WO2023149881A1

WO2023149881A1 - Determinations relating to printing fluid

Info

Publication number: WO2023149881A1
Application number: PCT/US2022/015040
Authority: WO
Inventors: Cristian DIEZ QUILEZ; Alberto Borrego Lebrato; Diana DE SOLER NAVALES
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2022-02-03
Filing date: 2022-02-03
Publication date: 2023-08-10

Abstract

A printing apparatus is disclosed. The printing apparatus comprises a check valve to restrict the passage of printing fluid in a direction along a printing fluid conduit between a printing fluid supply and a print head. The printing apparatus also comprises a pressure sensor to measure a printing fluid pressure in the printing fluid conduit, and a processor to: receive, from the pressure sensor, printing fluid pressure data indicative of a printing fluid pressure of printing fluid in the printing fluid conduit between the printing fluid supply and a print head; determine, based on the printing fluid pressure data, a first time point at which the printing fluid pressure falls below a first defined pressure level for a first time; determine, based on the printing fluid pressure data, a second time point at which the printing fluid pressure falls below the first defined pressure level for a second time; determine whether a duration between the first time point and the second time point is below a defined threshold duration; and generate an instruction signal if it is determined that the duration is below the defined threshold duration.

Description

DETERMINATIONS RELATING TO PRINTING FLUID

BACKGROUND

[0001] A print apparatus may include a check valve to restrict the flow of printing fluid in a direction through a line or conduit. If the check valve is defective, damaged or blocked, then it may not function as intended. A faulty or defective check valve can in some cases affect other components in the print apparatus.

BRIEF DESCRIPTION OF DRAWINGS

[0002] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0003] Figure 1 is a plot showing examples of how pressure may vary as a function of time;

[0004] Figure 2 is a schematic illustration of an example of an apparatus for determining whether there is a malfunction with a check valve;

[0005] Figure 3 is a schematic illustration of a further example of an apparatus for determining whether there is a malfunction with a check valve;

[0006] Figure 4 is a flowchart of an example of a method for determining whether there is a malfunction with a check valve;

[0007] Figure 5 is a schematic illustration of an example of a processor in communication with a machine-readable medium; and

[0008] Figure 6 is a schematic illustration of a further example of a processor in communication with a machine-readable medium.

DETAILED DESCRIPTION

[0009] An apparatus, such as a printing apparatus, may comprise a pump to move printing fluid from a printing fluid supply to a print head via a printing fluid line. The pressure of printing fluid in the printing fluid line may be maintained by the pump when the pump is active. A check valve, which may be located between the pump and the print head, may maintain the printing fluid pressure when the pump is inactive. The check valve may be arranged such that it prevents printing fluid from moving in a direction from the print head towards the print fluid supply. However, if the check valve malfunctions, printing fluid may be allowed to move through the printing fluid line in this way, which, in some examples, can lead to the print head ingesting air and, therefore, causing its damage. In this case, the pressure in the printing fluid line may decrease, which may be measured using a pressure sensor, such as a pressure sensor located between the check valve and the print head. According to this disclosure, a mechanism is provided by which a malfunction of the check valve may be detected, so that appropriate action can be taken.

[0010] To put the disclosure into context, an example application of the disclosure will now be described. During use, the temperature of a print head may increase above ambient, or atmospheric, levels. The print head temperature may increase to 70 °C, for example, depending on print mode, usage, print head nozzle firing frequency, and the like. During use (e.g. during a print job), the temperature of the print head may reach a steady state, for example 60 °C. During a period of non-use (e.g. after the print job has completed), the print head may start to cool down to ambient, or room, temperature. As the temperature of the print head decreases, the volume of any air located within the print head may decrease, which can lead to a reduction in pressure in the print head. In some examples, a valve, such as a regulator valve, associated with the print head (e.g. located in the print head) may be caused to open once the pressure in the print head falls below a threshold level (e.g. below a negative cracking pressure of the valve). Opening of the valve may allow a printing fluid delivery system, or ink delivery system (e.g. comprising a printing fluid reservoir, a printing fluid line and/or a pump) to compensate the falling pressure by topping up the print head with printing fluid. As a result of this process, the pressure of printing fluid in the printing fluid line may decrease.

