EP4222378A1

EP4222378A1 - Method for monitoring the oil level of an oil-lubricated compressor, oil level monitoring system for carrying out the method and compressor system having such an oil level monitoring system

Info

Publication number: EP4222378A1
Application number: EP21773489.6A
Authority: EP
Inventors: Ehsan Afshar; Stefan Lange; Adrian Kramlich; Rainer Stromayr; Michael Kramer
Original assignee: Knorr Bremse Systeme fuer Schienenfahrzeuge GmbH
Current assignee: Knorr Bremse Systeme fuer Schienenfahrzeuge GmbH
Priority date: 2020-09-30
Filing date: 2021-09-02
Publication date: 2023-08-09
Also published as: WO2022069147A1; KR20230070031A; JP2023543610A; CN116324167A; US20240200545A1; DE102020125500A1

Abstract

The invention relates to a method for monitoring the oil level of an oil-lubricated compressor, comprising the following steps: reading at least one oil level sensor (21, 21a, 21b), by means of which an oil level representing the compressor oil level is detected; checking compliance with at least one predetermined verification criterion for verification of the read-out oil level; and relaying an oil level signal if the read-out oil level complies with at least one relaying criterion (10a, 10b) according to whether said oil level reaches or falls below a predetermined oil level and with the at least one verification criterion.

Description

DESCRIPTION

METHOD FOR MONITORING THE OIL LEVEL OF AN OIL-LUBRICATED COMPRESSOR, OIL LEVEL MONITORING SYSTEM FOR CARRYING OUT THE PROCEDURE, AND COMPRESSOR SYSTEM

WITH

SUCH AN OIL LEVEL MONITORING SYSTEM

The invention relates to a method for monitoring the oil level of an oil-lubricated compressor, an oil level monitoring system for carrying out the method and a compressor system with such an oil level monitoring system. For example, the oil level of oil-lubricated compressors for rail vehicles is currently checked manually at regular intervals in order to avoid consequential damage and/or operational failures due to an insufficient oil level. This is associated with a corresponding maintenance effort and can result in compressor damage as a result of overheating, particularly in the event of an unexpected loss of oil outside of the maintenance intervals. In principle, however, a manual check of the oil level is also error-prone, since the reading may not be carried out correctly or even accidentally not at all.

Reading errors can also be encouraged by the fact that the specific process for reading the oil level, such as warming up the compressor, shutting it down, observing a waiting period and only then taking the reading, is not followed. The oil level indicator provided for reading is usually a sight glass or sight tube on the oil tank with a specific display height. This can also result in possible reading errors due to individual viewing angles or accessibility. In view of the above statements, it is therefore the object of the present invention to provide a method for monitoring the oil level of an oil-lubricated compressor, an oil level monitoring system for carrying out the method and a compressor system with such an oil level monitoring system in order to enable cost-effective automated and reliable oil level monitoring.

The task is solved by a method for monitoring the oil level of an oil-lubricated compressor, an oil level monitoring system for carrying out the method and a compressor system with such an oil level monitoring system according to the independent claims. Advantageous developments of the invention are contained in the dependent claims.

According to the invention, a method for monitoring the oil level of an oil-lubricated compressor comprises the steps:

- Reading out at least one oil level sensor, via which an oil level representing the compressor oil level is detected,

- Checking the fulfillment of at least one specified verification criterion for verifying the read oil level and

- Transmission of an oil level signal when the oil level read out meets at least one transmission criterion according to whether a predetermined oil level has been reached or fallen below and the at least one verification criterion.

An oil level representing the compressor oil level can be an oil level of the compressor itself or an oil level that corresponds to the oil level of the compressor. For example, a portion of the oil can be discharged from the compressor to monitor the oil level, in order to enable detection that is independent of the compressor operation and/or structural framework conditions.

The oil level can be detected in both an analog and digital manner. An analog oil level detection is understood as a quantitative detection, that is, for example, a determination of a specific fill level or a fill volume. In contrast to this, according to a digital detection, it can only be determined in each case whether a qualitative oil level characteristic, such as compliance with a minimum oil level, is fulfilled.

