Which DAQ for temperature and force

[picante]

Hi, I've very new to this, but I have a need to measure temperature of water (between 10 and 70 degrees Celsius) over a couple of hours with 4 probes at the same time. I also need for this device to be able to measure force or weight on a platform, the weight on this will be less than  500kg. Ethernet is prefered but USB will be suitable if there is a considerable cost diference.

Please let me know if you have any suggestions.

I should also add that in terms of sample frequency, the temperature needs to be sampled at least every 3 seconds. The force needs to be sampled 5 or more times per second.

Low cost is important.

TIA.

[Henrik_Volkers]

Low cost 🙂

Out of the box? How much time do you want to spend on soldering, building, breadboarding, calibration?

Tolerances +-10%??

What type of balance / force sensor do you want to use?

temperature with NTC sensor and a resistor is cheap but migth need individual calibration..

Low cost Ethernet.... Raspberry???

[picante]

I know you that in many aspects, you get what you pay for, my budget is of around 500, and I know that these systems can be very expensive.I'm new inventor, and very new to data acquisition, and wanting to get the best value for money. I'm no electrical engineer, but have some knowledge in electronics and in depth knowledge to computer systems and networks. Ideally the system would be ready to be used (no building needed), but if the budget does not allow for this, then I'm happy to spend the initial time building soldering, etc, as long as I end up with a system that it is easy to setup and reliable and flexible. Let me know if my expectations are not realistic. In terms of sensors, I'm open to suggestions. I need to measure if the force on a platform will go close to or below a specific force, this needs to be done while someone is walking on this platform.The sensor needs to be able to be exposed to outdoors and likely to be submerged in water. -+10% is too high -+2% would be more suitable. The temperature sensors need to measure water temperature and air, so it needs to be able to be measure 0.1 of a degree, and be able to adjust to the temperature within 30 seconds, it needs to be able to be exposed to those elements also.  I have heard of the term isolation used around data acquisition systems, is this relevant with sensors and the risk of people being close to sensors and water.

[Henrik_Volkers]

30s to 0.1K in air?  Do the physics and you end with very small sensors. Well, with classical sensors.... measure the speed of sound ? (related to air temp) ...

2% for force .. no problem for static measurements, but already more challenging with dynamic ones, and easely eat up your budget 😉

4 force transducer IP65 to build a platform.... maybe from CE  (china engineering) to stay inside the budget.

Sound like animal 'check' station. reliable weigth platforms for such enviroment are hard to get for <500 bucks ...

Depending on your application (inventors don't like to be too specific 😉 ) maybe a hunt for a balance at ebay ? 

So, solve the force measurement hardware task, and then look for the DAQ. Maybe you need to read a DMS bridge, or a 10V or a 20mA analog value , or a rs232 rs485 datastream ...

[picante]

Thanks for the feedback Henrik, you are right about not being specific 

That is all very good information, much appreciated, I had not realised that the cost of a force transducer was that high.

To clarify, I want this to actually collect data to validate a theory not so much to actually make long terms part of a device.

With that in mind, and because the most important aspect of this experiment would be to measure the temperature of water, would you have any suggestions that will allow me to measure temperature of air but most important water with the requirements of measuring every 30 sec with an accuracy of 0.1 of a degree centigrade, where the temp range is between 10-70 degrees C. And that will allow me to have test from a force transducer at a later stage.

I have heard of the terms agitation and isolation thrown around in the context of data acquisition, I’m not sure if these are relevant to what I’m wanting to do.

Please calrify, a DSM bridge, would this be a "distance meassuring system" sensor that would convert displacemnt instead of force to a voltage?

Also please let me know if you have any specific web sites for china engineering.

Thanks again.

[picante]

That line meant to say...

I have heard of the terms excitation and isolation thrown around in the context of data acquisition, I’m not sure if these are relevant to what I’m wanting to do

[johnsold]

I like that - "agitation" as equivalent to "excitation."  Translation can be a wondrous thing.  I have certainly seen some sensors which were quite agitated after being excited, especially when the excitation voltage was too large.

