Technical Article


Analysis and Best Practices for Viscosity Measurement


by Michael J. Hendricks, Applications Engineering Director; David Berta, Product and Application Specialist, Sam Duncan, Product and Application Engineer, Ransom & Randolph


V


iscosity measurement is likely the most widely conducted test in the Precision Investment Casting


(PIC) industry. This simple procedure is well suited for slurries used in the PIC industry, as an operator or technician can simply take the flow cup directly to a slurry tank and easily measure the efflux time. In the PIC industry, the measured efflux time is accepted as representing the viscosity of the material. The viscosity measurement can then be compared to specifications. If viscosity doesn’t meet specifications, the slurry can be adjusted back into range. Typical procedure for determining viscosity with a flow cup: 1. Insert a clean, dry flow cup into the fluid, filling the cup completely; then empty the cup. This step is often referred to as wetting the cup.


2. Reinsert the cup into the fluid.


3. Extract the cup and as the cup exits the top of the fluid, start a stopwatch.


4. Hold the cup vertically and observe the fluid.


5. Stop the stopwatch when the draining endpoint is reached.


6. Record the time. There are several brands of standard


dip style viscosity cups available,


including EZ (Equivalent Zahn), Zahn Signature, Ford and ISO cups. Each of these cups come with various hole sizes


32 ❘ April 2020 ®


in the bottom. For example, the Zahn Signature cups have 5 cups in the range (#1, #2, #3, #4 and #5). A larger number indicates a wider bottom hole diameter. The more viscous the material tested, the larger the hole should be in the cup. It is important to note that cups are not universally sized to a standard. #5 EZ and #5 Signature cups will not give the same results when used to test the slurry. In addition to the variation in readings based on the cup type and hole size, the endpoint, or when the stopwatch is stopped, also influences the viscosity reading. There are three endpoints for viscosity testing used in the industry, as shown in Figure 1: through the hole (TTH), one inch below (1 in) and break at the bottom (bottom). When using each of these endpoints, the viscosity is tested using the procedure above, but the endpoint (described in step 5) would be either through the hole, one inch below or break at the bottom. For the through the hole method, the operator looks down into the cup as it drains. Once the hole in the bottom of the cup opens and becomes visible, the endpoint is reached. When using the one inch below method, the stream of slurry exiting from the bottom of the cup is observed. As the cup empties, the stream will change. When the stream turns from steady to droplets one inch below the cup, the endpoint is reached. The break at the bottom endpoint is reached when the stream of droplets is at the bottom of the cup.


Testing Ensuring slurry viscosity homogeneity


across the surface of a tank is important in the PIC process so that every part on a tree is encountering the same slurry. This can be done by taking ten viscosity readings across the surface of the tank. Homogeneity


is achieved when all


ten readings fall within one second of each other. This was done before any of our testing began to reduce possible variation. For the purpose of our testing, we used the Zahn Signature 5, EZ Zahn 5 and ISO Mini 6 cups. A total of 2800 viscosity readings were taken. The testing methods used on the Zahn Signature 5 and EZ Zahn 5 cups were industry accepted methods: through the hole, one inch below and break at the bottom. The only testing method used on the ISO Mini 6 cup was break at the bottom, as this is the only method recommended by the manufacturer. Four operators took ten readings at each loading with each cup and each method, resulting in 70 different variable combinations. This is summarized in Table 1.


Results & Discussion The readings from each variable were evaluated by their standard deviation to determine the precision of each operator, cup and endpoint. The cumulative viscosity reading time for the 2800 readings was nearly thirteen hours, with the fastest test at five seconds and the longest at thirty seconds.


Operator What was found in our testing was that each individual operator had a relatively low deviation for their own


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