The test of at least one verification criterion is used to verify the oil level read out. In this case, the verification criterion does not serve to evaluate the oil level read out per se, but rather to evaluate the suitability of the oil level read out for forwarding an oil level signal, provided that a correspondingly predetermined forwarding criterion is met. The transmission criterion therefore corresponds to the information content of the oil level signal to be transmitted, while the Verification criterion verifies the applicability of the information or at least its sufficient probability.

According to the method, the transmission of an oil level signal is therefore linked to the fact that both the at least one verification criterion and the transmission criterion are met according to whether a predetermined oil level has been reached or fallen below. The oil level signal must first be distinguished from the signal read out by the oil level sensor, ie the sensor signal. If the oil level signal does not have to contain any information other than the sensor signal, for example because the sensor signal can be processed directly as an oil level signal when passed on, the oil level signal corresponds to the sensor signal. In many cases, however, it can be advantageous if it is not the sensor information per se, but information derived from it that is passed on as an oil level signal. In such a case, the content of the oil level signal differs from the content of the sensor signal. The sensor signal can therefore be converted into an oil level signal.

The method can be used advantageously in particular for oil-lubricated compressors for rail vehicles.

In one embodiment of the method, the transfer criterion corresponds to a low oil level and/or a critical oil level.

If the oil level is low, the compressor can continue to be operated for a certain period of time without the likelihood of compressor damage due to overheating, for example. By forwarding a corresponding oil level signal, the refilling of oil can be scheduled within the still tolerable operating time. If the oil level is critical, consequential damage can no longer be ruled out if the compressor continues to be operated, so that it is advisable to suspend compressor operation. A distinction is therefore preferably made not only between a low and a critical oil level, but in particular correspondingly different oil level signals are also passed on.

In particular, the verification criterion corresponds to at least one time criterion, in which the oil level corresponding to the transfer criterion is read out over a predetermined period of time, a frequency criterion, in which a predetermined Number of individual reading processes that fulfill the transfer criterion is present, and/or an operating state criterion in which a predetermined operating state is determined.

As already explained above, the at least one verification criterion is used to check the suitability of the oil level signal for transmission if the transmission criterion is met. For example, an oil level that corresponds to the transfer criterion can only occur for a short time and is not due to the actual oil level, but to a vibration of the oil tank or other general conditions not related to the oil level. To reduce the probability of incorrect conclusions being drawn about the actual oil level from temporary fluctuations in the oil level, a time criterion can be used as a verification criterion, for example. In order to meet the time criterion, a check is made as to whether the oil level corresponding to the forwarding criterion is read continuously over a predetermined period of time or at least at a predetermined frequency.

As an alternative or in addition, a frequency criterion can also be taken into account as a verification criterion, in which case there is a predetermined number of individual read processes that meet the forwarding criterion. A readout process corresponds to reading out the at least one oil level sensor over a predetermined period of time. A readout process that satisfies the transfer criterion occurs when the oil level corresponding to the transfer criterion is read out continuously or at least at a predetermined frequency throughout the individual readout process. In other words, the frequency criterion corresponds to a predetermined number of fulfilled time criteria. In order to meet the frequency criterion, it can be provided that the predetermined number of read processes that meet the forwarding criterion, for example five read processes, must follow one another directly or must be present within a predetermined period of time.

Furthermore, alternatively or additionally, an operating state criterion, in which a predetermined operating state is determined, can also be defined as a verification criterion. Accordingly, it is checked whether the compressor during the reading of the at least one oil level sensor in a predetermined operating status. For example, the operating condition criterion is only considered to be met if the compressor is in operation or is at a standstill. If the operation of the compressor is assumed as the operating condition criterion, a specific power level of the compressor operation can also be additionally assumed. Since start-up phases and compressor run-down phases have to be taken into account when the compressor is operated, it can also be advantageous to take into account not only the operating state itself, but also a corresponding time component for a stable operating state to be reached when defining the operating state criterion, for example "Compressor has been running for ... seconds" or "Compressor has stopped for ... seconds".

The probability of a reliable measurement of the oil level can be increased by combining the various verification criteria.

In a development of the method, a pressure sensor is read out to determine the predetermined operating state.

