Within the catalog of questions we are asked is a category related to Calibrating duNouy Ring Tensiometers. The subject matter ranges from how, why and what is proven?
I guess the immediate and wise-assed answers are:
- To see if the Tensiometer is working.
- To determine if it is working correctly
- Because it gives a reference for ISO traceability
Some of the calibration schemes pertain specifically to the unique design of the CSC Tensiometer and its application of torsion balance concepts.
Our tensiometers determine vertical force by measuring the twist or torsion in a wire. Starting from an equilibrium point where our scale dial is at zero:
1. We pull a duNouy ring through a liquid surface
2. This twists the wire as it resists the surface tension force and
3. We measure the amount of twist or torsion in the wire needed to pull the ring completely through the surface.
A calibration procedure is needed to produce a measurement in accepted units of surface tension (dynes/cm), as well as obtaining a reading that relates to a traceable measure or value. Thus, the real answer is all three of the above; the Tensiometer is working, working correctly and the results are traceable.
I shall describe the procedure which assures that a test reading is accurate.
The procedure goes as follows:
- Set up the Tensiometer so the indicator is on the “0” line in the mirror and the dial reads Zero. Like a Seesaw or Teeter Totter, in balance with equal sized kids (Ralph and Rachel) on either side. The ring (Ralph) on “teeter” side and the arm (Rachel) on the “totter” side.
- Now we add a known weight to the ring which pulls down the indicator. (We hand Ralph some free weights).
- We now turn our dial until the indicator goes back to Zero. (We give Rachel some stones until until it balances again).
- At this point we apply a magical formula that lets a wizard tell us what the dial should read if everything is in correct balance.
- We reconcile any difference between that number and the dial reading by adjusting the arm length. Thus, the wizard says Ralph should have more weights or that Rachel should move in or out on her side of the “totter” until the Seesaw balances again.
- Now we know that for a given weight, along with Ralph’s and Rachel’s specific position, the seesaw will be in balance. In other words, we know how much force will be needed to move the dial by one dyne. This magical formula makes the translation of the weight added to the number the dial should read.
We have a short video that shows how this calibration process works. You can get to it by clicking on the bubble.
I had fun putting this together and hope it simplified understanding of the tensiometer calibration process (except for the magical formula the basis of which is a designer’s secret).
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A befuddled as usual,
P.S. Hope you enjoyed meeting Ralph and Rachel. By the way sign up and subscribe to our Test Equipment Rants.