Hi. I’m Bill Griffith, an applications engineer from Agilent Technologies, and today we’re
going to talk about making humidity measurements. I have several devices here today to measure
humidity. Let’s start with the German weather house. The way the German weather house works
is on a wet day, the male will come out of the house and the female will retreat back
inside, whereas on a dry day, the female will come out of the house and the male will stay
inside the house. The way an antique weather house worked is they actually used a human
hair and so as the humidity soaked into the hair, the hair would actually stretch, allowing
the male to come out of the house, and on a dry day it would contract, allowing the
female to come out of the house. Another example of a mechanical humidity sensor is this weather dial.
It contains a barometer as well as a thermometer and a humidity sensor known as
a hygrometer. It’s a mechanical device that changes. It’s a spring that gets longer
with the amount of humidity applied to it and then as the humidity dries out, then the
spring shrinks and the dial moves backwards. So, several examples of the electronic hygrometer.
These are three pin devices. They work very well in the type of conditions you’d have
in a home or office environment. The way it works is you apply five volts to the first
two pins and then the third pin gives you a voltage that is based on the relative humidity.
Here’s another example of one of these three pin devices. I like this one in that you can
see the resistive device that changes resistance based on humidity and then on the back of it,
you can actually see the circuit that’s used to create the proportional voltage.
The final example I have is attached to the back of the 34970. It comes in a nice case and
it also contains a thermistor so it can measure temperature along with humidity. This is a
very accurate sensor. It has two percent accuracy over a broad range of temperatures and humidity.
So let’s go ahead and make some measurements. The first thing we want to do is we want to
set up our first channel which we connected our thermistor to. So I’m going to go ahead
and configure that channel. The 34972 uses universal channels, so you can hook up your
DUT or sensor to any channel, and then you get to select the type of voltage or resistance
that you have connected to it. I have connected again a temperature sensor so I’m going
to select temperature and then it will ask us the type of sensor we have connected, thermocouples,
RTD, or thermistors. I’m using a thermistor so I’ll select thermistor and it’s a 10K ohm
thermistor so I’ll go ahead and select 10K ohms. We’ll measure in degrees C with
0.1 degree resolution. Okay, our second channel is just measuring the five-volt supply,
so I’m going to go ahead and just stick with the defaults on that channel. On Channel 3,
we’re again going to measure the voltage that’s proportional to the humidity, so again
I can stick with the defaults, but I’m going to add in some scaling. So from
the data sheet, I have the scale factors so I’m going to set the gain to 37.5.
I’m also going to set an offset. Again from the data sheet, the offset I need to set is a -37.5.
Lastly, I want to set the units. Instead of being DC volts, I want to set a unit value
that makes more sense for the measurement that we’re making. Since it’s humidity,
I’ve selected HUM as our label for this channel. So as you can see right now, we have
53.6 degrees – or excuse me, 53.6 percent humidity in the room. So that’s an example
of how we can set up from the front panel. We can also set up a scan. So we first started
off with setting the scan interval so right now I have it set for a five-second interval.
One trick we can do is we can set up the number of scans you want to make or if you go past
zero, you can go into the continuous mode and so it will continue to scan until you
stop it. We could also set up alarms. We’re not going to do limit testing for this particular
example. Let’s go ahead and start our scan. So right now every five seconds – you may
be able to hear the relays – it will go off and do a scan of the temperature in the
room and of the voltage that’s being applied to the sensor and also reading back the voltage
and scaling it. Each of these readings is time stamped and being stored in non-volatile
memory inside the unit. And again, we can go ahead and stop it anytime, so I’ll go
ahead and hold this to stop so the scan is stopped and the data is being stored in the instrument.
So that’s an example of data logging humidity from the front panel of a 34972.
I thought we’d go ahead and move along and take a look at making the same measurements
using the free BenchLink Data Logger software that’s included with the product. So I’ve
gone ahead and installed the software. We’re connected with a USB cable and the first thing
we want to do is find the instrument. I’ve done that. And then the next thing is to go
ahead and configure the channels just as we did from the front panel. The channels, again,
are universal, so on the first channel, I get to select what type of sensor we have
connected and again, I have the 10K thermistor. So I’m going to go ahead and select that
from the pull down menu. Again, we’ll measure in degrees C, so this channel is configured.
Again with Channel 2, I’m measuring the five volts so I’m just going to go ahead
and stick with the defaults. So I’ll turn on that channel. And then with the third channel,
again it’s a DC voltage so I’m good with the defaults but I do want to add in the scale
factors, so I’ll go ahead and enable the scale factors. And if I wanted to, again,
we could set high and low limits or just a high or low limit, so these are the limits
and the alarms that we can set along with our measurements. Okay, moving along we’re
going to go ahead and set up our scan interval, so again, we can select the amount of time.
I have it set for five seconds. I can again select the number of scans I want to do or
I could just ask it to be continuous until we stop. So with our scan set to eight,
we’ll go ahead and start our scan and we’re going to overwrite the data that’s currently
in the 34972 and we’ll go ahead and look at this as a graph. So as the data is taken,
it’s coming back to the PC and being plotted so that we can verify the data in real time
and make sure that the data as it’s being scanned makes sense.
So after about eight scans, we will complete our graph.
Okay, so that completed our scans. So as you can see,
it’s very easy to set up the 34972 either from the front panel or using the free BenchLink
Data Logger software to make humidity measurements.