FDC1004: FDC1004 Sensitivity and Settling issue

Hi Team,

In terms of sensor prospective, how the sensor performance reacts to the rinse solution(detergent). Because most of the reference designs I see for the FDC1004 are based on water as a liquid.

 So please give me a update ASAP

Sathya,

Our reference designs and app notes focus on pure water because it is not particularly conductive, which is a necessity if you are trying to form a reliable capacitance for measurement. 

Additives in the water can impact the readings in multiple ways. An additive can alter the electrical properties or the "mechanical" properties of the water.

An additive can cause the liquid to coat the sides of the tank, which can complicate readings.
It sounds like your additive is a rinse agent, so that may not be the issue here.

An additive can make the liquid conductive, which drastically impacts the capacitance you are trying to create - and measure - with your sensors and the liquid. The system with a conductive liquid can be thought of as a really poor electrolytic capacitor. 

An additive can cause air bubbles to be trapped in the liquid for a longer time by making it more viscous - compared to water -  after the liquid is disturbed in some way, like shaking or stirring. This will cause the liquid to have electrolytic properties that are a combination of liquid + air, and they will vary over time as the bubbles dissipate. Foam on top of the liquid could have a similar effect..

Regards,
John

Sathya,

I see you clicked "This did not resolve my issue".

What other information are you looking for?

John

What is the best approach / algorithm to measure the conductive liquid of the capacitance of FDC1004 sensor.

Current PCB layer stack up is attached here.

Used Basics of Capacitive Sensing and Applications approach, 

  on top layer used the Reference liquid sensor for level sensing (CIN1) and Environmental sensor (CN2).

let me know how to reduce impact of additives in the water. To get accuracy and resolution in the capacitive measurement.

Sathya,

What do you think the reasons are that the additive is causing challenges with the readings?
Is it because the additive is causing otherwise pure water to become conductive?
Or is the additive altering the properties in some other way?

Regards,

John

Probably it is because of Tank thickness and more viscosity in the liquid.  Bottle thickness  and 2ml of rinse additive (surfectant ) with 1ltr of deionized water is our the requirement and  cannot be changed. Please let me know to improvise the current design to sense the capacitance reading to get accuracy and resolution for the viscosity in the liquid.

Sathya,

What are the goals for your application?
Are you trying to detect the liquid level in the tank, or something else?
What role does shaking the bottle play with respect to the application goals?
Regards,
John

In our application, two different sensor board design (same FDC 1004 design) and two different bottle.

- Rinse bottle (1000ml)

- Waste bottle (1200ml)

After the test is completed, use the rinse solution to clean the tubes and the used/waste water is stored in the waste bottle.

We use FDC1004 IC to measure liquid level in rinse and waste bottles. Therefore, the rinse solution decreases, whereas the waste bottle increases.

Once the rinse bottle drops below 15% of 1000 ml, the system will receive a warning indication that the waste bottle is empty. The waste bottle level goes over 90%, the system will get a warning indication because the waste bottle is full (biohazard).

When calibrating to low and high volume bottles. After calibration, immediately 50% of the volume in the bottle should be removed. Because the water in the bottle oscillates, the settling time of the sensor's capacitance value must be known.

Hope you are clear with the application.

Sathya,

Thank you for the add'l information.

Could you expand on the use cases for your statement "Because the water in the bottle oscillates, the settling time of the sensor's capacitance value must be known."

You mentioned shaking the bottle earlier. Is that one of the use cases?

John

Sensor will be stick to the bottle and measure the capacitance value by pouring 1000 ml of 15% of the rinse solution into the bottle and performing a low volume  calibration.

Different capacitance values ​​were observed for 15% 1000 ml of rinse solution when the bottle wall was dry and wet. So, after pouring 15% of the rinse solution into bottle. we shake the bottle and wait for the capacitance value to get settle down on the wet wall of the bottles. So to find the settle down time of the capacitance, shake the bottle and measure the raw capacitance value from FDC1004.

Another use case is when I insert the bottle into the slot. The rinse solution in the bottle will oscillate for 2 second. So why shake the bottle and find the settling down time of the source capacitance value from the FDC1004?

Thank you Sathya.