In measurement systems excitation refers to the voltage or current which is supplied to a passive sensor.  To measure the temperature using RTD sensors requires applying (exciting) the sensor with a known current and measuring the voltage across it.  Or voltage can be applied to a bridge configuration where the sensor is one leg of the bridge and the others are fixed resistor.

Isolation can mean different things in different contexts. For temperature sensors it typically refers to a sensor which by design and construction or by mounting method has no electrical connection between the sensor and the object being sensed. For example your submerged sensor should be isolated so that the conductivity of the water does not affect the measurement of the temperature.

The isolation also affects how quickly the sensor can respond to changes in the temperature of the sensed object because most materials which are good electrical insulators are also thermal insulators.

Getting an accuracy of 0.1 degree C is NOT simple or inexpensive. Look at the limits of error for thermocouples or permissible deviations for RTDs. None of the standard products has limits as low as 0.1 degree. Similarly semiconductor sensors have errors in degrees, not small fractions of a degree.  Resolution of 0.1 degree is simple, accuracy is not.

Finding a submersible force transducer will require some care (and expense) also. Do you need to account for buoyancy effects?

Lynn

[Henrik_Volkers]

Sorry, DMS is a german arconym for  Dehnungsmessstreifen -> strain gauge

And Lynn is so rigth, resolution is not accuracy !! That's why I mentioned the calibration.

However you can reduce the uncertaincy with a visit at a pharmancy  🙂    Buy a clinical thermometer. They are (at least in europe) calibrated for an accuracy of 0.1 °C in a range of 35°C to 42°C and give a good reference.

[picante]

You guys are great thank you both,

The most important thing for me to meassure is change in termperature, so I now understand that I need a resolution of 0.1 of a degree ( with a ~+-0.3 degree would be acceptable).

I think that I will leave the force meassurament to one side for now, but if I may, can I ask some more newbie questions, I'm still not sure what to aim for on a data aquisition system.

Because there are so many variables, maybe I'll start with another newbie question, so it almost lokos like it might be beter to find the sensor then to look at the aquisition system.

So what do I look for on a data aquisition system to make sure that it will meassure what I need it to meassure, for example in the future I might want to meassure flow rate of fluids (liters per minute, or total volume) I've have found a sensor that can meassure the flow rate, there is one in the link below, from Lake monitors, it outputs a frequency in Hz, if I just wanted to just buy a simple USB device like the ni-6008 in the link below, would that be a suitable device?

NI 6008

http://www.ni.com/products/usb-6008/

Lake monitors

http://www.lakemonitors.com/documents/LakeFlowStatManualLFSM-905.pdf

[johnsold]

For any measurement system you will likely have interaction and possbily conflict between the sensors and the DAQ devices (and cost). It is important to get the specifications clarified early in the process.  That is in part what Henrik and I have been trying to do with our questions.

By recognizing that your requirement was for 0.1 degree resolution rather than accuracy, you have allowed a much wider choice of sensors and DAQ devices.

While it is useful to focus on one measurement at a time while getting started or evaluating sensors, you do need to keep in mind the total scope of the project (like your future need to measure flow rate).

To identify a suitable DAQ device you need to have some basic specifications on what you need to measure: For temperature you have specifications on temperature range (10-70 C), resolution (0.1 degree), sampling rate (1/3 second), and number of channels (4).  Since most DAQ devices do not measure temperature directly, you need to select a sensor (and signal conditioning circuits or equipment) which can meet the first three specifications and determine what kind of voltages will be produced (both range and resolution). Then you would look for a DAQ device which meets those requirements.  Of course this works the other way as well. If you know that many inexpensive DAQ devices have +/-10 V input ranges and 12 to 16 bit resolution, you can select sensors and signal conditioners with that range in mind.

The Lake monitor data sheet indicates that the frequency range is from ~0 to 50 Hz. Two things make the USB-6008 counter a poor choice: 1. It has only one counter and that is set up as an event counter. A frequency counter really requires two counters, one to establish a timebase and the second to count the unknown frequency. 2. At low frequencies it is often better to measure period and calculate the frequency as 1/period. The USB-6008 is not set up to measure period. Regardless of the device you need to pay special attention to how you make a measurement when the flow rate is zero or very low.  How long do you wait for a pulse ?

Lynn