TIDA-00317: Water level sensing solutions

Ryan Thompson

Part Number: TIDA-00317

Hi,

I have an application for sensing water level in a small reservoir (about a quart of volume). Ideally, the electronics would be located a foot or two from the sensing electrodes that are submerged in the water, so the connections are made via cable. Also, the electrodes need to be stainless steel, and submerging a printed circuit board is not acceptable. I will design the stainless steel electrodes.

I saw this on your website.

www.ti.com/.../tida-00317 level sensor&tisearch=Search-EN-Everything

Is this the best solution for my application?

Thanks,

Ryan Thompson

Expert 7200 points

Yibo Yu
- May 11, 2017 12:00 AM
Hi Ryan,

The reference design provides a good approach for sensing liquid level. In particular, it is immune to hand or other interference. However, in order for the out-of-phase approach to work well, the shielding electrodes and sensing electrodes have to be well balanced, which can be easily achieved on a PCB.

Regards,
Yibo
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Ryan Thompson
- May 11, 2017 1:49 PM
In reply to Yibo Yu:
Hi Yibo,

Thanks for the information. We are looking at the gerbers for the liquid level sensor board, and the stackup is for a 4 layer board, with the 2 outside layers not having any copper. What is the intent of this? Is a 2 layer board with soldermask not sufficient? Note that we want to use a rigid/FR4 2 layer board to do some initial testing, but we can go to 4 layers is the outside blank layers are needed.

Ryan
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Ryan Thompson
- May 11, 2017 3:23 PM
In reply to Ryan Thompson:
After talking to our PCB designers, I now believe the extra layers are to providing stiffening in the sensor area yet still allow the tail to be flexible. Please confirm. With that understanding, we may just build an 0.062" 2 later rigid board and live with the tail section being rigid.

Ryan
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Yibo Yu
- May 11, 2017 11:06 PM
In reply to Ryan Thompson:
Hi Ryan,

The sensor board is a rigid-flex PCB. The rigid portion is for the connector only which is located at the end of the tail. It can be used for connecting to the FDC1004 EVM.

The sensor portion was meant to be flexible in this design (though it does not have to be), so that it can be attached easily to containers of various shapes. The flexible sensor portion uses 2 layers only. The TIDA-00317 document has more information on how the inputs and shields are connected.

Regards,
Yibo
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Ryan Thompson
- May 23, 2017 6:49 PM
In reply to Yibo Yu:

Hi Yibo,

We have started testing using the TI reference design for the liquid level sensing PCB. Overall the testing is going well, but there appears to be some strange behavior either with the FDC1004EVM or the GUI software. Measurement 3 is the CIN3 differential with CIN4 per the PCB layout and documentation (reference enviromental sensor), but it periodically shows a capacitance value that is way off (~10pF). See screenshot below. Any idea what is going on here? Is there a known bug in the EVM firmware or the GUI?
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Yibo Yu
- May 25, 2017 1:14 AM
In reply to Ryan Thompson:
Hi Ryan,

It seems that the screenshot did not get attached. Could you attach the screenshot again?

Regards,
Yibo
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Ryan Thompson
- May 25, 2017 1:53 PM
In reply to Yibo Yu:

Hi Yibo,

I'll try again, this time as a .pdf. Please see attachment.

RyanLiquid_Level_Sensing_screenshot.pdf
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TIDA-00317: Water level sensing solutions