The predetermined operating state can correspond to a defined pressure window, the presence of which is determined via the pressure sensor. The term "pressure window" refers to a pressure range that corresponds to a pressure-relieved or reduced-pressure condition of the compressor after a predetermined time. By interrogating the pressure sensor, it can be ensured, for example, that the oil level is detected by the at least one oil level sensor when the compressor is warm but switched off, with operation no longer than a defined period of time ago.

According to one embodiment, the step of reading out the at least one oil level sensor takes place before the at least one predetermined verification criterion is checked.

Accordingly, the check of the at least one predetermined verification criterion is only carried out when reading the oil level sensor shows that the forwarding criterion is met. Thus, data acquisition and processing can be reduced to a minimum. Alternatively, however, the checking of the at least one predetermined verification criterion can also precede the reading out of the oil level sensor. If, for example, the operating state criterion is used as a verification criterion, an oil level signal can only be transmitted after the predetermined operating state has been reached. Accordingly, a previous readout of the oil level, at least in the sense of the method described here, does not lead to a usable result in every embodiment of the method.

When checking several verification criteria, for example, the operating condition criterion can be checked first, the oil level sensor can be read out if the corresponding operating condition criterion is presented, and the check of a further verification criterion, such as the time criterion, can be linked to the fulfillment of the transfer criterion. A verification criterion and/or a sequence of the method steps can also be selected as a function of the operating state. Thus, both during operation of the compressor and when the compressor is switched off, various verification criteria and/or the sequence of the method steps, but also transfer criteria can be defined in order to be able to carry out an adapted and therefore complete oil level monitoring in every operating state.

In one embodiment, two oil level sensors are read.

Reading out at least two oil level sensors can be advantageous, for example, when several oil levels define a transfer criterion or different transfer criteria, the several oil levels cannot be detected via one oil level sensor, or a detection is to be mapped redundantly. If, for example, both a low oil level and a critical oil level are to require the transmission of a or a respective oil level signal if the applicable verification criterion is additionally met, an oil level sensor can be used for each of the respective oil levels. In addition to using an additional oil level sensor, if an additional oil level detection cannot be covered by an oil level sensor, different detection parameters such as measuring sensitivity, reaction times, measuring principles and the like can also be adjusted as required via independent detection. In particular, a float is used as the oil level sensor.

A float or a functional unit that interacts with it, such as a float switch, which can be actuated by a float, offers a cost-effective way of monitoring the oil level. The inclusion of at least one verification criterion for monitoring the oil level improves the reliability of the use of a float, which is otherwise usually associated with considerable uncertainties.

In a further development, depending on the transmission criterion, the oil level signal is forwarded as a message to a monitoring unit, in particular to a driver's cab and/or a control and/or maintenance device, and/or as a control parameter for a control device of the compressor, in particular when the oil level is critical .

The oil level signal can trigger an optical, acoustic and/or tactile signal in the sense of a message via the monitoring unit. For example, an explicit text message such as "low oil level" or "critical oil level" is displayed to an operator via a display in the driver's cab and/or a corresponding warning light is activated. Especially in critical situations, such as a critical oil level, an acoustic signal can be output as an alternative or in addition to increase the probability of detection regardless of the viewing direction. The monitoring unit can also be part of a maintenance system, so that depending on the transmitted oil level signal, a maintenance interval is adjusted and/or a corresponding maintenance instruction, such as "top up oil", is stored in the maintenance log for the next maintenance.

Depending on the transmission criterion of the oil level signal, however, a control parameter for a control device of the compressor can also be adjusted in such a way that the probability of a malfunction or consequential damage due to overheating of the compressor or other consequences of a lack of oil is reduced. For example, the maximum compressor output can be throttled to ensure that the remaining oil allows temporary emergency operation. In the case of critical oil levels, provision can also be made to completely stop the compressor operation. According to one embodiment, the method is carried out while the oil-lubricated compressor is at a standstill.

This makes it possible to find the stable, in particular constant, conditions required for reliable detection of the oil level. If, as described above, a cessation of the compressor operation should be provided when a corresponding oil level signal is passed on, a start of the compressor can then be suspended. The compressor is thus not switched off during ongoing operation. In other words, the compressor is only started when there is no oil level signal transmission. This is to be provided as a transfer criterion in particular for a critical oil level.

