The carbide manometer is a simple device which measures the water content of oil or biodiesel accurately in parts per million (ppm)
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 metres 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 coloured surface or wall.
Mix a little water with food colouring and 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.
It comes in the form of stones and must be kept stored in its airtight tub.



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 righthand 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.


In my next post I will describe how to calibrate the manometer.

My manometer reads 183mm if I test a sample containing 500ppm of water. If you construct your own carbide manometer you could use this figure as a rough guide but it is much better if you calibrate your apparatus.
Here is how I calibrated my carbide manometer. I began by bringing 2.5 litres of biodiesel up to 125 degrees C and kept it there for 2 hours. Steam stopped rising of the surface after an hour but I wanted to be sure it was completely dry..
Using a clean dry graduated cylinder I measured out a litre and added 2ml of water and immediately put it into a sealed container. This would be my 2000ppm reference sample
I then did the same with another litre but this time added half a ml of water. This would be my 500ppm reference sample.
I used a new clean 5ml syringe, I was very careful and I wore my closeup glasses, but I reckon this method cant do better than 10% accuracy. However I would be happy with that.

All of the following tests were carried out at 16 degrees C (289degrees K)
All samples were 100g to maximise accuracy and the vessel was shaken for 5 minutes or until no movement was seen for a minute
As a control I tested some of the totally dry bio and got a result of 9mm.
This may have been caused by the screwing closed of the cap or it may be a true reading caused by atmospheric moisture in the vessel. I dont know for sure but it is so small a reading it does not really matter.

I tested the sample with 2ml per litre or 2000ppm first. The reading was 622mm,

The sample with .5ml per litre or 500ppm gave a reading of 183mm.

On a A4 page of squared paper I marked off the vertical scale from zero to 3000ppm and on the horizontal scale I marked off from zero to 1000mm. I plotted my two results and drew a straight line through them with a ruler. With this graph I can translate any manometer reading into ppm instantly.