coming off of this sensor and that is a winding underneath the stainless steel sheath; that’s a platinum winding. And the circuit heats that and what happens is the other sensor, right here, is measuring the gas temperature. So as you see these are RTDs, resistance temperature detectors, they essentially are so stable they don’t drift the NIST uses platinum for their resistance standard. So they don’t drift. So, one of the one of the RTDs is self-heated, and that’s your flow sensor, that’s the longer one, and I can actually hand out some sensors if you haven’t seen it and the other RTD is your reference sensor. Let me just take a moment and pass this around. I’m going to hand out and pass this around, this is the sensor, and this is the circuit that drives the sensor (excuse me I’m just going to hand this over to this side). And this is the board that the circuitry is on, and that’s that called the SAB, and that has the heart of our technology, and that’s the magic. So, what we’re doing with that circuit, that I’m passing around, is we’re maintaining a constant temperature between the heated sensor and the reference sensor. So that’s what happens when it shifts The heated sensor is a small temperature about the reference sensor and when molecules start flowing in a pipe or in a duct, if your probe is in there, the heated sensor cools down and within one second that cooling due the energy carried away, it gets heated up by the circuit. The sensor is being cooled by the energy carried away from the molecules and that energy needs to be replaced so within one second, the circuit is imbalanced, it know something’s wrong and pumps more current out and replenishes the energy and heats that sensor back up The flow gets higher, cools it down again Heats it up further, cool it down again, I’m sorry it doesn’t heat it up
further and heats it back up meanwhile the output of the meter is the energy required to heat this up in milliwatts. So the more flow rate, the more heat carried away, the higher the milliwatt output of the meter. Everybody get that? Now in a perfect world one sensor would be enough. But the temperature of the gas is changing also. So how does it know the temperature is changing? I didn’t know but everything that’s the other sensor — that’s your reference sensor so let’s take a very simple example, an example where there’s constant mass flow controlled by some controller upstream of a 1000 pound an hour. Somebody has put in some system and always guarantee 1000 pounds per hour an hour, but throughout the day, that 1000 pounds per hour, which is kept at a thousand pounds per hour that … air, let’s call it air, is getting warmer ,and warmer, and warmer. It’s still a thousand pounds an hour but the other sensor drives it up, and if it gets cooler during the day (or night) it drives it down. There will not be a change in the output. You don’t know anything but that it’s a thousand pound an hour. The circuit is taken care of its job to correct for temperature changes at constant mass flow. Okay? So we have a reference sensor correcting for temperature changes and we have a heated sensor measuring the flow rate. (inaudible) Well the other one is nothing more than a thermometer Paul, it’s just measuring the temperature of the gas. The reference sensor is just measuring the temperature of the gas and telling the other senor to drive up or down. Now, there is … well what’s your question? (inaudible) Oh, it’s about 20 degrees roughly it could be less it could be more and I’ll tell you why. When it shifts it depends on your application as to whether it’s a low delta T or a high delta T. I’ll talk about that in just a moment. So, the current required to maintain that overheat is your mass flow signal. If we are to work the math, which I’m not going to do, the milliwatts that you see coming out, which is a signal available on all meters, which by the way we linearize so that you have a nice linear display in SCFM or pounds per hour or order twenty that many your that raw signal represents the mass flow signal.