In addition, it should be noted that the transfer criteria, such as a low oil level or critical oil level, can be adapted to framework criteria. If, for example, a navigation system is used to monitor the oil level based on a comparatively longer route or a comparatively longer time until the next maintenance interval is stored, a higher oil level can already form a transfer criterion than with a comparatively shorter distance or shorter time until the next maintenance interval.

In a further aspect, the invention relates to an oil level monitoring system for carrying out the above method, comprising:

- At least one oil level sensor for detecting an oil level representing the compressor oil level of an oil in an oil tank of a compressor and

- an evaluation unit for reading out the sensor signals of the at least one oil level sensor, in which at least one transfer criterion and at least one verification criterion are stored, the evaluation unit being configured in such a way that it transfers an oil level signal if the at least one transfer criterion is based on the read oil level and the at least one verification criteria are met.

Compliance with the verification criterion is checked via the evaluation unit itself, for example via the evaluation of the oil level read out in terms of the time criterion and/or frequency criterion described above corresponding comparison of the evaluation result with the stored verification criterion. However, such an evaluation result or also an operating state with regard to an operating state criterion can also be transmitted to the evaluation unit for comparison with the stored values and/or states.

In a further development, the oil level monitoring system has a pressure sensor, in particular a pressure sensor that can be arranged on an oil container that represents a compressor oil level, via which a pressure that represents an operating state of a compressor can be detected, and the evaluation unit takes at least the detected pressure into account as a verification criterion.

The detection of a pressure representing the operating state of the compressor and its consideration as a verification criterion goes hand in hand with the procedure described for the method. For this purpose, the pressure sensor is preferably arranged on an oil tank representing a compressor oil level, in order to be able to minimize distance-related interference. The oil level in the oil tank corresponds to an oil level in the compressor. This is the case, for example, when the oil tank is an oil tank of the compressor or the oil level in the oil tank is connected to the compressor in such a way that the oil level in the oil tank can be used to make statements about the oil level in the compressor. Accordingly, with respect to the oil tank, the expression "oil tank representing compressor oil level" is used.

In particular, the at least one oil level sensor is a float.

The advantages of a float as an oil level sensor are analogous to the explanations already given for the method.

According to one embodiment, the evaluation unit is configured in such a way that it distinguishes at least two transfer criteria and transfers different oil level signals depending on the transfer criteria.

For example, reaching or falling below a first limit value of the oil level, which corresponds to a low oil level, is defined as the first transfer criterion. Reaching or falling below a second The second transfer criterion is the limit value of the oil level, which corresponds to a critical oil level. The second limit value is lower than the first limit value. If the first limit value is reached or fallen below and the at least one verification criterion assigned to the first transmission criterion is met, the evaluation unit forwards the oil level signal "Oil level low" as a corresponding message to a driver's cab of a rail vehicle as an example of an application and/or a maintenance facility. If the oil level read out reaches or falls below the second limit value while at the same time fulfilling the verification criterion assigned to the second limit value, the evaluation unit forwards the oil level signal "oil level critical" as a corresponding message to the driver's cab of the rail vehicle and/or a maintenance facility. Alternatively or additionally, the evaluation unit can also forward the oil level signal as a control parameter to a control device of the compressor, so that the output of the compressor can be throttled as a precaution or the operation of the compressor can be stopped entirely.

In a further aspect, the invention relates to a compressor system with an above oil level monitoring system, the compressor system having an oil tank representing a compressor oil level, and the at least one oil level sensor being arranged in the oil tank, in particular in the middle.

The positioning of the oil level sensor affects the probability of detecting a temporary fluctuation in the oil level in the oil tank compared to an actual oil level. If a float is arranged as an oil level sensor on the edge of the oil tank, for example, a fluctuation in the oil level in the sense of a fluid movement at the edge of the oil tank causes larger amplitudes and thus greater uncertainties with regard to the distinction between temporary level movements and the actual oil level. Accordingly, possible differentiation uncertainties can be reduced by an arrangement that is as central as possible, ie an arrangement in an area with comparatively little level movement or level fluctuation amplitude. Consequently, it is also possible in this way to reduce the at least one verification criterion to lower requirements. Alternatively or additionally, the oil tank can also be used as a be designed as largely decoupled from fluctuations container to keep unwanted influence on the oil level detection low. In order to have to carry out such a fluctuation decoupling for the compressor, provision can be made here in particular for the oil level to be detected via a separate oil container. The separate oil tank, whose oil level corresponds to a compressor oil level, can be decoupled from fluctuations as a smaller unit with less effort.

