Inline pickling bath monitoring

The LiquiSonic® measuring technology provides a solution for online measurements of pickling bath concentrations facilitating redosing of the required quantity of fresh acid. This ensures a continuous, optimum pickling bath quality. Delays in time as a result of sampling and lab analyses are avoided.

Pickling baths are used downstream of the hot rolling process, but also in many other fields of the metal-working industry to remove, modify, passivate or clean surfaces in a defined manner. To this end, pickling solutions are employed, mainly consisting of mineral acids, like hydrochloric acid.

The concentration of these acids decreases during the pickling process, whereas the degree of disturbing components such as contaminations and carry-over increases.

For the measurement of tertiary mixtures, it is possible to calculate a further measurand in addition to the sonic velocity sensor. Thus, the individual components such as acid and metal salts can be determined separately by the LiquiSonic® system.

Benefits of LiquiSonic® measurement technology

Two problems often occur during the pickling process:

    1. Pickling the batch for too long, which makes it impossible to use the batch.
    2. Pickling too short, what leads to a poor galvanization quality.

What if the perfect pickling time was known at all times?

The LiquiSonic® measurement technology allows a clear dosage of acid and salt content during pickling bath monitoring. By continuously checking the pickling bath concentration in real-time, LiquiSonic® detects process deviations reliably. Thus, precise automated re-dosing and acid replenishment is possible.

Monitoring of acid concentration and salinity

When determining the concentration of the acid in the pickling bath, measurement with only one measuring method is often used. As a result, the measured value is influenced by the dissolved salts and the measurement result is falsified. For precise monitoring of both components of the mixture, the combination of sonic velocity and conductivity is required as the measuring method.
Both measuring signals are sent to the LiquiSonic® controller. The controller evaluates the information within seconds and shows the concentration of the acid and the salts separately on the display. This clear determination of both concentrations enables efficient process control. Influences of the salts are now not reflected in the measurement of the acid concentration.


Pickling removes impurities from the steel surface. However, the acid also attacks the iron oxide on the metal surface and ferrous salts are generated (e.g. iron (II)). It is therefore of great importance to precisely control all process-relevant parameters, such as dwell time, bath temperature and bath composition.
Only in this way, pickling bathes can be operated in an economic and resource-saving manner.

The measuring system LiquiSonic®  allows the parallel detection of two physical measurands, such as sonic velocity and conductivity, and therefore the determination of the HCl concentration in the pickling bath as well as the iron content. This leads to the elimination of expensive laboratory analysis.

The inline LiquiSonic® sensors enable the user to monitor the process concentrations precise in real time. Fluctuations can be seen and corrected immediately and very flexibly.

Measurement with LiquiSonic®

With LiquiSonic®, the solution from SensoTech, product quality is increased because the relevant key figures in the pickling process are precisely known at all times.
Measurements are not taken punctually, as in the case of sampling, but at one-second intervals over the entire course of the process. This means that there is no time lag between the measurement and the result. Deviations can be reacted to very quickly, e.g. in the form of subsequent dosing. In addition to saving time, inline measurement has another advantage: the influence of human error is reduced to a minimum, since measured values only have to be read from the display. The dosing of hydrochloric acid is more accurate and thus more efficient. This reduces acid consumption, which in turn reduces costs. Multiple sampling and waiting for measurement results of the pickling bath concentration are a thing of the past.

Measurement without LiquiSonic®

Without continuous concentration measurement by LiquiSonic®, only snapshots of the concentration values are available. As a result, the condition of the pickling baths is insufficiently known due to the missing values. The manual sampling increases errors caused by human hand. The time lag between sampling and the availability of the results makes for an insufficiently controllable process. As a result, quality defects often occur or one more pickling processe is necessary. This unnecessarily takes up additional time or makes the batch unusable. Without LiquiSonic® there is a waste of resources, which additionally increases the costs for the concentration measurement.

Installation of LiquiSonic® technology in the pickling bath process

The LiquiSonic® analyzer is installed into the transport pipelines (mostly DN80, 3“) or the bypass of the pickling bath. Even for installation either in the transport line of the rinsing bath or in the fresh acid feed, SensoTech offers proven solutions for decades.

The robust sensor construction and the optional special materials, like Halar or PFA, ensure long process life. The installation of the LiquiSonic® flange type sensor DN80 and the corresponding conductivity sensor in a piping system DN80 is simplified through the use of the installation adapter DN80. The total length of the measuring distance can be shortened by the adapter, to 0.70 m.

Typical measuring range:
concentration of free HCl: 0 - 120 g/l; 180 - 250 g/l
concentration range of Fe: 20 - 140 g/l
temperature range: 60 - 95 °C (140 - 200 °F)

Customer value

The LiquiSonic® analyzer provides a precise inline bath concentration measurement with real-time monitoring and continuous data logging. This allows the process to be automatically controlled in the optimal concentration range with stable, sufficient pickling results. LiquiSonic® is used to avoid a underdosage or overdosage of HCl. The analyzer reduces extensive lab measurements and replaces the time-consuming sampling process:

  • time saving: 1 h per day

LiquiSonic® saves costs by reduction of hydrochloric acid consumption.

Downloads for inline pickling bath monitoring

Hydrochloric acid Pickling

Hydrochloric acid Pickling