The Carbide Manometer

By Rick Da Tech


Testing oil for water using a Carbide ManometerASTM 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. It does have a drawback however, it cannot measure the water content of methanol.

Building the Manometer

Calcium Carbide Manometer to test for water in oil

To make a carbide manometer you will need a 1/2 pint mason jar with lid.  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 2.2lbs (1kg) from for about $100 USD. This is enough for hundreds of tests. It is sometimes available for under $10 a pound on ebay and at some specialty stores.

Crushing the Carbide

warningCalcium 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.

Calcium carbide comes from the store in rocks. We need to crush that into a powder. If you don't crush it to a fine powder, the reaction will be very slow, and can take over an hour to complete, and will give you unreliable readings. 

The easyist way to crush calcium carbide is to build a variant of the mortar and pestal called a 'Tube Mortar'. I like to call mine a carbide crusher. This is basically a pipe capped with a heavy rod plunger inside. I made mine from a 1" galvanized pipe with caps on each end. For a plunger, I purchased a "cold rolled steel" "round stock" 1" diameter and 4" long from a machine shop for a couple dollars.

To use the tube mortar, put a peice of calcium carbide in the mortar, screw on the cap and shake the mortar tube so the plunger goes up and down and crushes the calcium carbide. Continue until you have about 3g of fine powder. I use a window screen and funnel to seive out the big stuff for more time in the crusher. The picture below shows both the carbide before and after crushing. 


Carbide Crusher

Using the Manometer

To use the carbide manometer, place the plastic cup on a digital kitchen scales and pour in precisely 100 grams of biodiesel. Fill a plastic drink 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, you will need a bottle of new store bought vegetable oil.

  1. Test a sample of the oil straight from the bottle and record this reading.
  2. Prepare a 2000 ppm sample by adding 1 ml of water to 500 ml of oil and mix thoroughly.
  3. Test a sample of the 2000 ppm oil and record the reading
  4. Plot the two recordings on a graph with ppm marked along the bottom and cm along the left side of the page.
  5. Draw a temporary reference line between the two points.
  6. Draw a second line parallel to the temporary reference line that also crosses the 0cm, 0ppm point on the graph.
  7. Erase the temporary reference line you drew in step 5 above.

If the 2000 ppm sample creates too much pressure for the manometer to read, then you need to start over using a 50g sample. Once you go to a 50g sample you will need to always use a 50g sample size on every test or your calibration will no longer be valid.

When you buy oil at the store, it has some water in it. In this test we ignore that initial water in the beginning of the calibration. When we draw the parallel line, we are adjusting for the fact that the purchased oil has water in it. The advantage of this method of calibration is that we no longer need to “dry” our calibration oil.

When performing the calibration, make sure all the tests are performed at the same temperature. Room temperature (68F) is best.

Calcium Carbide Manometer to test for water in oil

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 from, 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.

Related Links

Simple test for water in fuel - Inopop thread on carbide manometer.

More Carbide Manometer Pictures


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