Clarification about this Water proof ultrasonic distance sensor

[Wond3rboy]

Hello

I am working on a project for a chemist where I have to measure the liquid level of a basic pH value. The liquid will be present in a cylinder which can have varying diameters. I was looking to use ultrasonic sensors to measure the level and stumbled upon this one
https://www.evselectro.com/ultrason...f-dc-5v-7119?keyword=ultrasonic&category_id=0

which is available locally. However, I wanted some clarification on the parameter named 'blind=25cm'. Does that mean that the sensor is not sensitive within 25cm from its head?

Thank you

 

[Nigel Goodwin]

I would imagine so - it's perfectly normal for ultrasonic ranging to have a minimum limit (for sound and obvious reasons).

 

[jpanhalt]

Will the composition of the liquid be constant?

Are the cylinders clear or opaque?

John

 

[Wond3rboy]

Thank for the reply Nigel and John.
Nigel: I had thought so, but 25 cm seemed to be a too large to minimum distance to me. This effectively means that we cannot use this in our project.
John: The composition of the liquid will remain constant, the cylinder is sealed actually. The cylinders are clear.

The whole assembly is used to produce methane (if I remember correctly) and the idea is that as more methane is produced, the liquid in the cylinder rises up and we are supposed to measure that level and relate it to the methane produced.

I will try to look up other sensors now. I did find other ultrasonic sensors too which are waterproof(I go for water proof because of the protection they offer, I dont want any erosion due to the base in the cylinder). I will replace these in the normal ultrasonic sensors and see what I get. Resolution is what I am concerned about here though.

Do you guys have a better suggestion?

 

[Nigel Goodwin]

Nigel: I had thought so, but 25 cm seemed to be a too large to minimum distance to me. This effectively means that we cannot use this in our project.

Presumably it's intended as a car reversing sensor?, so the minimum distance is low enough. It also looks to use a single transducer, which is also likely to limit the minimum range as well.

 

[alec_t]

The main problem with using an ultrasonic sounder at short distances is that you need to 'ping' the transducer for some minimum period. If that period is, say, 2mS then in that time the ultrasound travels a distance of ~2ft =~ 50cm. That establishes the minimum measurable distance as ~1ft =~25cm. Because of a finite beamwidth and multiple reflections the pulse echo is unlikely to have a well-defined rising edge; hence timing errors become significant at short range.
An ultrasonic ranging system will not be very accurate unless you can compensate for the effects of temperature and humidity on the speed of sound in air. What sort of accuracy are you hoping for?

 

[Wond3rboy]

Ah! Nigel thanks for the explanation.

Alec: That makes sense, thank you. The accuracy I am looking for is 0.5 cm. However, I think the chemist is a little wrong in the accuracy he is asking for as he has said that the cylinder can be narrowed or thickened in the future which will in turn change the displacement for the same unit of gas produced so I am looking for as much accuracy as possible (I would think ideally in 1mm). No off the shelf ultrasonic sensor seems to provide that though, I wanted to do a test run for him and show the principle.

I am also looking at the detection of pressure in the cylinder and relate that to the change in level. I will have to work more on that.

 

[Tony Stewart]

I imagine a digital scale would be more accurate. with 1 or 0.1 g resolution.

 

[Wond3rboy]

Thank you for the reply Tony. We aim to create an automated system for this task so we are excluding all manual options. I am currently working on the pressure thing. Will update on this as it progresses.

 

[alec_t]

Compensation for liquid density change with temperature will, of course, be necessary with a pressure/digital-scale/strain-gauge system.

 

[Tony Stewart]

Thank you for the reply Tony. We aim to create an automated system for this task so we are excluding all manual options. I am currently working on the pressure thing. Will update on this as it progresses.

Ultrasound for fish-finders has a big blind range due to the lower frequency, but has longer range used than Medical transponders used in the >>1MHz range.

Methods I would consider are dielectric frequency response with vertical electrodes and Infrared surface reflance with very narrow aperture emitter/detectors with nanosecond response is easier to restrict diffusion, diffraction and scattering than acoustic but harder due to propagation velocity demand on signal BW.

I would think of all the errors from scattering of 3D signals in a near-field emitter and stay away from those methods that are difficult to control and filter out invalid reflections.

Scales can be integrated real-time with the same sensors with some reference for calibration required for any method. This is far more accurate than measuring volume or height.

 

[Wond3rboy]

Thanks very much Tony. I will try to work on the scales suggestion

 

[cowboybob]

Have you considered this option:

using the is device: https://www.analog.com/en/search.html?q=AD7746 ?

(Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/AD7745_7746.pdf)

 

[Tony Stewart]

Depending on cylinder size, ( beaker?) Cheap scales have 1gm resolution, I have a $30 one that does 400 gm +/- 0.1 with auto-zero for the empty container, then you can re-zero after adding each ingredient. One peppercorn is 0.1g

Interfacing to an existing scale is much easier than DIY from scratch.

The capacitance method ought to work as a linear height scale, but far less accurate than weight using strain gauges, but perhaps easier on the mechanical design.

 

[Wond3rboy]

Thank you bob and tony. You guys have really given me a new direction to work in. Thanks a ton!

I will evaluate both options.