[0011] It is intended that the printing fluid delivery system is to remain pressurized (e.g. pressurized relative to the print head) in order to provide the print head with printing fluid and/or to compensate the change in pressure in the print head due to the print head cooling down. If the pressure of printing fluid in the printing fluid line is not maintained, then during a subsequent use of the print head, there may not be sufficient printing fluid to replenish any printing fluid used (e.g. delivered) by the print head and/or to compensate for a subsequent print head cooldown, which can cause damage to the print head, e.g. through air ingested by the print head through a nozzle to fill the void space caused by the cooldown.

[0012] Due to the time taken for the print head to cool down, the pump may not be activated continuously. The pump may activate more than once as the temperature of the print head, and the associated pressure within the print head, falls. As described in more detail herein, the pump may activate when the printing fluid pressure in the printing line falls below a first, lower, threshold level and/or may deactivate when the printing fluid pressure increases above a second, upper, threshold level. This process may be referred to as regulator compensation.

[0013] If the printing fluid pressure does not fall when the pump is deactivated, then it may be understood that the check valve is functioning as intended by preventing a flow of printing fluid in a direction from the print head towards the print fluid supply. While a decrease in printing fluid pressure may be indicative of a potential fault with a component, such as a component of the check valve, the pressure of printing fluid in the printing fluid line may decrease for reasons other than a component malfunction, such as the reasons outlined previously. In some circumstances, for example, a decreasing pressure of printing fluid in the printing fluid line may be indicative of a flow of printing fluid in a direction from the print head towards the print fluid supply. This phenomenon may be indicative of a malfunctioning check valve. Printing fluid flowing in a direction from the print head towards the print fluid supply may be referred to as backflow. The rate at which the printing fluid pressure decreases may be indicative of the severity of the problem associated with the check valve. For example, particles (e.g. contaminants) within the printing fluid may become lodged within the check valve, resulting in the check valve not being able to seal or function properly (e.g. due to a valve within the check valve not being able to fully close). In this case, the printing fluid pressure in the printing fluid line may decrease relatively slowly when there is only a small build up of particles within the check valve (e.g. one particle). The printing fluid pressure may start to decrease relatively quickly as more particles build up within the check valve over time. The check valve may malfunction for other reasons, such as a mechanical failure of one or more components of the check valve, or the like. If the check valve fails mechanically, the rate of decrease of the printing fluid pressure may be relatively high.

[0014] The present disclosure provides a more reliable apparatus (e.g. a print, or printing, apparatus). Examples in the disclosure can be used to provide a more reliable printing fluid delivery system (e.g. ink delivery system), which may be used to prevent early, or premature, print head failures. For instance, if the check valve is allowing a backflow of printing fluid, then the printing fluid delivery system will not be able to replenish the printing fluid in the print head and/or add printing fluid to the print head in response to the pressure in the print head falling due to print head cooldown. In this case, the print head may suck in air through the print head, which can result in de-prime issues and early print head starvation. Furthermore, as a result of the print head being starved of printing fluid (e.g. ink, an overcoat, a pre-treatment fluid and/or an optimizer), components of the print head may become damaged when the print head is activated, or powered, in the absence of printing fluid in the print head (e.g. in the print head firing chambers). By detecting when there is backflow due to a malfunctioning check valve, repair of any faulty component(s) may be possible before further, potentially irreparable, damage occurs.

[0015] When examples of the present disclosure are used in associated with a printing apparatus, the quality of printing may be improved, fewer consumables may be used, and/or data may be collected from a plurality of apparatuses (e.g. printers) to allow tracking and troubleshooting of issues on a large scale.

[0016] Referring to the drawings, Figure 1 is a plot showing examples of how pressure may vary as a function of time. The x-axis represents time and the y-axis represents pressure. More specifically, Figure 1 illustrates examples of how printing fluid pressure in a printing fluid line of an apparatus, such as a printing apparatus, may vary as a function of time, as represented by lines 102 and 104. Line 102 represents an example whereby the pressure of printing fluid decreases relatively slowly, and line 104 represents an example whereby the pressure of printing fluid decreases relatively quickly. If the pressure of printing fluid falls below a first threshold pressure level 106, a pump may activate thereby increasing the pressure of printing fluid in the printing fluid line (e.g. by pumping more printing fluid into the printing fluid line from a print fluid reservoir). The pump may deactivate in response to the printing fluid pressure reaching a second threshold pressure level 108. In other examples, the pump may activate when the pressure of printing fluid falls below a pressure level other than the first threshold level, such as a pressure level below the first threshold level. The first threshold pressure level 106 may be referred to as the first defined pressure level 106. The second threshold pressure level 108 may be referred to as the second defined pressure level 108.

