Reference laboratory for viscosity

The calibration laboratory at Poulten Selfe & Lee Limited maintains a primary viscosity scale. In the whole world, nobody can measure viscosity more accurately. All PSL Rheotek glass viscometers and viscosity oil standards are supplied with certified values directly traceable to the primary scale.

The primary Reference

In the world of viscosity, the primary reference is the kinematic viscosity of double distilled water measured at 20 degrees C. This is the gold standard, which all certified values can ultimately be traced back to. 

How does that work? How can the viscosity of distilled water tell me anything about a heavy fuel oil sample?

The answer is not as complicated as you would imagine. When measuring kinematic viscosity in a calibrated glass viscometer, you will determine the so-called flow time. This is the time it takes for the surface or meniscus of a sample to pass between two lines on the glass viscometer flowing under gravity. 

When you know the flow time in seconds and the calibration constant of the viscometer in question it is easy to calculate the kinematic viscosity. The equation is t (flow time in seconds) multiplied by c (constant) and the result is v (kinematic viscosity) in mm2S/cSt.

Let us now return to the double distilled water. The internationally agreed viscosity value for the water at 20 degrees is 1.0034 mm2s/cSt. When you then run this water through the appropriate Master Viscometer, you can then determine the flow time in seconds.

With a known flow time and a known viscosity, it is now easy to calculate the calibration constant of the Master Viscometer used.

C=viscosity divided by flow time.

From the constant on the smallest size of Master Viscometers, the process is then repeated with larger and larger size viscometers to create a scale. All this work is conducted in compliance with the protocol of ASTM D2162.  

This is known as a primary viscosity scale and this type of work is typically conducted by National institutes of Metrology around the world. At Poulten Selfe & Lee Limited, we maintain our own Primary Viscosity Scale and all certificates of calibration issued for PSL Rheotek Viscometers and oil standards are directly traceable to this scale.


Patience, patience and more patience

It takes a steady hand and a patient mind set to calibrate Master Viscometers. Laboratory Technician Liam Cooper from our calibration laboratory can testify to that. Typical flow times observed for the first measurement of double distilled water are around 850-900 seconds. Flow times are observed by the human eye and recorded using a calibrated stopwatch.

I enjoy the precision, he says although the long flow times are a downside. 

The longest flow times seen in Master Viscometer work can be up to 3000 seconds. 

Another time consumer is the cleaning and drying of the Master Viscometer tubes. A rigorous protocol of repeated flushing with solvent and deionised water consumes hours. 

There is a lot of cleaning, says Liam. 

Absolute attention to detail is required for working to the level of confidence required for scientific references. And it does not end there. During the work, it is also necessary to observe good practice routines. If the smallest speck of dust manages to get inside the capillary tube, it could affect the flow time and trigger several hours of additional work. Liam Cooper has experienced this first hand.

I wore a jumper one day, which was fluffy, and I ended up taking it off, he says.

All flow times are performed three times over. This to establish the highest level of confidence in the result as per the protocol. Each run needs to agree with the other runs inside the tolerance band defined in the method document. If agreement is not obtained, the viscometer must be removed, thoroughly cleaned and the entire process repeated.

Master Viscometers are much longer than normal viscometers and require extra care when handling.


Searching for custom instrumentation for your unique requirements?

Each PSL Rheotek viscometer system is configured for the owner! A wide variety of international test method protocols are readily available in the software and company specific method protocols are also supported.