We don't have any specific guidance on how to use the FDC1004 to deal with liquids that need time to settle.
Hopefully you can budget the time needed for the liquid to run down the sides, and/or stop moving in the container.

Also, we don't have any guidance to offer on using the FDC1004 to measure conductive liquids.
A strategy will need to be derived thru experiments with your liquid.

Regards,
John

Thank you John,

Can you suggest an alternative part that can be used to measure conductive fluids?

Hello, 

One of our alternatives for liquid level sensing is to use an ultrasonic solution. Here are a few app notes we have on using ultrasonic for this: 

• https://www.ti.com/lit/an/snaa270/snaa270.pdf
• https://www.ti.com/lit/an/snaa266a/snaa266a.pdf
• https://www.ti.com/lit/an/slda058/slda058.pdf

Best Regards, 

Justin Beigel

Hello Sathya, 

I saw you clicked "This did not resolve my issue". 

Are other technological approaches to liquid level sensing able to be considered for this application or can you only use capacitive?

Is there anything else you would like our input on? 

Best Regards, 

Justin Beigel

We only use it with capacitive sensing solution.

Is this TI part suitable for FDC2x1x (FDC2112, FDC2212, FDC2114, FDC2214) application??

Application states – A wide frequency range is especially useful for reliable sensing of conductive liquids such as detergent, soap, and ink.

But I don't see any of your reference designs and application notes that focus on the conductive liquid?

Sathya,

We do not recommend devices from the FDC2xx family of products.
Many customers have struggled to get to production with these parts for a variety of reasons, especially EMI.
The FDC1004 is a much mode reliable solution and is our recommended device for capacitive sensing.

I can't comment on the the claim of the FDC2xxx being useful for detecting the level of conductive liquids because there is no supporting info on this in our archives, in spite of the fact we have published comments on this topic.

It sounds like there may be two issues at work here:
1. To some degree, residual liquid tends to cling to the side of the your tank for some period of time, and then settle into the rest of the liquid.
2. To some degree, the liquid you are trying to measure is conductive.

While we do make the FDC1004 and are experts in how the device functions, we can't in any way present ourselves as experts in tank materials, fluid properties, or how fluids interact with tank materials. 

Perhaps one approach to this problem is to try a set of experiments with your tank assemblies, starting with pure water and assessing how the FDC1004 works with your tank and sensors. If everything works well, this condition will serve as your baseline case. It it does not work, other options for the tank, sensors, or both will be needed.
If your system works well with pure water, the next steps would be to add minute but increasing amounts of your cleaning fluid and assess the system performance at each step. The goal will be to find what amount of cleaning fluid causes the the system to no longer provide reliable fluid levels. 
If you can identify an amount or concentration of cleaning fluid + water that allows accurate fluid level estimates and meets your other requirements, then you will have your solution. 
If the cleaning fluid concentration is too low when the fluid level estimates become unreliable, then perhaps a different cleaning fluid could be considered. You could also experiment with FDC1004 device settings, but,as I said, we don't have any guidance to offer about how to adapt the settings to accommodate conductive fluids, or if that is even possible. 

Regards,
John

Hi Team,

Yes it works with pure water and gives good result. 

We can not change the concentration of the rinse solution and use it for urine sample testing.

So it is recommend to measure conductive liquid. unless if i don't touch the bottle accuracy was good. 

If the user wants to refill the rinse solution, the test can be started immediately, whereas after the waste liquid is removed from the waste bottle, have to wait for the settling time. It goes to maximum capacitance value and slowly it is getting setting down  (1-2 hours) which is not acceptable. 
In other situations, removing the waste bottle from the slot and re-attaching it will take time (1-2 hours) for the liquid to settle which is not acceptable. 

so what is going wrong on the sensor?

Hi John,

I need TIDA-00317 EVK and when I search on distributor website it is not available.

So, can you arrange an EVK for us?(its urgent)

Sathya,

I am sorry to say my access to the TIDA is the same as everyone else's.
We don't have any stock available here, which is unfortunate because I need a few as well for another application.

regards,
John

Hi John,
Since we dont have time to send the gerber file for fabrication, is it possible for you to reproduce the test with the conductive fluid and observe the settling time for the conductive fluid with TIDA-00317 EVK. If so, use deionized water with 2 mL of rinse solution.