Paint Viscosity Chart / Cross-reference of Ford cups to Zahn cups

A. Hi Deshmukh. That formula is from ASTM D1200, and is only valid when the efflux time is between 20 and 100 seconds. It's an empirical formula for the best fit across that time range for the Ford #4 cup. The best fits for other cups use different multiplicands and subtrahends.

Luck & Regards,

A. You should be able to obtain tables with the conversions from the company that sold you the cups. Typically the EZ series Zahn cups come with a blue booklet that has these tables in them.

A. Not very difficult, really. I just googled "viscosity conversion chart" and got 489,000 hits. The very first one on the list.
has a comprehensive conversion chart available for free download at a click of the mouse button. No doubt there are many others among the 489,000.

A. The chart below may help you.

Chart for Viscosity Conversion for Reference

     cps    Ford#B3    Ford#B4    Zahn#2    Zahn#3
            (sec.)     (sec.)     (sec.)    (sec.)
	 
     10                  5         16
     20      12         10         18
     30      19         14         20
     40      25         18         22
     50      29         22         24
     60      33         25         27
     80      41         31         34
    100      50         34         41         10
    120      62         41         49         14
    140                 45         58         16
    200                 58         80         23
    240                 65                    28

The comparison given is for reference purpose only.

Viscosity Tester

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A. Try www.visco.co.za/downloads/ford-conversion-chart.pdf for Ford cup conversion chart - efflux seconds to Centistokes - then convert CPs = cSt X S.G.

viscositycup.com/ViscosityCupConversionChart.pdf

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Ed. note: That one is broken now as well, but both the table offered by Bill Reynolds and the one offered by Meghan Manjrekar give you that comparison.

A. Hi, Mohan.

The B4 cup offers you an estimate of the absolute viscosity of a liquid in centistokes based on its efflux time. The viscosities of liquids decrease with increasing temperature. But regardless of whether you specify the viscosity in terms of centistokes or Ford #4 efflux time or anything else, I think you would need to specify exactly what liquid you are speaking of before anyone could offer a chart to show the relationship between its viscosity and its temperature because different materials react so differently to temperature (when honey is hot it runs almost like hot water; when it's cold it's very much thicker than cold water). Good luck.

Regards,

A. Hi, Aniruddha.

Meghan's posting was from 3 years ago so he might not be around to answer.

But I think the basic issue is that there are practical limits to such rough conversions. Using a cup with a large hole to measure the viscosity of thin liquids that will drain in a flash would be like timing a jackrabbit with a sun dial; and using a cup with a tiny hole to measure the viscosity of very thick fluids would be like using a stop watch to time the lifespan of an tortoise :-)

The viscosity which you are trying to measure is beyond the workable range of an F4 cup. According to the previously referenced chart at www.visco.co.za/downloads/ford-conversion-chart.pdf, it looks like the efflux time for an F5 cup would be about 48 seconds.

Luck & Regards,

A. Dear David and other friends,
Try www.viscosity.com
-- Viscosity conversion tables by Norcross Corporation. I think it has wide range of useful information.

? Hi, Manikantan. Can you clarify in what way Meghan Manjrekar's chart does not answer your question? Thanks!

Regards,

A. Hi again Manikantan. Method ASTM D1200 covers viscosity measurement with the Ford Cup. Please also try to also get hold of the ASTM Gardner/Sward Paint Testing Manual [on Amazon affil links] as it has an excellent chapter on the subject of viscosity and its measurement with the various cups and devices. It includes a chart by Euverard showing a conversion factor of 3.7 for the Ford #4 cup. That is,

centistokes = efflux time x 3.7
centipoise = centistokes x specific gravity

But as previously explained to Aniruddha, you're using the wrong cup. A Ford cup with efflux time "more than 200 seconds", is meaningless. It's an extrapolation way off the chart :-(
A carpenter doesn't drive tacks with a sledgehammer, nor landscape spikes with his tack hammer; he uses different hammers for different jobs. Nor is there a single cup that allows good measurement of all possible viscosities. I think the basic problem is that you are trying to measure a high viscosity liquid with the wrong cup for the job; the first step would probably be to get a cup which is designed for higher viscosity liquids than the #4 cup . . . like a #55 cup. Best of luck!

Regards,

A. Hi, Siva. In light of the previous answers and referenced charts & tables, I'm not fully understanding wherein the difficulty lies. Can you please try to re-cast your question in terms of the answers already supplied? Sorry! I understand that the information supplied is deficient for your needs in some way, but I don't know in what way :-) Thanks!

