There are two RH Sensors connecting to the CR1000. One of them connects to G,12V and H1 channel but the other RH is connecting to VX1, L2, H2, G, and H3 while a resistor connecting L3 to H3.
I am wondering what is the reason for using BrHalf4W and why there is such a wiring system? and when we use this system?
Thanks
The advantage of the BrHalf4W circuit is that lead resistance does not affect the measurement. The disadvantage is that it requires two differential (4 SE) voltage input channels and 4 wires to the sensor.
Some sensors have 4 wires and are sensitive enough that the lead resistance would cause too great an error.
The BrHalf2W does not compensate for lead resistance.
The BrHalf3W lead compensation assumes that the leads are of the same resistance.
See the help in the CRBasic editor for more info.
With that said, the other RH sensor would use a voltage (VoltSE) measurement rather than a bridge measurement. Lead resistance for this sensor probably does not affect (or has very little affect) the measurement result.
Bottom line, some sensors require a bridge (ratiometric)measurement, while others require a voltage.
We need more details about the sensors in use, but one explanation for what you report is the sensor is a Temperature and Humidity probe that has an unconditioned platinum resistance thermometer for its temperature measurement.
This would usually be measured with a 4 wire half bridge.
Most T&RH sensor manufacturers often this option for their meteorological probes although Campbell Scientific has not generally sold probes with that configuration.
Thanks!
I did understand that sometimes a sensor(Like RH) is used for both Temperature and the humidity measurement. This link is an example of two RH sensor with different wiring.
http://img25.mediafire.com/817705aea7d7670307ffb1ad0b708ace69d00078db54c4522f3e1a2b2b466da55g.jpg
Can you in a simple way explain;
1- why we use the resistor and how we choose the resistor value?
2- why we use VX1 and why the H channel is connected to the L with a resistor?
3- which L and H in the blue box (Image) are for the temperature ?
I greatly appreciate your help
That link does not work for me - error 404, not found.
Here is the question and the link again. The link is working pretty fine, If it wont work again please let me know.
I did understand that sometimes a sensor(Like RH) is used for both Temperature and the humidity measurement. This link is an example of two RH sensor with different wiring.
http://img25.mediafire.com/817705aea7d7670307ffb1ad0b708ace69d00078db54c4522f3e1a2b2b466da55g.jpg
Can you in a simple way explain;
1- why we use the resistor and how we choose the resistor value?
2- why we use VX1 and why the H channel is connected to the L with a resistor?
3- which L and H in the blue box (Image) are for the temperature ?
I greatly appreciate your help
1. the resistor is used for a reference measurement to compare against the measurement from the sensor. The value can be chosen as being equal to the midpoint resistance of the range expected from the sensor.
2. VX1 is used to provide power to the sensor. H & L are connected to ground via the resistor for the reference measurement described above.
3. from the diagram, it is too difficult to work out which wire is for what purpose, but the way it is wired, it looks like a 4 wire half bridge (BrHalf4w programming instruction) if that is the case, neither would be for the temperature.
Cheers
More details are needed about the sensor(s) involved, as the labelling is confusing.
As I mentioned before this could still be the temperature measurement from temperature and humidity probe, that has an unconditioned PT100 sensor built into it for temperature measurement. All major humidity sensor manufacturers produce probes with this configuration although Campbell Scientific do not normally sell them.
It could even be a wet bulb temperature sensor, which some still use for RH measurements. However if that were the case you would expect a dry bulb sensor too.