In a development, the compressor system has a control device to which the oil level signal is forwarded by the evaluation unit, the control device controlling the operation of the compressor as a function of the oil level signal.

As already explained with regard to the method and the oil level monitoring system, the compressor can thus be controlled as a function of the oil level signal, for example in order to reduce oil consumption when the oil level is low or to completely prevent operation that could possibly cause damage when the oil level is of this type, especially when the oil level is critical. The control device can therefore, for example, switch the compressor to an operating mode with lower oil consumption or suspend the operation of the compressor.

The invention is explained in more detail below on the basis of an embodiment with reference to the accompanying figures. The figures show in detail:

FIG. 1 shows a schematic representation of an oil level monitoring system according to an exemplary embodiment of the invention.

FIG. 2 shows a flow chart of a method for monitoring the oil level according to a first embodiment of the method.

FIG. 3 shows a flow chart of a method for monitoring the oil level according to a second embodiment of the method.

FIG. 4 shows a flow chart of a method for monitoring the oil level according to a third embodiment of the method. FIG. 1 shows an oil level monitoring system 1 for carrying out a method for monitoring the oil level of an oil-lubricated compressor based on oil level monitoring of an oil-lubricated compressor for rail vehicles, as can also be used for the method configurations shown in FIGS. The oil level monitoring system 1 includes an oil level sensor 20 arranged in an oil tank 20 representing the oil level of the oil-lubricated compressor. The oil level sensor 20 is designed here as a float. In addition, the oil level monitoring system 1 has an evaluation unit 30 which is connected to the oil level sensor 21 via a signal line 40 in terms of signals. The signaling connection can also be formed via wireless transmission types. In the embodiment shown, evaluation unit 30 not only receives the sensor signals from oil level sensor 21, but also sensor signals from a pressure sensor 22, which is arranged on oil tank 20, in order to be able to derive an operating state of the compressor from the detected pressure. Here, too, the signal connection is made via a further signal line 40.

In addition, a low oil level 10a is shown in FIG. 1 via the dashed line. The dash-dotted line 10b represents a critical state 10b. According to the representation of the oil level sensor 21 shown, the oil 10 in the oil tank 20 has a low oil level 10a. With the specification of the transfer criterion as reaching or falling below the low oil level 10a, this is therefore fulfilled in the state shown. This is checked by the evaluation unit 30 either by comparing the sensor signal of the oil level sensor 21 with a stored limit value for the transfer criterion or by transmitting a correspondingly positive value in terms of fulfillment of the transfer criterion by the oil level sensor itself. For this purpose, the oil level sensor can have its own data processing unit. If the oil level sensor 21 is digital, ie only sends a signal when the transmission criterion is met, the presence of the sensor signal at the evaluation unit 30 can have sufficient information content.

However, since the fulfillment of the transfer criterion by the oil level read out does not have to be sufficient in itself for the actual oil level to be reliable specify, the evaluation unit 30 also checks a predetermined verification criterion. In the embodiment of the oil level monitoring system 1 shown, for example, the sensor signal of the pressure sensor 22 is used by the evaluation unit to detect whether the compressor is at a standstill. An oil level signal is forwarded from the evaluation unit 30 via a further signal line 40 to the driver's cab 50 only when the pressure signal fulfilling the corresponding verification criterion is presented and the transmission criterion is simultaneously fulfilled by the sensor signal from the oil level sensor 21 . The message "Oil level low" then appears there, for example, on a corresponding display device. As is also made clear with reference to the further description of the figures, the verification criterion is not limited to an operating condition criterion that can be checked here by the pressure sensor 21, but can also alternatively or additionally depict a time criterion or frequency criterion, for example.