[0017] Figure 2 is a schematic illustration of an example of an apparatus 200, such as an apparatus for detecting a potential malfunction in a component. The apparatus 200 may comprise or form part of a print, or printing, apparatus. The printing apparatus 200 comprises a check valve 202 to restrict the passage of printing fluid (e.g. ink) in a direction along a printing fluid conduit between a printing fluid supply and a print head, a pressure sensor 204 to measure a printing fluid pressure in the printing fluid conduit, and a processor 206. In some examples, the check valve 202 may allow the passage of printing fluid from the printing fluid supply to the print head while preventing, or restricting, the passage of printing fluid from the print head to the printing fluid supply. Ink may be referred to as print fluid, or the like. The printing fluid conduit may be referred to as a tube, pipe, a print fluid line, a printing fluid line, an ink line, or the like. The printing fluid supply may be referred to as a printing fluid reservoir, a print fluid supply, or the like. The print head may be referred to as a print fluid delivery unit, or the like. In some examples, a single pressure sensor 204 may be used to measure a printing fluid pressure within the printing fluid conduit whereas, in other examples, a plurality of pressure sensors may be used to measure the printing fluid pressure at one or more positions within the printing fluid conduit.

[0018] The processor 206 is to receive, from the pressure sensor 204, printing fluid pressure data indicative of a printing fluid pressure of printing fluid in the printing fluid conduit between the printing fluid supply and a print head. The printing fluid pressure data may comprise one or more pressure measurements, which may correspond to a printing fluid pressure measurement in the printing fluid conduit. The printing fluid pressure data may be generated, measured and/or recoded using the pressure sensor 204. In some examples, the printing fluid pressure data may comprise a time at which a printing fluid pressure measurement was generated.

[0019] The processor 206 is further to determine, based on the printing fluid pressure data, a first time point, or first time instant, at which the printing fluid pressure falls below a first defined pressure level 106 for a first time. The first defined pressure level 106 may be a preset pressure level (e.g. 0.5 bar, 1 bar, 2 bar, or the like). In some examples, the first defined pressure level 106 may be set by a user (e.g. a user of the printing apparatus). It may, for example, be possible to change the first defined pressure level 106. In some examples, the first defined pressure level 106 may be a preconfigured value hardcoded into the printing apparatus.

[0020] The processor 206 is further to determine, based on the printing fluid pressure data, a second time point, or second time instant, at which the printing fluid pressure falls below the first defined pressure level 106 for a second time. The first time point and the second time point may be different times. The second defined pressure level 108 may be a preset pressure level (e.g. 0.5 bar, 1 bar, 2 bar, or the like). In some examples, the second defined pressure level 108 may be set by a user (e.g. a user of the printing apparatus). It may, for example, be possible to change the second defined pressure level 108. In some examples, the second defined pressure level 108 may be a preconfigured value (e.g. hardcoded into the printing apparatus 200).

[0021] In some examples, the first time point and the second time point may correspond to consecutive time points at which the pressure fell below the first defined pressure level 106. In other words, there may have been no other instances where the printing fluid pressure fell below the first defined pressure level 106 in the time between the first time point and the second time point. In other examples, the first time point and the second time point may correspond to non-consecutive time points at which the pressure fell below the first defined pressure level 106. In other words, there may have been one or more instances of the pressure falling below the first defined pressure level 106 between the first time point and the second time point. [0022] The processor 206 is further to determine whether a duration between the first time point and the second time point is below a defined threshold duration. The duration between the first time point and the second time point may be referred to as a time interval between the first time point and the second time point. The defined threshold duration may be set by a user (e.g. a user of the printing apparatus) or may be a preconfigured value (e.g. hardcoded into the printing apparatus 200). A duration between two time points may be expressed as: t(n) — t(n — 1) < threshold, where t(n - 1) is the first time point at which the printing fluid pressure falls below the first defined pressure level 106 and t(n) is the second time point at which the printing fluid pressure falls below the first defined pressure level 106.

