We investigated garages in South Africa for adulterated diesel with interesting results

MyBroadband recently tested the diesel quality at several garages in Pretoria and Centurion using the Yateks Viscopen and found no correlation between price and quality.

Our tests also showed that some garages may be selling poor-quality diesel or diesel that could have been adulterated with paraffin.

The Yateks Viscopen is a portable electronic diesel quality sensor, which gives users an accurate reading of the density and viscosity of a test sample.

Density and viscosity are among the first indicators available to consumers of diesel quality and a flag for possible paraffin content.

According to SABS standards, diesel in South Africa needs to have a density of 0.800–0.847 kg/ℓ at 20 degrees Celsius. Kinematic viscosity at 40 degrees Celsius needs to be 2–4.5 cSt.

Diesel density and viscosity need to fall within these parameters regardless of the sulphur content, typically labelled in parts per million at the pump.

Illuminating paraffin generally has a density of around 0.790–0.800 kg/ℓ and a kinematic viscosity of around 1.3 cSt.

By measuring the density and viscosity of diesel before filling up, consumers can spot very poor-quality diesel or diesel that may have been adulterated with illuminating paraffin.

MyBroadband tested 11 diesel samples from 9 different garages to compare their prices, viscosities, and densities.

The garages included common brand names found across South Africa, some smaller local brands, and a private garage that had the cheapest diesel we could find.

While petrol prices are regulated at the pump, diesel is regulated only at the wholesale level in South Africa. Each garage can set its own retail price.

At the time of our tests, the wholesale price of 50ppm diesel was R26.90 per litre, while the least expensive garage we found charged R26.98 per litre.

The most expensive 50ppm diesel we found cost R29.26 per litre, while 10ppm diesel ranged from R29.29 to R29.73 per litre.

We bought a small diesel sample from each garage in separate containers and used the Viscopen to measure the density and viscosity.

This is as simple as switching on the pen, keeping the sensor end submerged in the fluid for around 10 seconds, and reading the values off the screen on the side of the device.

Diesel quality tested with the Yateks Viscopen

The Yateks Viscopen gives density, temperature and viscosity values, as well as a standardised kinematic viscosity at 40 degrees Celsius.

It makes it easy for consumers to decide whether to buy diesel from a specific garage before filling up their vehicles.

The lowest kinematic viscosity we recorded was 2.2 cSt, with a density of 0.821 kg/ℓ. That sample was also the second-most expensive diesel we tested and was from a smaller brand.

The best diesel we found, according to these measurements, was also from a small brand and the third-cheapest we tested. It had a density of 0.840 kg/ℓ and a viscosity of 3.9 cSt.

The cheapest sample we tested was from a private fuel station, and cost only 8 cents per litre more than the wholesale price. It had a good density of 0.832 kg/ℓ and viscosity of 3.4 cSt.

While none of the diesel samples we tested fell below any standards, some were definitely reading too low for comfort.

That could indicate poor-quality fuel or some level of diesel adulteration. However, to confirm this, the samples would need to be sent to a lab for further testing.

The most interesting finding from our test was that there was no correlation between diesel quality and price.

Some garages with cheap diesel offered excellent quality, while several expensive ones sold poor-quality diesel. The table below summarises all the diesel samples tested.

Garage typeSulphur Content (ppm)Price/ℓDensity (kg/ℓ)Kinematic viscosity at 40°C (cSt)Local brand10R29.730.8303.7Local brand10R29.45

0.821

2.2

National brand10R29.290.8313.2National brand50R29.26

0.824

2.5

Local brand50R29.18

0.823

2.6

Local brand50R28.99

0.822

2.4

National brand50R28.830.8363.6National brand50R28.820.8293.0Local brand50R28.300.8403.9National brand50R27.31

0.826

3.5Private Garage50R26.980.8323.4