DE3118771A1 - Apparatus for electrochemical measurements, particularly pH measurements, having a device for cleaning the measuring electrode and/or recalibrating a measuring cell - Google Patents

Abstract

The invention relates to a sensor for electrochemical measurements, in particular pH measurements, having automatic cleaning and calibration of the measuring cell. In order to minimise the consumption of cleaning agent and calibration solutions, the measuring cell is installed in such a way that it is immersed in a small recess which promotes flow and which serves as cleaning and calibrating vessel after the test solution has been drained. The cleaning agent and calibration solutions are fed into the recess so as to displace the residue of the test solution in accordance with an individually specified program. As a preparation for cleaning and calibration, the test solution is drained from a flow sensor after the circulation has switched off automatically. In an immersion sensor, this is effected by compressed air in conjunction with a downcomer tube, so that the automatic cleaning and calibration can also be applied to this type of sensor.

Description

Encoder for electrochemical, especially pH measurements with

automatic cleaning and calibration of the electrode.

Electrochemical measuring sensors, especially glass electrode measuring chains, Consisting of glass and reference electrodes, for determining pH values, in only limited service life in practice. Your display will change over time for several reasons: 1. Soiling by the sample solution 2. Zero point drift 3. Loss of steepness Against the disorder 1. Continuous cleaning sometimes helps, which is applied mechanically or acoustically without interrupting the measurement. she does not help against all soiling, so dry cleaning is often necessary with hydrochloric acid or another cleaning agent is necessary. To do this, the Electrodes are removed from the fitting, the so-called transmitter. - There are flushing rings and spray nozzles for chemical cleaning agents, which are also within the measuring solution filled sensor can be operated; but the cleaning agent consumption is proportionate high and the effect only weak because of the dilution by the measuring solution.

Errors, which after a shorter or longer period of time for the reasons 2. or 3. occur, require regularly repeated calibrations with one or two calibration solutions and corrections to the corresponding devices of the measuring attachment. This work, depending on the type of measuring solution or the electrode at an interval of a few hours to Week, always requires an expansion of the measuring chain and happens currently always by hand.

There are several suggestions for both effective dry cleaning, and to carry out recalibrations automatically.

The easiest way is to install an ordinary in-line encoder in the way that via appropriately switched valves in a certain time cycle Cleaning agents or calibration solutions are fed instead of the measuring solution. One However, such an arrangement has not been able to be introduced because of the consumption of Auxiliary solutions is too big to flush through flow sensors of the known types.

The aim of more recent efforts is therefore to construct pass-through encoders in such a way that that as few auxiliary solutions as possible can be managed. - After a suggestion (DP 25 57 542) the electrodes are installed in ball valves, which when changing position Separate the electrodes from the measurement solution and move towards cleaning or calibration solutions release. - According to another proposal (OS 27 12 159) measuring and calibration solutions are sprayed in small quantities through fine nozzles onto the electrodes.

Automatic cleaning and calibration are now particularly advantageous for aggressive and heavily soiled measurement solutions, in which these operations i.n short intervals are necessary. Such measuring solutions are encoders with precision mechanical Parts and narrow nozzles, however, are very prone to failure and only allow short operating times expect. The free cross-section of a continuous encoder should also pass through these devices are not restricted, because then larger dead times get into the measuring section. Nominal widths of 25 mm are common. The application of the known devices on immersion or built-in encoders is not possible.

According to the invention, a transducer is proposed that without precision mechanical Parts or narrow nozzles gets by, whose need for auxiliary solutions is minimal and whose Cross-section is not limited. The suggestion is also on built-in and immersion transducers applicable.

The continuous encoder in Fig. 1 differs from the known one straight through-flow encoder through one located in the middle on a base (1) small recess (2) into which the membrane of the measuring electrode (3) is immersed. the Arrangement in the middle of the pipe with the greatest flow velocity means during of the operation an optimal exchange of the measuring solution. After emptying the encoder A small volume of 0.5 to 1.0 ml of liquid remains in this well. Via one of the supply lines (4) and (5), which can be branched further if necessary cleaning or calibration solution is supplied according to a specified program, until it has displaced the rest of the previous solution. You don't need to do this more than 5 to 10 ml, i.e. less than in general manual calibration.

