Neither do I, and obviously
@Rajinder doesn't either.
Thanks for your help.
I have tried ti contact the manufacturer for some more information and an application note/cinditioning circuit example/help.
So will see if they come back with anything.
I will keep everyone, who have helped updated to what they come back with.
So, the first thing I would do is beg, borrow, or <gasp> buy one to try it out on a breadboard setup. It is especially important to get the pulsed application of V
H and V
C performed properly. The datasheet offers a pretty good starting point on what you need to supply V
H followed by V
C power pulses.
Note that the tin-oxide sensor bead is both thermally and electrically connected to the low-resistance (typically only 1.8 Ω) heater element. The V
C power is applied
after V
H power is removed because of this common connection. You don't want the voltage drop in the heater element to interfere with the resistance measurement of the tin-oxide bead, but you can't wait too long to measure that resistance using V
C because the sensor will begin cooling as soon as power to the heater is removed. How fast it cools depends on factors you have little or no control over: gas flow through the vent holes in the ceramic carrier and thermal conductivity of the ceramic carrier from sensor bead to ambient temperature. The fact that the sensor resistance has a logarithmic response to gas concentration probably also means its response to temperature is probably highly non-linear as well.
Oh, well, just having the gas-concentration sensing capability in a non-destructive, low-power, sensor element is probably worth any extra effort required to utilize it. Does anyone here remember when Bosch oxygen sensors had to be heated to exhaust manifold temperatures before they started to work? This meant a rather long delay after starting the engine before a stoichiometric fuel mixture could be provided. Bosch later added resistance heaters to their line of oxygen sensors so they would begin to provide good data long before the ICE (Internal Combustion Engine), whose fuel/air mixture the sensors were monitoring, reached operating temperature.
This little Japanese sensor appears to respond rather quickly, albeit with a lengthy duty cycle of 0.06 seconds on, followed by 1.94 seconds off, or a minimum cycle time between data readings of 2.00 seconds, or a maximum of 30 measurements per minute. The datasheet also says that this heating/cooling regimen be followed for at least 48 hours of "preheating" before the data is reliable under the manufacturer's test conditions. I think this is probably a CYA statement, but it wouldn't hurt to provide a "warm up" period before making measurements. You can record the data to spot any drift attributable to temperature changes in the ceramic carrier before deciding whether this is something you have to worry about.
Based on information you gather from inspection of the datasheet, you need to specify the
range of gas concentrations and the
resolution to which you want to measure those concentrations. This is an analog sensor, so the output varies in a continuous fashion from about 10 kΩ in clean air to about 1000 Ω at a concentration of about 40 ppm for acetone or ethanol vapors. Other ratios for other gasses. See Figure 4, Sensitivity Characteristics, in the datasheet. This means the analog voltage you measure during the V
C pulse is going to be highly non-linear as a function of gas concentration. Also, resolution is not the same as accuracy, but unless I missed it, I did not see an accuracy specification listed anywhere on the datasheet. I would design for at least 1% resolution of the raw sensor data measurement, V
S, over a sensor resistance, R
S, range of 200 Ω to 10,000 Ω. Keep the load resistance, R
L, as high as possible but always greater than 200 Ω.
Since your data acquisition rate is low, you should look for an embedded sigma-delta analog-to-digital converter with 22 to 24 bits of resolution and a full-scale input voltage of between one and ten volts. That should allow you to directly connect to, and digitize, the V
S output signal. After digitization, use floating-point arithmetic to remove the DC offset and to scale the output to the maximum and minimum concentrations you want to measure. What you do with the floating-point results is up to you... maybe pass them along to a threshold algorithm that lights up green, yellow, and red LEDs, color depending on gas concentration. Or maybe just sound a buzzer alarm.
