses the pestle to tap off the end cap with the powder in it it can easily be poured into the manometers pressure vessel.
Using an acorn nut on the "business end" of the pestle concentrates the force and mixes the powder/granual mixture around during pulverization so it all becomes very finely powdered.
Acorn nut
I also question the need to add any oil or biodiesel to a calibration sample, why not use 99.9% heat for a base line, the add graduated amounts of water for calibration purposes. Tom
Dana
-Is your carbide soft? the carblide I have (over 1000 lbs) is in rock form the largest being 1" it is extremely hard and takes a good wack with a hammer to break, the rocks are much softer if they have bean exposed to damp air, this also reduces the amout of gas that you get from the carbide/water reaction. I have been placing the stones in a 1 1/4 chunck of tube and smashing them with a piece of 1 1/4 round bar. the problen is not with dust created at this piont, but in subsaquent handeling, you don't want acetylene being produced in your lungs. Tom
The tutorial for biodiesel suggests using heat to drive all the water from a sample of biodiesel.
I am skeptical this will produce a water free sample of either biodiesel or VO.
Why not simply take some unused cooking oil or biodiesel and mix in enough CC to turn any water present to gas instead? Is there any reason this would not produce a completely water free sample of which can then be used to calibrate the manometer by adding known amounts of water?
A simple and cheap to make enclosed mortal/pestle which quickly turns carbide lumps into a very fine powder. This nearly negates the hazard of carbide dust/bits contacting damp tissue (like eyes and lungs). The resulting very fine powder speeds up the reaction.
The Carbide I have had in the past was in lumps no larger than 1/2". 1" lumps might not fit..but the unit can easily be made slighly larger by using large pipe and bolts/nuts.
To make this an enclosed mortar and pestle single take a 1" long 3/8" bolt with 2 nuts screwed all the way on and an acorn nut screwed on the end (inside a 6" length of 3/4" PVC pipe with caps on each end). This allows you to make a very fine and uniform CC powder in about 30 seconds of vertical shaking without any airborn dust being produced. Note:The is placed in the bottom an the Acorn nut end of the bolt assembly is pointed down.
I have an old acetylene generator that used carbide, yes carbide and water = acetylene! Please read the MSD sheets before you start breaking the carbide stones up with a hammer. Respiratory protection IS required!! Tom
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