[0023] In some examples, an average of durations between two or more pairs of time points at which the printing fluid pressure falls below the first defined pressure level 106 may be compared to a defined threshold duration. For example, for two pairs of time points in which the printing fluid pressure level falls below the first defined pressure level 106, the following equation may be used: t(ri) — t(n — 1) + t(n — 1) — t(n — 2) - < threshold,

2 where t(ri) and t(n - 1) correspond to a first time point at which the printing fluid pressure falls below the first defined pressure level 106 and a second time point at which the printing fluid pressure falls below the first defined pressure level 106, respectively, and where threshold is the defined threshold duration.

[0024] In some examples, a cumulative duration between two or more pairs of time points at which the printing fluid pressure falls below a first defined pressure level 106 may be compared to a defined threshold duration, using, for example, the following equation: t(n) — t(n — 1) + t(n — 1) — t(n — 2) < threshold.

[0025] The processor 206 is further to generate an instruction signal if it is determined that the duration is below the defined threshold duration. The instruction signal may comprise an alert signal, or may comprise and instruction signal to generate an alert. For example, the instruction signal may generate an alert (e.g. a message, an alarm, or the like), indicative of the duration being below the defined threshold duration. The instruction signal may deactivate (e.g. remove a supply of power to) one or more components of the printing apparatus (e.g. a component of a print head, a print head, or the like).

[0026] In some examples, the processor 206 may be further to activate the pump 202 in response to a determination, based on the printing fluid pressure data, that the printing fluid pressure falls (e.g. has fallen, or fell) below a defined pump activation pressure. In some examples, the defined pump activation pressure may be different to (e.g. lower or higher than) the first defined pressure level 106. In some examples, the first defined pressure level 106 may comprise the defined pump activation pressure.

[0027] The processor 206 may, in some examples, be further to deactivate the pump 202 responsive to determining, based on the printing fluid pressure data, that the printing fluid pressure exceeds a second defined pressure level 108. In some examples, the second defined pressure level 108 may be different to (e.g. higher than) the first defined pressure level 106. In other examples, the second defined pressure level 108 may be the same as the first defined pressure level 106. The second defined pressure level 108 may comprise a pressure value above atmospheric pressure, such as 1 .5 bar 2 bar, 3 bar, or the like. The second defined pressure level 108 may be set by a user (e.g. a user of the printing apparatus 200) or may be a preconfigured value (e.g. hardcoded into the printing apparatus 200).

[0028] In some examples, the processor 206 may be further to generate an alert signal if it is determined that the printing fluid pressure is less than, or equal to, a second defined pressure level 108 after the pump has been activated for a first defined time period. In other words, if the printing fluid pressure has not increased to at least the second defined pressure level 108 within a defined time period (e.g. 10 second, 30 seconds, 1 minute, or the like), then an alert signal may be generated and/or the pump deactivated.

[0029] The generated instruction signal may be, for example, generated, or selected, based on the duration between the first time point and the second time point. For example, if the duration between the first time point and the second time point is relatively long, then a relatively less severe instruction signal may be selected (e.g. an instruction signal to generate a warning message), whereas if the duration between the first time point and the second time point is relatively short, then a relatively more severe instruction signal may be selected (e.g. an instruction signal to prevent the supply of power to a component of the print head). In some examples, the generated instruction signal may comprise an alert signal to be delivered to an operator. In some examples, the generated instruction signal may comprise a silent system error.

[0030] Figure 3 is a schematic illustration of a further example of an apparatus 300, such as a printing apparatus. The printing apparatus 300 may comprise one or more elements of the printing apparatus 200. In some examples, the printing apparatus 300 may comprise a pump 302 to move printing fluid along the printing fluid conduit between the printing fluid supply and the print head. In some examples, the printing apparatus 300 may comprise a printing fluid supply 304 and/or a print head 306. The arrangement of the components shown in Fig. 3 is an example of a possible arrangement. The components shown in Fig. 3 may be arranged differently. For example, the check valve 202 may be located downstream or upstream of the pump 302. Similarly, the pressure sensor 204 may be located downstream or upstream of the check valve 202. In some examples, the printing fluid supply may be downstream of the pump whereas, in other examples, the printing fluid supply may be upstream of the pump.

[0031] Figure 4 is a flowchart of an example of a method 400. The method 400 may comprise a computer-implemented method. The method 400 comprises, at block 402, receiving data corresponding to a print fluid pressure in a print fluid line between a print fluid supply and a print fluid delivery unit. In some examples, the received data may be incident from the pressure sensor 204 directly, while in other examples, the received data may be incident from a memory of a computer (e.g. which may correspond to an earlier pressure measurement).