Regards,

A. As per the ASTM D1200, "Standard Test Method for Viscosity by Ford Viscosity Cup", the following formula is used to convert efflux time in seconds (unit s) to Kinematic viscosity (unit cSt.) for Ford Cup Number 4:

Kinematic Viscosity (cSt) = 3.85 (t - 4.49), where t is efflux time in seconds.

Please remember that this ASTM method covers the determination of Newtonian or near-Newtonian liquid materials. For non-Newtonian, Method ASTM D2196 should be used.

A. Hi, Amit. Unfortunately my copy of the book I quoted is out on loan so I can't double-check but, as I recall, "3.7" is simply an empirical factor that is reasonably close for a narrow range of efflux times. If you look at the previously referenced chart at www.visco.co.za/downloads/ford-conversion-chart.pdf, you will be able to see that the centistokes is reasonably close to 3.7 times the B4 efflux time in seconds.

The ASTM D1200 formula quoted by Dr. Jamal is presumably a better fit.

Regards,

A. Hi Shah. Yes. But I can't tell whether there is something here which is specifically confusing you, or whether you haven't read the page :-) Please explain what it is about Meghan's chart that you are not following, then we will be able to clarify it. Thanks!

Regards,

? Hi Dinesh. Why is Meghan's chart insufficient for you? Please detail the issue for me. I am trying to understand this continuous repetition of a question that I thought was already very clearly answered several times. Please clarify what happens when you look for your answer on that chart. Thanks!

Regards,

A. Hi Dinesh. As noted, Meghan Manjrekar has already posted the relationship between Zahn #2 Cup and Ford B4 cup on this page so you can compare your Zahn readings vs. the Ford B4 readings and decide if they are close enough. To me they don't seem to be close enough because for sample A the Zahn reading was lower that the Ford reading, which goes against the whole direction of the chart.

To my knowledge, the only B4 cup is the Ford B4 cup, but I think you can't resolve this unclear communication by asking a 3rd party what the party of the first part was thinking :-)   

To me the answer is for the two lab to use the same cups. Good luck.

Regards,

A. Hi Ruchir. Although I am not an expert on this stuff, I am confident from simply reading this page, and looking at the several conversion charts on it, that your problem is the same as previously explained to Aniruddha and then explained again to Manikantan, i.e., that you have a cup that is designed to measure a lower viscosity fluid, and you are trying to use it to determine the viscosity of a higher viscosity fluid.

You're not finding the conversion factor that you're looking for because extrapolation will lead to huge errors and THEY DON'T WANT YOU TO EXTRAPOLATE. The charts tell you the viscosity of fluids that drain in 5-60 seconds, probably most accurately when the drain time is in the 20-40 second range. So when you ask the viscosity of a fluid that takes 140 seconds to drain from a Ford B4, the chart writers and cup makers don't want you to hazard wild & unrepeatable extrapolated guesses ... they want you to use a cup with a bigger hole :-)

Luck and Regards,

A. Paint viscosity is not a simple subject. Most paints are deliberately formulated to have non Newtonian characteristics. i.e. they typically contain thixotropic agents which give non drip properties. The flow rate (viscosity) depends on the shear speed so that the paint goes on thin and does not immediately flow off again before the solvent evaporates.
Ford, Zahn and similar measures are simple devices for use in the paint shop to confirm that under a very limited set of conditions the paint is thinned to a degree suitable for spraying. They are only comparators.
If you need a more accurate assessment you could start with
http://www.brookfieldengineering.com/download/files/Paint_Viscosity_Temperature.pdf
Welcome to the world of Rheology.

Ed. note: That link is broken now as well. Please use google to search for the conversion charts you need if you find broken links here. Hopefully the internet will stabilize soon :-)

A. From literature, I find that the kinematic viscosity (cSt) tapers off with increasing flow times in Ford cups or Zahn cups, above 40 - 60 secs flow times. So, it does not make much sense to measure flow times that exceed 60 secs, and try to correlate to cSt / cPs.

A. Hi Purushothaman. There is a 2-step conversion process presented at https://www.gardco.com/stormer_krebsconv_PU-G271.pdf … first you convert Krebs Units to centipoise from a graph they offer you, then you convert centipoise to centistokes by dividing by the specific gravity (S.G.) of your liquid. You can work it in reverse by multiplying your centistoke value by the S.G. to obtain centipoise, then look on the graph for the corresponding Krebs Units value. As that page will warn you, and this page already warns you, such conversions are only accurate for newtonian and near-newtonian fluids (where shear force is directly proportional to speed) and within very limited ranges. Good luck.

Regards,