I have a Vaisala HUMICAP® Humidity and Temperature Probe HMP155 that i connect to CR1000 in mode single ended...the probe provides me values of temperature that in the range of the first decimal it is not stable in the measure (the measure ranges too much).Verifying with a digital multimeter, it is estable, but not in cr1000.
Which can be the reason of this oscillation?
Thanks.
Two possibilities:
a) you are trying to make a fast integration measurement which being fast will pickup noise on the sensor output (which most multimeters will not see). Try the 50 or 60 Hz integration times and try the default settling time or even a longer settling time as some Vaisala sensors are sensitive to switched loads (as happens when the logger switches the sensor in to be measured).
b) you are not allowing enough power up time, if you are switching the power on and off. You need ot allow at least 2 seconds for that sensor.
Thank you FOR your recommendations!
I made this program:
'CR1000
'Created by Short Cut (2.8)
'Declare Variables and Units
Public BattV
Public AirTC
Public RH
Units BattV=Volts
Units AirTC=Deg C
Units RH=%
'Define Data Tables
DataTable(Table2,True,-1)
DataInterval(0,0,2,0)
Minimum(1,BattV,FP2,False,False)
sample (1,AirTC,FP2)
EndTable
'Main Program
BeginProg
Scan(10,Sec,1,0)
'Default Datalogger Battery Voltage measurement BattV
Battery(BattV)
'HMP155 (panel switched power) Temperature & Relative Humidity Sensor measurements AirTC and RH
PortSet(9,1)
Delay(0,5,Sec)
VoltSe(AirTC,1,mV2500,3,0,0,_50Hz,0.1,-40)
VoltSE(RH,1,mV2500,4,0,0,_50Hz,0.1,0)
CallTable(Table2)
PortSet(9,0)
If RH>100 AND RH<108 Then RH=100
'Call Data Tables and Store Data
NextScan
EndProg
with this program I obtain a mesasure more stable of the HMP155.
APS, Julio
We have observed the same "problem" with the HMP155.
The initial measurement of humidity was as follows:
(HMP155A model with 0.0 to 1.0 VDC output, 0-100 % humidity)
Every 10 minutes the CR1000 program would
- turn on HMP155 for one minute (switched 12V)
- take measurement (250 micro second sample)at end of one minute.
- turn off HMP155A
we observed also an oscillation in the measured humidity that when plotted appeared as a modulated sawtooth pattern.
There were periods where this did not occur.
We then used a Beckman model 310B digital voltmeter to monitor the output of the HMP155 during the one minute period it was powered up by the CR1000.
The output appeared stable and the returned value by the CR1000 agreed to within 0.1 VDC of the voltage measured by the 310B meter.
We continued to monitor the voltage
Time 310B measurement CR1000 returned value
1:20 pm .373 .374
1:30 .364 .364
1:40 .393 .392
1:50 .371 .373
2:00 .394 .391
2:10 .393 .396
We the switched to the 60Hz measurement mode
2:30 .386 .385
2:40 .401 .405
2:50 .376 .381
3:00 .409 .409
3:10 .372 .373
3:20 .398 .397
so switching to the 60Hz mode did not improve the results.
We have concluded that the CR1000 is returning a valid voltage and is what we observe with 310B meter.
When the HMP155 was powered continuously we also observed some variations.
We conclude there is an additional issue here. When the HMP155 is powered up each time there results the saw tooth pattern in voltage outputs.
As an additional note, we have observed the same thing with the Vaisala PTB110 (CS106) barometer.
Our investigations will continue and we will post results on our web site so that interested parties can comment.
I will be notifying Vaisala of these observations as well.
George Weidner
Antarctic Automatic Weather Station Program
Department of Atmospheric and Oceanic Sciences
University of Wisconsin - Madison
Vaisala are aware of high frequency noise problems with the analogue outputs of the HMP155. My understanding is they are changing the design to reduce the problem. I am not sure if that change is in production or not.
With an older probe if you sample the outputs with a short integration in the logger you will definitely see noisy readings. If you use a longer integration the problem is reduced but may not be removed entirely.
The jumps in the readings you are seeing would seem to be a different problem. Unless the noise is worse in your probe and the apparent sawtooth is a result of sampling repetitive noise. If possible I would look at the outputs of the probe with an oscilloscope.
BTW when you said:
"The output appeared stable and the returned value by the CR1000 agreed to within 0.1 VDC of the voltage measured by the 310B meter."
Do you not mean 0.001 VDC?
aps,
Yes, the correct value was 0.001 VDC.
More recent investigations with a HMP155D (with passive PRT and active 0.0 to 1.0 VDC humidity output) we found that by placing a resistor across the humidity output, one obtained a reduction in the amplitude of the oscillations. The amplitude of the observed "noise" was only twice (versus up to 10 times for the HMP155A probe) that observed with an older HMP45D probe.
However, there was some offset (a lower voltage) of approximately -0.02 VDC with a 1.0 Kohm resister. When we then tried a 10Kohm resister the offset was reduced to less than -0.01VDC and with a 1.0 Mohm resister the offset was negligible and the amplitude of the noise reduction was still the same as the 1Kohm according to the CR1000 data.
This really speaks to the very high input impedance of the input channels of the CR1000.
Our connections were still for a SE measurement with the analog ground of the HMP155D to the analog ground next to the SE channel used to measure the output of the HMP155D.
The response from Vaisala mentioned that they placed a PTC (positive temperature coefficient) current-limiting resistor in the power ground line of the "older models". I assume they refer to HMP45's as the older models and they no longer place PTC resistors in the HMP155's.
They also recommend that we use a differential measurement.
they also noted that a 3-wire measurement would be less noisy but that a 4-wire connection is preferred. It was suggested that the "digital" HMP155 had a more variable current than the "all analog" HMP45.
Regards
George Weidner
George said:
The response from Vaisala mentioned that they placed a PTC (positive temperature coefficient) current-limiting resistor in the power ground line of the "older models". I assume they refer to HMP45's as the older models and they no longer place PTC resistors in the HMP155's.
APS:
I would go back to Vaisala and double check this. My understanding is that initial problems with the HMP155 with noise were found to be due to a component in the ground rail which they subsequently removed.
So "older models" may actually mean earlier versions of the HMP155.
Using a differential measurement is likely to reduce the noise and spurious offsets if you have that component fitted.
aps,
I contacted Vaisala to find out when they removed the PTC on the ground. Sensors manufactured after 28th Oct, 2009 (serial numbers after E4430000) should not have this noise problem on the analog outputs for temperature and RH.