Measuring mercury vapour

We regularly receive questions about mercury, sometimes also called quicksilver. In the Netherlands, several tonnes of mercury are recovered each year as a by-product of natural gas extraction. In this article, BaSystemen covers the essentials, briefly and to the point:

• Where do you encounter mercury vapour?
• What do the Dutch regulations say about mercury vapour?
• The mercury vapour meter and the measuring principle

Where do you encounter mercury vapour?

Mercury is a naturally occurring element found mainly in oil and gas. As mentioned above, mercury can also be brought to the surface during oil and gas extraction, in gaseous or liquid form. Levels can range from a few ppb (parts per billion) to several ppm (parts per million). Mercury also reaches the surface during coal mining. However, the last coal mine in the Netherlands closed decades ago. The coal-fired power stations still operating in the Netherlands import their coal from countries such as Russia, South Africa, Colombia and the USA.

In the oil and gas industry, as much mercury as possible is removed from the fuel during the condensation process. There are situations in which mercury forms an amalgam with other metals such as aluminium while being transported through the pipelines. This can then cause a hole in the pipeline, with all the dangerous consequences that entails. Because mercury is an extremely toxic substance, it is, together with benzene, one of the most troublesome substances the O&G world has to contend with. Other places where mercury is frequently measured include steel recycling, coal-fired power stations, gold production and mining, waste processors and, for example, universities and laboratories.

Regulations on mercury vapour

In industry, limit values are applied to toxic substances. These are the maximum concentrations at which work may be carried out without respiratory protection. For mercury, that limit value is 0,02 mg/m3 or 20 μg/m3. However, these limit values are intended for adults in occupational circumstances and take account of exposure during working hours (8 hours a day / 40 hours a week). In domestic situations where mercury has been spilled, exposure can be even more critical.

Measuring mercury vapour

At BaSystemen we sell, rent out and service the Nippon EMP-2 portable mercury vapour meter and, more recently, the new Nippon EMP-3. Many companies in the oil and gas world choose this meter for a number of reasons. The main reason is that the instrument measures by means of atomic absorption spectrometry, in contrast to the measuring principle based on the gold-film resistance sensor used in other devices. Thanks to this measuring principle, the EMP-3 does not become saturated and can measure continuously in real time with extremely high accuracy. A further advantage is that the device has a high tolerance to false positives such as H2S, SO2, NH3, hydrocarbons or moisture, and can verify during the measurement whether one of these elements is distorting the reading.

The internal pump draws the mercury vapour into the measuring cell. A source of ultraviolet light with a wavelength of 253,7 nm then shines into the absorption cell. Atoms can absorb radiation, and mercury happens to be one of the few atoms that absorbs this wavelength. Unfortunately, interfering substances do still exist, which is why there is also a ZERO filter. Broadly speaking, this is a block of gold past which the gas is led when the user chooses to do so, since mercury vapour adheres to gold. This serves as a validation of whether the detected gas really is mercury vapour.