One of the most common methods of measuring the viscosity of a Non-Newtonian fluid is via the use of flow cups, viscosity flow cups & viscosity dip cups. Just like all measuring equipment, flow cups require routine calibration to ensure correct results.
Though a simple device, flow cups must be manufactured to very tight tolerances and specifications in order to be accurate. Even a brand new device should be calibrated to ensure there are no manufacturing faults or errors.
It is possible to calibrate your own viscosity flow cups but like many things its best left to a professional AML calibrate 1000's of flow ups every year, or you may want to perform your own in house checks in between external calibrations. The information in this guide describes how you could perform these calibrations.
For more information on viscosity measurement, see the following guide: What is Viscosity and Why is it Important to Measure it?
Flow cup principle of operation.
The length of time required for an exact volume of fluid to flow through a precision nozzle is measured. This time can be reported in the original format or can be converted into a Kinematic viscosity value in Centistokes (cSt). Centistokes equals 1 millimetre squared per second (mm²/s).
Flow cup design
Machined from a solid billet of aluminium, flow cups have a hollow central portion that holds the fluid under test. A high precision nozzle controls the flow of the fluid and has a gutter to catch excess fluid once the device is filled.
Dip cup design
Friikmar Dip Cups possess a similar design to flow cups, however do not have a gutter. Wheres as Zahn Dip Cups are manufactured from stainless steel. Both types have a long handle, a precision nozzle and a hollow central cylinder that contains the fluid being tested.
Types of Flow & Dip Cup
Procedures for Flow Cup Calibration / Viscosity Cup Calibrations
Required test equipment
- Certified reference oil of appropriate viscosity, typically the calibration takes place at approximately mid-way through its published operating range of the cup
- Calibrated reference thermometer with an accuracy of greater than 0.1°C
- Calibrated stopwatch
- Stand for flow cups
- Level for levelling the cup
- Water bath or formula to correct reference oil for temperature
Calibration procedure for viscosity flow cups
- Level stand.
- Visually inspect the flow cup for damage or contamination, clean if required.
- Ensure flow cup and reference oil is at the required temperature, usually 20°C or 25°C.
- Place finger over the nozzle.
- Fill with oil while ensuring no air bubbles are present, it should be slightly overfilled.
- Using a glass plate, slide carefully across the top of the cup to remove the excess oil into the gutter.
- Remove finger from the nozzle.
- Simultaneously remove the glass plate horizontally and start the stopwatch.
- Stop the stopwatch precisely the moment you see a break in the flow. This is called the efflux time and should be taken note of.
- Following the relevant ISO, DIN, ASTM or other standard, ensure that the expected efflux time and observed efflux time are within the specification stated.
- Repeat steps 3 to 9, three times and calculate the mean time. If any times are greater than 0.5 seconds apart, repeat this timing until you have three satisfactory results.
As with all viscosity measurements, the key to accurate and repeatable measurements is close temperature control. This applies to both calibrations and during measurement of fluids on a daily basis. A temperature difference of 1°C could amount to a variance of 3 to 8% in the observed time, depending on the viscosity of the reference oil.
Zahn cup calibration
zahn cup calibration is similar the the procedure above but you dip the Zahn cup into a beaker of oil and you start the timer as the cup is raised out of the oil. zahn cup calibration is straightforward however the accurate timing can be difficult and takes practice to achieve accurate and repeatable results.
As with all forms of instrument testing, certification is integral to maintain high standards. We are fully certified to carry out flow cup calibration services. You can find the certification below for further information. AML use high quality ISO 17025 UKAS certified calibration oils and UKAS certified temperature probes and timers.
AL441732318-08Dec2020-Zahn Flow Cup Efflux Time-114002-CAVI0046
This post is an overview of the methods listed in the appropriate ISO, DIN, ASTM, BS or AFNOR standards. The relevant standard for your flow cup should be studied in detail with requirements fully understood and followed in full. If you need more info or want to get your flow cups calibrated contact us today.
