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1216  Forum 2005-2010 (read only) / Interfacing / Re: increase sensor to arduino sensitivity on: November 25, 2010, 01:31:59 pm
Welcome back, I see you managed to source a 250ohm resistor.

You have inadvertantly fallen into the old trap whereby the resolution of your measurement/display unit is an order of magnitude lower than that of your sensor device.

On the basis that the arduino ADC has 1023 points of resolution and you want to scale for a detector ranged 0-10000 it follows that the best resolution you can display from your 10,000 point input is 10000/1023 or just under 10 measured points per arduino point.

However the matter is even worse due to your 4ma / 1 volt zero elevation which equates to 0 input so you lose the first 206 arduino points.  In effect therefore your true resolution is 10000/817 or 12.24 measured points per arduino point.

However, you have to be cogniscant of the fact that 1 part in 817 is 0.122%.  There are few bog standard sensor devices that can guarantee accuracy or lineraity better than 0.5%  so you really should appreciate that the arduino is doing a good job for you - even if you think that a resolution of 12 parts in 10000 is poor.

As Mike says, you could use an opamp with a gain of 1.25 and biased to remove the 1 volt elevation. This will give you an output of 0 to 5 volts for the 1 to 5 volts input.  However the limiting factor is still the 1023 point resolution of the arduino.

Smile and accept the limitations of the digital display.  Also bear in mind that, if you were using an analogue display 1 part in 817 would be less than the width of half a gnats hair, something that would be impossible to see.   One of the problems with the digital age is that we think resolving to the Nth degree gives us better information - nothing could be further from the truth.

Kind regards
1217  Forum 2005-2010 (read only) / Interfacing / Re: Arduino LED Voltage Meter on: November 21, 2010, 04:59:17 am
That really is looking at solving a problem in an expensive way.

If all you want to see is a certain voltage being reached then use a simple zener diode, resistor and LED.

If you want to see a varying voltage then use an LM3914 circuit.  See the attached for full details


1218  Forum 2005-2010 (read only) / Interfacing / Re: Automated fluid dropper? on: November 21, 2010, 05:44:21 pm
How fast do you want the droplets to drop  ie how many per minute and what degree of precision do you want on ensuring 100% drop count.   Bear in mind that drop size and speed will be influenced by water temperature since it is surface tension that creates drops.

One idea you could use is to have a wick dippled into your supply and this will create a constant stream of droplets (determined by the wick permeability) then under the wick you have a shutter/diverter mechanism controlled by a servo motor.  Drops demanded, servo rotates to show aperture, no drops demanded, servo rotates to block aperture and reroute droplets to a secondary reservoir.

Main problem with that would be synchronising drops to shutter.  You'll loose some and cut some.  However you could create a drop sensor such that the shutter could be synchronised to drops about to fall.

And as in all previous requestor requests, please tell us what you are trying to do - this may influence the use of brain cells

1219  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 13, 2010, 07:10:59 am
Congratulations, it seems your controller supply organisation is very customer orientated.

As to resistor tolerance :-

Basically you buy a precision resistor, typically 0.1% for use with 4-20 measurements.

Unfortunately you will NOT find the likes of that in Radio Shack or Tandy.  However they might stock 1% resistors, which is many orders better than what you are using.

I suggest you try and  "borrow" a 0.1% resistor from a colleague who works in an industry where they are universally available, typically the chemical or manufacturing industry. The value they normally use is 250 ohms, to produce the desired 1 to 5 volts

I previously mentioned about protecting the arduino input against going overscale, hence my suggestion about fitting a lower value resistor.  However, from reading elsewhere, the arduino has an intermal protection diode which safeguards the system providing the input current is kept under 1ma.   I therefore suggest that you instal a resistor in the +ve line that connects to the input sense point on the arduino.   Assuming you are protecting against say 10 volts, then this resistor should be in the order of 10kohms.   This shouldn't affect normal measurement calibration since the current demanded by the input port is negligable until the input voltage exceeds the system tolerance level, after which it's a "don't care" situation as you are outwith sensible measurement capability.

1220  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 05, 2010, 03:27:35 pm
Here's a set of hook-ups for your next set of tests

1221  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 05, 2010, 05:31:23 am
Hope I make it before the next 'quake hits.

Keep me informed on how you get on and let's get all the wiring sorted out before you apply power !

Use your new meter as a reference standard to do some calibration checks of your existing ones.  And before the purists state that a cheap multimeter is not a true standard, it makes not one jot for hobby purposes.  At the end of the day measurement is a relative or comparative business.  As long as you have a defined working standard to work against you know how your other meters fair.


1222  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 05, 2010, 04:57:45 am
Congratulations, you're getting there

That's the good news, now the bad :

Because the meter is showing only 0.42ma it is quite clear that your controller's 4-20 output stage is damaged.  The minimum reading should be 4ma.

Suggest you try sweet-talking the manufacturer into changing it.

Just out of idle curiousity, what exactly are you trying to measure - or did you mention that several pages ago.  This topic is now on its 5th page so we should soon be getting an endurance award from the administrator.

