Hello,
I am trying to hook up a PT100 to the Campbell C1000 datalogger. I know the wire's resistance and prefer to do a 2-wire measurement on the resistance, correcting afterward for the wires resistance. I am using the BrHalf command for this.
I try to do a simple 2-wire bridge measurement using a 120 ohm reference (I understand the resitance needs to be faily equal to the resistance to be measured). However, I do not get measurements out of the sensor, closing or opening the loop gives me a reading of about 0.08 mV.
My first question is how to hook it up: the C1000 manual shows Vx (I hook this one to the excitation channel), V1 (I hook this one to a H channel) and Grnd (Which I hook up to ground). Is that OK?
Thanks,
Henk Witte
Henk,
As you stated the sensor is a variable resistor whose resistance changes with temperature. The PT100 are packaged in a variety of ways.
The most basic package would have two leads, one for each side of the resistive element. CRBasic help for the BrHalf measurement instruction shows a schematic for a half bridge circuit. In this example, the matching/known/120 Ohm resistor is "Rf" and the PT100 is "Rs". With that in mind, one leg of your 120 Ohm resistor should be connected to VX on the logger. The other leg should be connected to one of your single ended channels. One leg of your PT100 should be connected to that same single ended channel. The other leg of the PT100 will be connected to ground.
This configuration is creating a half bridge or voltage divider with one half being a known resistance and the other half a variable resistance. By knowing your voltage drop across both resistors (VX) and measuring the voltage drop across the variable resistor (PT100), you can calculate the resistance of the PT100 and subsequently your temperature.
Sam
* Last updated by: Sam on 10/8/2009 @ 1:56 PM *
Hello Sam,
Thanks for your answer, I have connected the sensor according to the schematic in the help file. Also, I have measured the voltages and I can see the pulses of the excitation channel in the circuit.
However, I do not get good results from the BrHalf statement, in particular:
- Any value other than mV2500 for the range results in NAN (independent of the excitation level supplied)
- The value of X depends on the excitation voltage supplied, as X is the ratio between the supplied volt (Vx) and measured volt (V1) this should not be the case!
- If I change the sensors temperature I can see a change in the voltage going through the circuit as the resitance changes, however I do not see a corresponding change in X
Any further help would be appriciated. The resistance in the sensor + cable is 114 Ohms @ 0 oC.
Henk Witte
Henk,
Could you post your code? Maybe even email me a digital picture or electrical schematic of your setup? Does your code look like the following?
sutley a/t campbellsci d.o\t com
Public VoltRatio As Float
BeginProg
Scan (1,Sec,0,0)
BrHalf(VoltRatio,1,Autorange,1,Vx1,1,2500,True,0,_60Hz,1.0,0)
NextScan
EndProg
Hello,
With the code below I tried quite a fiew different paramters, the schematic is as in the example: a resistor between EX3 and channel 8 (I tried both H and L), channel 8 H or L hooked up to the PT100 and the return from the PT100 to ground. I suspect the resistor may be too small and a too large current is flowing to the input channel, I will try some larger resistors.
The main difference with the code is that I did not define VoltRatio (X in my program) as a Float explicetely.
However I also have to start my measurements, so I have decided to use the thermocouples only and use the PT100 manually to obtain some reference temperatures with a hand-held multimeter.
The code is (as you see I also measure 25 thermocouples on a multiplexer):
Public Tref,PT100,X
Public TC(25)
DataTable (DatSec,1,-1)
DataInterval (0,10,Sec,10)
Sample(1,PTemp,IEEE4)
Sample(1,X,IEEE4)
Sample(1,PT100,IEEE4)
Sample(25,TC(1),IEEE4)
EndTable
BeginProg
Scan (5,Sec,0,0)
BrHalf(X,1,
mV2500 (tried different settings, autorange also)
,8,Vx3,1,
50, (tried different between 50 - 2500)
false (tried true and false)
,100 (tried different settling times)
,250,1.0,0)
PT100=125.2747*ABS(X/(1-X))
AM25T(TC(),25,mV2_5C,1,1,TypeK,Tref,1,2,Vx1,True,0,_50Hz,1.0,0)
CallTable DatSec
NextScan
EndProg
I suspect the resistor may be too small and a too large current is flowing to the input channel, I will try some larger resistors.
I doubt this is the case if the you are using 120 Ohm resistor with a sensor of resistance 114 Ohm. If both legs were equal, and the excitation was 2500mV then the voltage drop across the PT would be approximately 1250mV.
You mention that are using Channel 8 Hi or L?
In the BrHalf instruction you are using Single Ended inputs. So Channel 8 is referring to SE Channel 8, (which is Diff Channel 4). The SE channels number sequence is on the Row marked SE above the Diff channels (I believe that the text color is blue).
I missed the fact of SE numbering in the manual, if I use channel 15 or 16 it works! The logger is built into a housing, so seeing the numbers etc. is difficult (I can just manage to get the screwdriver in if I loosen the frames screws, so that is my excuse)
Thanks very much,
Henk Witte
You would be better off using a higher value resistor (10k) and measuring the voltage across the PRT on a lower range, e.g. 25 mV, both to reduce power consumption but more importantly avoid self heating of the sensor, which could be significant when putting 10 mA through you bridge, albeit for a very short time.
I believe that you meant to say to use the 250 mV range code with a 10k ohm resistor and a 100 ohm PRT when using 5 volts for excitation?
If the CR1000 could output 5V excitation maybe, but it is limited to 2500 mV max. Even at 2500 mV there is a risk of over-ranging with 25 mV, so point taken, but for best measurement resolution (which is limited for half bridge measurements on PT100s) you need to balance the optimum range (normally 25 mV) and excitation voltage used so the output at the maximum temperature you want to measure is at the top of the input range on the logger.
This is discussed in some details in section 4 of this manual:
http://www.campbellsci.com/documents/manuals/3whb.pdf
The excitation voltage versus temperatures given in that table will still apply to a simple half-bridge, although you need to add on an allowance for the cable resistance. The program needed will be a little different to allow for the correction for the cable too.
Thanks very much for all usefull suggestions and comments.
One other question I have: how can I measure easiest a 0-10V signal (this seems to be fairly standard for most sensors)?
I understand the input channels are limited to -5 +5 V?
Henk Witte
We offer a precision divider for this that divides either one differential or two single ended inputs.
See:
http://www.campbellsci.com/vdiv2_1
There is also a 10:1 divider too.