FIG. 2 shows a flow chart of a method for monitoring the oil level according to a first embodiment of the method. In this case, the oil level sensor 21 is first read out. If the read-out oil level satisfies the "low oil level" transmission criterion, that is, if it reaches or falls below a predetermined limit value corresponding to this transmission criterion, the fulfillment of defined verification criteria is checked. Otherwise, the oil level sensor will continue to read. In the embodiment of the method shown, when a low oil level is read out, it is first checked whether a time criterion has been met. The time criterion here is the continuous measurement of a low oil level, ie the oil level that meets the transfer criterion, over x seconds, ie a predetermined period of time or minimum period of time. If the time criterion is met, a frequency criterion is checked as a further verification criterion, or else the evaluation is aborted and the oil level sensor continues to be read. According to the frequency criterion used here, it is checked whether there is a predetermined number k of successful measurements. The evaluation unit 30 is programmed in such a way that it recognizes instantaneous changes in the oil level read out over a period of t<x*ms as an oil level measurement error and does not evaluate them. The frequency criterion is considered to be met if the oil level meets the transfer criterion, ie a low level in this case Oil level, is read within a predetermined period of time in the predetermined number k, for example five times. However, the frequency criterion can also be defined in such a way that, in order to meet it, the time criterion must be given in a certain number of consecutive times or within a predetermined period of time. If the frequency criterion is met, an oil level signal "oil level low" is passed on. According to this oil level signal, a corresponding message "Oil level low" is displayed, for example, on a display of the driver's cab 50 . Alternatively or additionally, the oil level signal can also be forwarded to a maintenance device, which then stores the refilling of oil as maintenance work in the maintenance log for the respective compressor. In one embodiment, the maintenance device can also adjust maintenance times as a function of the oil level signal, for example bring them forward.

FIG. 3 shows a flow chart of a method for monitoring the oil level according to a second embodiment of the method. The second embodiment of the procedure differs from the first embodiment of the procedure in that two transfer criteria are distinguished. Reaching or falling below a first limit value of the oil level, which corresponds to a low oil level, is defined as the first transfer criterion. Reaching or falling below a second limit value of the oil level, which corresponds to a critical oil level, is defined as the second transfer criterion. The second limit value is lower than the first limit value. Two digital oil level sensors 21a, 21b are read out in the flow chart shown for illustration purposes. Reading the first oil level sensor 21a corresponds to checking the first transfer criterion, ie a low oil level, and reading the second oil level sensor 21b corresponds to checking the second transfer criterion, ie a critical oil level. Alternatively, an analog oil level sensor can also be read out, which can then be used to detect both a low and a critical oil level. Thus, the analog oil level sensor can be used instead of the oil level sensors 21a, 21b. Irrespective of the number of oil level sensors used, it is now first checked whether the oil levels read out indicate that the oil level is critical, ie whether it has reached or fallen below the second limit value. If no critical oil level is detected, the further course of the method shown in FIG If a critical oil level is detected according to the read oil level of the oil level sensor 21b, a time criterion and a frequency criterion are checked analogously to the further method for a low oil level in addition to the submission of the second transfer criterion. In the exemplary second embodiment of the method, the time criteria and frequency criteria for the transfer criteria "critical oil level" and "low oil level" are the same except for the respective limit values and thus the oil levels to be used. In other variants of the method, the verification criteria can also be defined independently of one another. If the associated time criterion and frequency criterion are met in the case of a critical oil level, an oil level signal "oil level critical" is passed on. This is also forwarded here, for example, to the driver's cab 50 for display. In addition, the oil level signal can also be forwarded to a control device of the compressor in order to prevent further operation of the compressor in this case or to switch to another operating mode.

FIG. 4 shows a flow chart of a method for monitoring the oil level according to a third embodiment of the method. The third embodiment of the method according to FIG. 4 differs from the second embodiment of the method according to FIG. 3 in that the pressure sensor 22 is also read out. For example, the target range is defined as the pressure range that is present when the compressor is at a standstill. The oil level monitoring according to the second embodiment of the method is only continued when the pressure read out by the pressure sensor 22 is in the predetermined target range. In other words, the fulfillment of the operating condition criterion is a prerequisite for the further examination of the forwarding criteria and as a result of the respective time criterion and respective frequency criterion. If the pressure is outside the specified target range, the sensor values continue to be read out until the operating condition criterion corresponding to the pressure is met.