A known design is suitable as a glass electrode for pH measurements with dome mambran, in which only the lower arched part is made of sensitive glass consists. Other electrodes should have a flat membrane when using Combination electrodes must be membrane and diaphragm on the same level.

The reference electrode (6) can be in one of the embodiment shown in Fig.3 Storage container (7) described are installed, expediently in one of the containers with calibration solution. In this case, the reference electrode can also be a second measuring electrode which would emit a constant galvanic voltage at this point. There measuring electrodes are mostly not pressure-sensitive, then all expenses that were necessary for the pressure compensation of a conventional reference electrode would be necessary if The donor and storage vessel would be under pressure. The supply line (10) to the encoder is used at the same time as an electrolyte bridge.

The associated valve (ß) must then be bridged by a diaphragm (9) so that the measuring circuit is not interrupted when closing. The easiest this diaphragm is realized by a porous seal in the seat of the valve.

(s) The invention can also be applied to immersion and Use built-in encoder. Figure 2 shows the foot (3 of an immersion transducer with a recess (11) for the Mgßelctrode (12) in a plate (13) from the supply pipes (14) and (15) is worn. The encoder contains a drop cylinder inside the bell (16) (17), which floats in the operating position, as shown, and the exchange the measuring solution at the measuring electrode (12) is not hindered. - Will, however, at the point (1) Enter compressed air, then the liquid level in the bell (16) drops The drop cylinder (17) also sink until it rests on the sealing surface with its flange (19) rests, as shown in dashed lines in Figure 2.

The liquid level can then be lowered even further until the Plate (13) becomes free. - The supply of the help solutions is then carried out in the same way Way as with the pass-through encoder. If necessary, the supply lines are above the shown encoder part is shown further.

Fig. 3 shows the complete arrangement as an application example an automatic measuring point schematically. It concerns a pass-through encoder (21) with circulation pump (20) and free outlet, as is widely used for measurements in wastewater is.

- After switching off the pump (20), the encoder (21) runs up to the Remainder empty in the well. Via the supply line (22) and the M2 solenoid valve (23), controlled by a programmer, first the cleaning agent from the storage container (24) passed into the recess. After a certain time it takes place via the supply line (10) and the solenoid valve (8) the addition of the first calibration solution from the storage container (7). As soon as the calibration value is constant, the electrical zero point in the measuring attachment corrected. This is followed by the addition via the supply line (25) and the solenoid valve (26) the second calibration solution from the storage container (? 7). After using the second Calibration value the slope has been reset in the additional measurement, the supply of the Solution must be switched on again.

The invention allows a versatile expansion: the emptying of the in-line encoder can also be done with the help of several valves and compressed air This applies in particular if the measuring point is under overpressure stands.

In this case, the storage containers are also under the corresponding ones To set overpressure Instead of solenoid valves can also be pneumatic or mechanical operated valves or metering machines are used.

The program can be extended by inserting rinse cycles or simplify it by dispensing with a cleaning agent and the second calibration solution.

Instead of or in addition to dry cleaning, a continuous mechanical or acoustic cleaning can be used.

If the measurement solution has a different temperature from the environment, so can use the calibration solutions for the purpose of a correct temperature adjustment are brought to the measuring temperature by heat exchangers.

Claims (6)

Claims: 1. Transmitter for electrochemical measurements with automatic Cleaning and recalibration of the electrode, since it is clear that after emptying the encoder a small volume remains in a recess for the measuring electrode and that Cleaning and recalibration with supplied auxiliary solutions in this well be made. 2. Encoder according to claim 1, dad.gek. That the recess in the continuous encoder is brought on a pedestal in the area of greatest flow velocity. 3. Encoder according to claim 1, dad.gek. That the reference electrode or a Measuring electrode serving as a reference electrode installed in a storage vessel with calibration solution and that the connection from this storage vessel to the transmitter through a parallel to the valve switched diaphragm is kept open. 4. Encoder according to claim 1, dad.gek. That the execution as an immersion encoder is provided with a floating drop cylinder, which allows complete emptying makes possible except for the recess with compressed air. 5. Encoder according to claim 1, dad.gek. That the encoder also has a Contains continuous mechanical or acoustic cleaning. 6. Transmitter according to claim 1, dad.gek. That the calibration solutions through heat exchangers be brought to the measuring temperature.