1223  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 05, 2010, 02:58:03 am
Oh dear, oh dear
When measuring current you are measuring that which flows THROUGH the resistor.  So you have to put the meter in series with it, not in parallel as your drawing shows.  Without wishing to offend, you really need to think about what you are endeavouring to do before you blindly jump in.

The calibration sheet makes perfect sense and apart from a new meter you do not need any new resistors.  Simple maths will suit your needs, and you need to allow for going over 5 volts to protect the arduino, as previously mentioned

1224  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 04, 2010, 05:26:05 pm
Still more wrong things.  

According to your previous readings 3771ppm with 470ohms gives 0.94 volts.  That equates to a current of around 0.002 amps or 2ma.

But now you say the current reads around 4ma for slightly less ppm

I take it you have the meter in series with the resistor and not across it when you are measuring the current.

If so then measure the current again and calculate the voltage developed across the resistor using the measured current in accordance with V=IxR. Then measure the voltage across the resistor.  The two values should correspond .  If they don't then your meter has a fault.   Can you compare it with someone else's that they know is reasonably well calibrated.

Unless you can trust your measuring devices, you are fighting a losing battle.

1225  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 04, 2010, 03:53:04 pm
Does your test meter have three sockets on it.  One with a label saying "A" or "AMPS".  If so the red test lead should go into this when measuring current.   Note that the red lead must NEVER be in the "A" socket if you are measuring volts.

If there is an "A" socket and the meter doesn't read any current then the internal fuse may have blown (by someone trying to measure volts with the lead in the "A" socket)

1226  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 04, 2010, 03:36:11 pm
I like your last line  - true engineering philosophy

or to put it another way  "those who have never made a mistake  haven't done anything"

Out of interest, you should get the same voltage readings with the arduino disconnected - ie it should put "no" load on the controller.

Have you set your meter to ma range and looked at the current flowing through the resistor.  As previously said the minimum should be 4ma.  Note, with the sensor disconnected the output may fall to a predetermined "failed sensor" value, typically around 2ma, (called fail downscale) on the other hand it may fail upscale and set the output to 22ma.

Looking at the papework you've supplied it appears that the controller is a single-term device ie "proportional" only.  Most controllers are generally three-term, or PID, comprising proportional, integral and deriviative functions.  

If by any chance yours is a two or three-term device, both the intergral and derivative will have an unexpected effect upon the output (that's "unexpected" if you don't know about them).   If it is, let us know and we'll try and guide you on setting them up.

1227  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 04, 2010, 08:52:17 am
Forget about the 0ppm problem, that's the least of your worries.

There is something seriously wrong with your data.

I take it you understand that the controller output range is a minimum of 4 ma and a maximum of 20ma.  With that in mind, and using ohms law calculation of V=IxR then the minimum voltages you should be getting across your resistors (with a 0ppm output) are as follows

4ma @ 180ohm  = 0.72volts
           220ohm  = 0.88volts
           470ohm  = 1.88volts
           510ohm  = 2.04volts

Note that the 510ohm is outwith the specification of the manufacturer so may be ignored.  I presume you just tried it to "see what if..."

You state that the maximum voltage you are getting is only 0.94volts with the 470ohm resistor.

I put it to you therefore that either you have it incorrectly wired, you are misreading your meter (or it's faulty), or your controller is damaged.  I suggest the latter since your attempts at creating cold fusion almost worked (the trial by fire)
1228  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 03, 2010, 10:42:55 am
I suppose we all have to start somewhere, and for me I've got over 50 years of self-electrocution under my belt.

I learned in the days when you were permitted to kill yourself by experimentation without being charged for doing so, providing you didn't take anyone else with you.

1229  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 02, 2010, 06:46:25 pm
Is this HM unit originating from China or some other non-English speaking nation, because I feel their comments are losing something in translation.  Their quote of "maximum supply voltage" suggests an external source drive voltage.  And their instruction sheet - which is quite poor - suggests an external source voltage.  However your last hook-up clearly shows the system operation from an internal supply.

If the latter is the case, they are really saying that the internal supply can drive 20 ma into a maximum resistance of 500 ohms.  In which case the PD developed will be 10 volts.   They specify no minimal resistance which is not unusual as half decent 4-20 transmitters should be able to feed into a dead short.  So don't worry, the use of a 180ohm resistor will be OK.

1230  Forum 2005-2010 (read only) / Interfacing / Re: 4-20mA analog input to serial out on: November 02, 2010, 04:05:36 pm
Nice one Mike, Brit diplomacy is alive and kicking

No protection for the arduino/uno input has been discussed yet so here goes :
A typical 4-20ma current source is capable of going over and under range, typically as far as 24ma before the transducer saturates.  So it  might be better if the resistor is reduced to no more than 208 ohms (5volts/24ma).  
The nearest readily available lower value is 180ohms If that value is used the new measured voltage range for a 4-20ma source will be ;

  4ma = 0.72 volts input
20ma = 3.60 volts input

You still need to consider that the ADC count is 0v = 0 and 5v =1023 or 204.6 / volt  so your ADC range will be  147 to 737


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