[0032] The method 400 comprises, at block 402, determining, based on the print fluid pressure data, a first time instant at which the print fluid pressure falls below a defined threshold pressure level. The method 400 comprises, at block 404, determining, based on the print fluid pressure data, a first time instant at which the print fluid pressure falls below a defined threshold pressure level. The method 400 comprises, at block 406, determining, based on the print fluid pressure data, a second time instant at which the print fluid pressure falls below the defined threshold pressure level. The method 400 comprises, at block 408, calculating a time interval between the first time instant and the second time instant. The method 400 comprises, at block 408, responsive to determining that the time interval is below a defined threshold time, performing an intervention action.

[0033] In some examples, the intervention action may be selected based on the time interval between the first time instant and the second time instant.

[0034] The method 400 may further include blocks associated with or corresponding to functionality disclosed herein. For example, the method 400 may further comprise activating a pump in response to a determination, based on the print fluid pressure data, that the print fluid pressure has fallen below a defined pump activation pressure. In some examples, the method may comprise deactivating the pump responsive to determining, based on the print fluid pressure data, that the print fluid pressure has exceeded a second defined pressure level.

[0035] In some examples, performing an intervention action (block 408) may comprise generating an alert signal if it is determined that the print fluid pressure is less than, or equal to, a second defined pressure level after the pump has been activated for a first defined time period. The method may comprise deactivating the pump after a second defined time period. The method may, in some examples, comprise generating an alert signal if it is determined that the print fluid pressure is less than, or equal to, a second defined pressure level after the pump has been activated for the second defined time period.

[0036] The intervention action performed may be selected based on the calculated time interval. For example, if a relatively long time interval is calculated, then it may be determined that a relatively minor defect is affecting the check valve, and a non-urgent intervention action may be performed, such as generating an alert to be presented to an operator. However, if a relatively short time interval is calculated, then it may be determined that a relatively severe defect is affecting the check valve, and an urgent intervention action may be performed, such as cutting power to a component of the print fluid delivery unit, in order to limit the likelihood of damage to other components.

[0037] Figure 5 is a schematic illustration of a machine-readable medium 504 in communication with a processor 502. The machine-readable medium 504 comprises instructions which, when executed by the processor 502, may cause the processor to perform blocks of the method 400 disclosed herein. The machine-readable medium 504 comprises instructions (e.g. pump activation data receiving instructions 506) which, when executed by the processor 502, cause the processor to receive pump activation data for a pump in fluid communication with a printing fluid line, wherein the pump is to pump printing fluid through the printing fluid line between a printing fluid reservoir and a print head of a print apparatus. When the pump is activated, the pump may operate (e.g. pump printing fluid through the printing fluid line) for a period of 1 second, 2 second, 5 seconds, 10 seconds, 20 seconds, or the like, depending on the amount of printing fluid to be moved, and/or on the change in printing fluid pressure to be achieved. The pump activation data may include information on when the pump started pumping and/or when the pump stopped pumping. A pump activation may, therefore, be considered to be the event of the pump starting to operate, or the entire event, from the moment the pump started to operate to the moment the pump stopped operating.

[0038] The machine-readable medium 504 further comprises instructions (e.g. pump activation frequency determining instructions 508) which, when executed by the processor 502, cause the processor to determine, based on the pump activation data, a pump activation frequency. The machine-readable medium 504 further comprises instructions (e.g. alert signal generating instructions 510) which, when executed by the processor 502, cause the processor to generate an alert signal in response to determining that the pump activation frequency meets or exceeds a defined threshold frequency.

[0039] In some examples, the pump activation frequency may be determined based on two data points (e.g. a first time at which the pump activated and a second time at which the pump activated). In other examples, the pump activation frequency may be determined based on three or more data points (e.g. time points associated with three or more pump activations). In some examples, the data points may correspond to consecutive pump activations. In other examples, the data points may correspond to non-consecutive pump activations. In some examples, the data points may correspond to a mixture of consecutive, and non- consecutive, pump activations.

[0040] The frequency with which the pump activates may be used to determine, or quantify, whether there is a problem associated with the check valve 202 and/or a severity of the problem. The frequency with which the pump activates may be used to determine an action to be taken, such as generating a message to be delivered to a user (e.g. an operator of the print apparatus), generating an alert signal (e.g. an audible alarm), or deactivating one or more components of the print apparatus (e.g. a component of a print head, a print head, or the like).