The invention is not limited to the embodiments described. Even if, for example, the operating condition criterion in the third embodiment of the method is checked using a pressure measurement as an example, it can the respective operating state of the compressor can also be passed on to the evaluation unit 30 via the control device of the compressor and evaluated accordingly. In addition, the evaluation unit 30 can also be part of the control device of the compressor.

REFERENCE LIST

1 oil level monitoring system

10 oil

10a low oil level

10b critical oil level

20 oil containers

21, 21a, 21b oil level sensor

22 pressure sensor

30 evaluation unit

40 signal line

50 driver's cab

Claims

PATENT CLAIMS

1 . Method for monitoring the oil level of an oil-lubricated compressor, comprising the steps:

- Reading out at least one oil level sensor (21, 21a, 21b), via which an oil level representing the compressor oil level is detected,

- Transmission of an oil level signal when the oil level read out fulfills at least one transmission criterion (10a, 10b) according to reaching or falling below a predetermined oil level and the at least one verification criterion.

2. The method of claim 1, wherein the transfer criterion (10a, 10b) corresponds to a low oil level (10a) and/or a critical oil level (10b).

3. The method according to claim 1 or 2, wherein the verification criterion includes at least one time criterion, in which the oil level corresponding to the transfer criterion (10a, 10b) is read out over a predetermined period of time (x), a frequency criterion, in which a predetermined number (k) of individual the transfer criterion (10a, 10b) fulfilling read-out processes is present, and/or corresponds to an operating state criterion in which a predetermined operating state is determined.

4. The method according to claim 3, wherein a pressure sensor (22) is read to determine the predetermined operating state.

5. The method according to any one of the preceding claims, wherein the step of reading out the at least one oil level sensor (21, 21 a, 21 b) takes place before the check of the at least one predetermined verification criterion.

6. The method according to any one of the preceding claims, wherein two oil level sensors (21a, 21b) are read.

7. The method according to any one of the preceding claims, wherein a float is used as the oil level sensor (21, 21a, 21b).

8. The method according to any one of the preceding claims, wherein depending on the transfer criterion (10a, 10b) the oil level signal as a message to a monitoring unit (50), in particular to a driver's cab and / or a control and / or maintenance device, and / or as Control parameters for a control device of the compressor, in particular at a critical oil level according to claim 2, is passed on.

9. The method according to any one of the preceding claims, wherein the method is carried out during a standstill of the oil-lubricated compressor.

10. Oil level monitoring system (1) for carrying out the method according to any one of claims 1 to 9, comprising:

- At least one oil level sensor (21, 21 a, 21 b) for detecting an oil level representing the compressor oil level of an oil (10) in an oil tank (20) of a compressor and

- an evaluation unit (30) for reading out the sensor signals of the at least one oil level sensor (21, 21 a, 21 b), in which at least one transfer criterion (10a, 10b) and at least one verification criterion are stored, the evaluation unit (30) being configured in this way that it forwards an oil level signal when the at least one forwarding criterion (10a, 10b) according to the read oil level and the at least one verification criterion are met.

11. Oil level monitoring system (1) according to claim 10, wherein the oil level monitoring system (1) has a pressure sensor (22), in particular a pressure sensor (22) which can be arranged on an oil tank (20) representing a compressor oil level, via which a pressure representing an operating state of a compressor is detectable, and the evaluation unit (30) takes into account at least the detected pressure as a verification criterion.

12. Oil level monitoring system (1) according to claim 10 or 11, wherein the at least one oil level sensor (21, 21 a, 21 b) is a float.

13. Oil level monitoring system (1) according to one of claims 10 to 12, wherein the evaluation unit (30) is configured in such a way that it distinguishes at least two transfer criteria (10a, 10b) and depending on the transfer criteria (10a, 10b) forwards different oil level signals.

14. Compressor system with an oil level monitoring system (1) according to one of claims 10 to 13, having an oil tank (20) representing a compressor oil level, wherein the at least one oil level sensor (21, 21a, 21b) is in the oil tank (20), in particular in the middle, is arranged.

15. Compressor system according to claim 14, wherein the compressor system has a control device to which the oil level signal is forwarded by the evaluation unit (30), and the control device controls the operation of the compressor as a function of the oil level signal.