Written by Art Gatenby | Mar 29, 2012 7:55:00 PM

We get repeated queries about calibration of our DuNouy Ring Tensiometers. That makes me think that calibrating procedures to insure accurate surface tension measurment needs some clarification.

Our questions fall into the three general categories

1. Basic Calculations
2. Procedures
3. Special Problems

I 'll strive address the most pervasive.

Basic Calculations:

Where can I get gravity computations for my location?

The gravity information for most places in the world is available from the National Oceanic and Atmospheric Adminiatration (NOAA).

Why do I need to put a weight on the ring for calibration?

The weight provides a known force, which is part of a calculation that includes:

1. correction for gravitational pull at the testing location and

2. the ringâ€™s dimensions.

The calibration reading is developed by multiplying the weight times the gravity and dividing this result by twice the ring circumference (Reading = Weight x gravity/2 x Circumference). The ring circumference is printed on the ring storage box.

How much Weight should I use for the calibration process?

We recommend a weight between 500 and 800 milligrams (mg). If a standard weight is not available, a paper clip will suffice if weighed to the closest mg (.001 grams)

Procedures:

What do I do after calculating the reading?

This is a mulch-step process.

• First, turn the adjustment knob to line up the pointer with the black stripe on the mirror. Then, check the dial reading.

• If the reading is higher than the calculated number, shorten the arm by turning the adjustment nut CLOCKWISE. If the reading is less than the calculated number, lengthen the arm by turning the adjustment nut COUNTER-CLOCKWISE.

[Note: The adjustment is on the left end of the arm for the 70535 Precision Tensiometer. There are two adjustment nuts on the right side of the arm for the 70545 Interfacial Tensiometer. Each of these 70545 adjustment nuts must be moved the same number of turns.]

After turning the adjustment nut about 1 full turn, check to determine if the reading gets closer to the calculated number.

From here, make the necessary additional adjustment to bring the reading to within 0.5 dynes of the calculated number.

When this process is completed, check the zero point.

• Remove the weight and paper from the ring. Then turn the knurled knob to line up the pointer and the black stripe on the mirror.

• It is possible that the zero point will require adjustment by loosening the dial clamp and matching the zero with the Vernier zero.

• If the Zero need adjustment The calibration check must to redone with the weight.

The process may have to be repeated several times.

My distilled water result does not match the published value.

Assuming that the calibration procedure has been successful, there are several reasons that the distilled water test may not match the calculated value.

Things to be checked:

• Is the tensiometer Level?

• Is the ring clean?

• Have we checked the sample temperature?

• Is the ring damaged or misshapen?

• Is my distilled water clean?

One of these issues is usually reason for a discrepancy between the tensiometer value and the published value used for comparison.

Special Problems:

If you are having trouble getting to the calculated value because the reading is low and the arm length adjustments are not getting you close, the problem is usually caused by a wire that is too stiff. This often occurs after a new wire has been installed. The solution is to sand (emery cloth recommended) the wire.

The recommended procedure is:

• Clamp the arm.

• Remove the wire cover.

• Sand the wire gently on both sides of the point where the arm is clamped.

• Unclamp the arm.

• Move the pointer to the mirror stripe.

• Zero the dial.

• Repeat the calibration procedure.

If the sanding was sufficient, you will be able to bring the reading to the calculated value by using the arm-adjustment nuts. If the reading is still too low, repeat the entire procedure.

If the reading is above the calculated value and you cannot get enough adjustment, the wire is either not sufficiently tightened or it has been sanded too much. Check the wire replacement procedure to see how to tighten the wire. If the problem persists after tightening the wire, install a new one.

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We have endeavored to bring clear, intelligible answers to our most common questions about tensiometer calibration. If you have issues that have not been addressed here, please pass them on in a comment to this article.

We have a video demonstrating the CSC Tensiometer Calibration process. Click the button get it.

I hope that this ramble has been helpful. I continue to be confounded by the perplexing particulars of surface tension.

Art

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