Hot wire Annemometer

Thread Starter

Raunak Bahri

Joined Oct 25, 2015
6
hello
i am trying to make a hot wire annemometer , i want to build a constant temperature annemometer using PT 1000 wire as sensor. To have temperature compensation i will put 2 such wire of pt 1000 on 2 legs of the Wheatstone bridge .But i am not able to figure out the feedback circuit required for constant temperature method. Can anyone guide me ?
 

Thread Starter

Raunak Bahri

Joined Oct 25, 2015
6
There was a thread about this a while back.
Yes i tried to continue on that but ,the site said the post is too old to reply .But that post does not solve my query and i was actually trying to write to you directly as i saw you were giving valuable feedback on that post .Can you help me with this ?
 

joeyd999

Joined Jun 6, 2011
4,268
Yes i tried to continue on that but ,the site said the post is too old to reply .But that post does not solve my query and i was actually trying to write to you directly as i saw you were giving valuable feedback on that post .Can you help me with this ?
Not without any details.

Read the whole thread and try to understand the schematics I posted. I (and others, I am sure) will help where I can.

I also pointed out that the circuit I proposed does not compensate for temperature -- that will be a major source of error.

And, I do not answer PMs (private messages) for help. All your work needs to be done out here.
 

Thread Starter

Raunak Bahri

Joined Oct 25, 2015
6
Hello joey

Its been some time i was not working on the project for some time , now i am back i am thinking of implementing this circuit and have to order components , can you advise me if this is ok ? I have used to Pt100 sensors on the 2 legs of the wheatstone bridge and one is having higher current than the other so .The one with higher current responds to flow velocity while other is for temperature compensation as it is not affected by flow . I want to increase the current going into the circuit aswel as right now its just 100ma and i believe it will not be enough to heat the Pt100 enough for accurate measurement.Also i am not able to balance the bridge to 0V . Can you suggest some improvements to this circuit ?Annemometer.png Annemometer.png
 

Thread Starter

Raunak Bahri

Joined Oct 25, 2015
6
thanks bertus i have seen that thread , but i want to make it with Pt100 and most of the circuits in there are based on NTC also they do not involve temperature compensation .Can you guide me with the circuit i have given above .I modified it a little bit to have more current .Refer attachement below
 

Attachments

joeyd999

Joined Jun 6, 2011
4,268
I have not commented upon your circuit because this is not the way I would do it. Unfortunately, I cannot tell you how I would, but think of it this way:

Try to identify (and quantify) how ambient temperature affects the response of an uncompensated hot wire. This will require a great deal of thought, some algebra, and, likely, some actual experimentation.

Then, assuming you can measure both the output of the bridge and ambient temperature, see if you can discover a way to compensate for temperature in software rather than hardware.

This is possible, and amazingly easy.
 

Thread Starter

Raunak Bahri

Joined Oct 25, 2015
6
I have not commented upon your circuit because this is not the way I would do it. Unfortunately, I cannot tell you how I would, but think of it this way:

Try to identify (and quantify) how ambient temperature affects the response of an uncompensated hot wire. This will require a great deal of thought, some algebra, and, likely, some actual experimentation.

Then, assuming you can measure both the output of the bridge and ambient temperature, see if you can discover a way to compensate for temperature in software rather than hardware.

This is possible, and amazingly easy.
Hi joey

It will be really nice if you have a look at this video http://www.us.endress.com/en/Tailor-made-field-instrumentation/Flow-measurement-product-overview/Product-Thermal-flowmeter-t-mass-T-150-6TAB

This is the exact principle i am trying to implement , If its not so clear can you guide me with the following:
1. how can i increase the current in one of the legs ,at present the value is close to 100mA , if i want to make it more than that how do i achieve it .
2.Also i am getting a value in mv as offset when i am trying to balance the bridge . How do i get it to null .At least in the practical scenario.
 

joeyd999

Joined Jun 6, 2011
4,268
Hi joey

It will be really nice if you have a look at this video http://www.us.endress.com/en/Tailor-made-field-instrumentation/Flow-measurement-product-overview/Product-Thermal-flowmeter-t-mass-T-150-6TAB

This is the exact principle i am trying to implement , If its not so clear can you guide me with the following:
1. how can i increase the current in one of the legs ,at present the value is close to 100mA , if i want to make it more than that how do i achieve it .
2.Also i am getting a value in mv as offset when i am trying to balance the bridge . How do i get it to null .At least in the practical scenario.
I don't see a video. Just a product page.

1. You can define the operating current (more precisely, temperature) by adjusting the values of the fixed components in the bridge. It's just math.

2. Yes. An ultra-low Vos amp is an absolute requirement for any kind of accuracy/repeatability. Think chopper stabilized down to microvolts. Manual Vos compensation is going to be useless.
 

Thread Starter

Raunak Bahri

Joined Oct 25, 2015
6
I don't see a video. Just a product page.

1. You can define the operating current (more precisely, temperature) by adjusting the values of the fixed components in the bridge. It's just math.

2. Yes. An ultra-low Vos amp is an absolute requirement for any kind of accuracy/repeatability. Think chopper stabilized down to microvolts. Manual Vos compensation is going to be useless.
Hello

I tried to implemen my circuit which is posted above , i am running into trouble with the opamp , My opamp is always running into saturation .So with increase in Air flow, i.e. increase in bridge output there is no change in the opamp output. Kindly guide me how to solve this problem
.
 
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