[0041] Figure 6 is a schematic illustration of a machine-readable medium 602 in communication with a processor 502. In some examples, the machine-readable medium 602 may further comprise instructions (e.g. printing fluid amount determining instructions 604) which, when executed by the processor 502, cause the processor to determine, based on the pump activation data, whether an amount of printing fluid in the printing fluid reservoir is below a defined threshold amount. This may be used to determine that the printing fluid reservoir is running out of printing fluid, and an operator could be alerted that the printing fluid reservoir (e.g. a printing fluid supply or printing fluid cartridge) is to be replaced.

[0042] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0043] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0044] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0045] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0046] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams.

[0047] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure. [0048] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.

[0049] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0050] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.

Claims

CLAIMS A printing apparatus, comprising: a check valve to restrict the passage of printing fluid in a direction along a printing fluid conduit between a printing fluid supply and a print head; a pressure sensor to measure a printing fluid pressure in the printing fluid conduit; and a processor to: receive, from the pressure sensor, printing fluid pressure data indicative of a printing fluid pressure of printing fluid in the printing fluid conduit between the printing fluid supply and a print head; determine, based on the printing fluid pressure data, a first time point at which the printing fluid pressure falls below a first defined pressure level for a first time; determine, based on the printing fluid pressure data, a second time point at which the printing fluid pressure falls below the first defined pressure level for a second time; determine whether a duration between the first time point and the second time point is below a defined threshold duration; and generate an instruction signal if it is determined that the duration is below the defined threshold duration. The printing apparatus of claim 1 , further comprising: a pump to move printing fluid along the printing fluid conduit between the printing fluid supply and the print head; wherein the processor is further to activate the pump in response to a determination, based on the printing fluid pressure data, that the printing fluid pressure falls below a defined pump activation pressure.

3. The printing apparatus of claim 2, wherein the processor is further to: deactivate the pump responsive to determining, based on the printing fluid pressure data, that the printing fluid pressure exceeds a second defined pressure level.

4. The printing apparatus of claim 2, wherein the processor is further to: generate an alert signal if it is determined that the printing fluid pressure is less than, or equal to, a second defined pressure level after the pump has been activated for a first defined time period.

5. The printing apparatus of claim 2, wherein the processor is further to: deactivate the pump after a second defined time period.

6. The printing apparatus of claim 5, wherein the processor is further to: generate an alert signal if it is determined that the printing fluid pressure is less than, or equal to, a second defined pressure level after the pump has been activated for the second defined time period.

7. The printing apparatus of claim 2, wherein the first defined pressure level comprises the defined pump activation pressure.

8. The printing apparatus of claim 1 , further comprising: the printing fluid supply and the print head.

9. The printing apparatus of claim 1 , wherein the generated instruction signal is selected based on the duration between the first time point and the second time point.

10. The printing apparatus of claim 1 , wherein the generated instruction signal comprises a signal to prevent the supply of power to a component of the print head.

11 . The printing apparatus of claim 1 , wherein the generated instruction signal comprises an alert signal to be delivered to an operator.

12. A computer-implemented method comprising: receiving data corresponding to a print fluid pressure in a print fluid line between a print fluid supply and a print fluid delivery unit; determining, based on the print fluid pressure data, a first time instant at which the print fluid pressure falls below a defined threshold pressure level; determining, based on the print fluid pressure data, a second time instant at which the print fluid pressure falls below the defined threshold pressure level; calculating a time interval between the first time instant and the second time instant; responsive to determining that the time interval is below a defined threshold time, performing an intervention action.

13. The computer-implemented method of claim 12, wherein the intervention action is selected based on the time interval between the first time instant and the second time instant.

14. A machine-readable medium comprising instructions which, when executed by a processor, cause the processor to: receive pump activation data for a pump in fluid communication with a printing fluid line, wherein the pump is to pump printing fluid through the printing fluid line between a printing fluid reservoir and a print head of a print apparatus; determine, based on the pump activation data, a pump activation frequency; and generate an alert signal in response to determining that the pump activation frequency meets or exceeds a defined threshold frequency.

15. The machine-readable medium of claim 14, wherein the processor is further to: determine, based on the pump activation data, whether an amount of printing fluid in the printing fluid reservoir is below a defined threshold amount.