Please help Selecting the optimum sensors

[Rishabh]

Hi Everyone, 

My VI is supposed to take input from a pair of infrared sensors, A Photosensor and a Thermocouple.

Can Anyone suggest some models for above mentioned sensors. I have read somewhere that photosensorsare not directly connected to DAQs. How far does the truth go?? I don't need a very costly one.

Thankx!!!

[Albert.Geven]

This depends completely on the speed and accuracy that you need.

thermocouples also need a good more than 16 bit input and appropriate amplifying system.

[Rishabh]

Hi,

 I need a windows based system with a DAQ of sampling rate as low as 8-24 samples/sec. I am sampling the signals by statistical methods(taking the mean). My output requirement consists of purely On/off signals which will be used to activate/deactivate certain devices. 

 I require my thermocouple to be sensitive for some 8-50 degree celcius range with minimum resolution being of the order of 2-3 degrees. 

I had initially selected USB 6008 for my purpose as it was a cheap and optimum performer. but, i have read that the thermocouple reslution will rise to somewhere around 10 degrees. How far does the truth goes?? 

I hope this info might suffice to give u the clear idea about my requirements. 

Regards,

Rishabh

[Albert.Geven]

Hi

You need at least 14 bit for this thermocouple resolution. So a 6009 would do.

It has also digital IO and is relative fast 50kHz so a lot of statistics can be done. Look also at the median function, I really like it.

On the internet you can find other devices, maybe cheaper.

[Rishabh]

Thanks Albert, 

6009 seems to fit in the job. But, I have one serious doubt. How do we calculate the resolution of a DAQ based on our requirements. For Example, in my case, I don't need a very precise measuring apparatus.

How have u decided that it will require a 14 bit device to suit the need? Furthur, I want to know if a DAQ can accept input in LUX(lumen per square metre)  directly. 

Regards,

Rishabh

[Albert.Geven]

Hi

In the case of the thermocouple it was experience but you can calculate this.

What you want is a resolution result say: it must be possibleto measure 1 degree.

1 degree means in a thermocouple ( k type) a 41 microvolts change.

To measure this with a 6009 with a pga set to 20 (only use differential mode) you can measure 20v divided by 20 gives a full range of 1V.

With 14 bits you get  about 16000 possible combinations meaning 0.0000625 Volt per step is 62.5 microvolt per step so about 1.5 degree (celsius) for a Ktype couple.

Now about Lumen... A measurement with all measurment machines I know is voltage or current. But we have converters for light into current an converters for current into voltage (called a resistor) that make it possible to measure Lumen with a 6009.

So with a 6009 you always measure voltage but with multiplying conversion factors we get Lumen.

good luck

[Rishabh]

That was really a good revealer.

Thanks for all the HELP!! 

Regards,

Rishabh

[Henrik_Volkers]

Why stick with a thermocouple? Is space critical? If not I would choose a PT1000 or a NTC, much better SNR.

Even cheaper after rough one point calibration: the bias voltage of a diode is -2mV/°C ( a 470k R to 5V in series with a 1N4148 to GND is always worth a try 😉

Message Edited by Henrik Volkers on 09-01-2009 02:16 PM