Prosser Instruments anenometer: info required

[John_BS]

I've just obtained an anemometer which appears to work on the "hot bead" principle. The type number is AVM 500, and I have no information whatsoever. It is powered from two internal batteries with PP3-size connectors, and judging from the dimensions of the clamps, they would have been PP6's.

Can anyone help me with
1. handbook, or schematic etc and / or
2. where to obtain PP6's, or is it a question of building something which will fit?

Many thanks

John

where to obtain PP6's

Modern alkaline PP3s would be up to the job.

[John_BS]

I've now managed to trace the circuit, and it does appear to work. It can be powered from PP3's (thanks merlinmaxwell) : the one supplying the +ve HT is loaded by about 12mA quiescent, rising to 20mA or so when measuring high wind velocities.

I might try to modify the circuit to allow it to run on just one PP3, as the -ve rail is only used to power the -ve supply of the 741. Everything else is hung between the +9 and 0v.

If anyone is interested in how it works, I'll make a tidier copy of the schematic and post it.

Does anyone know anything about Prosser Scientific Instruments? They were based in Hadleigh, Suffolk.

John

[yesnaby]

http://books.google.co.uk/books?id=Q...dleigh&f=false

[Alistair D]

I assume many of the members of the forum are similar to me in that the schematic of an instrument they have never come across before is always interesting. Maybe it is just adding another little bit of knowledge to my portfolio. For me please post a copy of the circuit if you have the time.

Al

[John_BS]

Al:

OK: and yes, it is interesting! In this instance, you'll need to puzzle out why the designer decided to stick a 47nF capacitor from the Offset Null 1 terminal of a 741 to the +ve supply!

I was also a bit worried to find the OP-amp output connected to the +ve input with a 56ohm resistor......

yesnaby: thanks for the link: fascinating. I wonder when they ceased trading.

John

[Alistair D]

"I was also a bit worried to find the OP-amp output connected to the +ve input with a 56ohm resistor"
That sounds more like potential oscillator than an amplifier to me.

Al

[John_BS]

Here is some more information on the meter.

NB all the resistors are 5%.

It's based around a bridge, with the bead in one leg, and the op-amp detecting the unbalance and driving the bridge supply in such a way that the resistance of the bead is held constant at about 125 ohms. The bead is very small (see photos), and there is sufficient self-heating ( in still air ) to reduce the resistance from about 1k7 down to the 125 ohm range with just 10mA flowing (12.5mW). With the bridge balanced, the feedback voltage from the BC 318 emitter is 1.80v, and this voltage is similarly set up with the 10k pot to zero the meter.

There is mention in the New Scientist article (which yesnaby found ) to a resistance in the "flex" of each probe, so the probes become interchangeable between meter bodies. The probe is wired with what looks like screened twisted pair, but there are 4 connections wired to the meter socket. Inside the probe DIN plug there's a 745ohm resistor, which is connected in series with the meter, and hence adjusts the sensitivity of the entire device. There is a 390ohm resistor wired across the 1mA meter connections, but no provision on the PCB, which suggests that this is an AOT device (so the meter is now 1.06mA fsd).

I'm contemplating swapping out the 741 for a more recent op-amp which includes 0v input terminal capability, and hence would avoid the need for a second battery.

John

[PaulHarman]

Hi

I am a bit late coming to this thread but I thought I might add a bit of info anyway.

I I worked for Prosser in the mid '80s and the AVM500 was a legacy instrument then, it had been superseded by the AVM501TC which had a second thermistor for temperature compensation, and there was the low flow rate AVM502 as well.

I was a little shocked when I saw the design with the pair of PP6 batteries providing 18V split rail and the 741 op-amp which was pretty old hat by then, but any discussion of improving it was thwarted by the fact that these instruments sold very well and the price people paid for them meant that they were not too worried about the cost of the expensive batteries.

That said for new instruments steps were taken to improve things. The 741 was replaced with a TL071 to remove the need for a negative supply (you can just drop one in and lose the negative battery!). There were rechargeable versions sold which used a 12V Ni-Cad pack custom made by Ever Ready where the negative supply was generated with a 3.3V Zener diode to give +9V and -3.3V which is adequate to run the AVM500. In the lab we tended to use PP3 alkaline batteries because they were cheaper, lasted longer and the voltage did not drop gradually. The PP6 is a pretty poor battery for high current devices like this.

While the probes were supposed to be interchangeable, and if you are not too worried about accuracy they are, in practice the AVM50x needs to be calibrated with the probe that is going to be used. You can only do a two point calibration since the scale on the meter is pre-printed and unfortunately the zero point is not a good one to use. These meters are very accurate over a 10-1 range, so they can be good for 3m/s up to 30m/s but not very good between 0m/s and 3m/s. If calibrating set the slope control at 30m/s and then set the zero control at 3m/s and repeat. You cannot get accurate setting at zero because of the power dissipated by the thermistor heating the local area around it.

It was an interesting time. The later instruments which I don't think were widely sold (customers were happy with what they already had and just wanted more!) used the TL071 and an analogue lineariser circuit based on a CA3046 transistor array to use a Ax^2+Bx+C transformation with analogue multipliers all run from 9V to produce an output on a 3.5 digit 7-segment display with an accuracy of 0.1% when newly calibrated over a 10-1 range.

The 'rugged' probe with the ring round it has a few airflow problems that make it a lot less accurate than the unprotected version, and obviously has to be positioned carefully in the direction of flow.

It is not unusual to find capacitors in odd places in Prosser instruments. A lot of what they produced was leading edge in terms of working at high frequency and a lot of the op-amps used had a very high gain/bandwidth value and as a result were very unstable. Quite a few innovative methods were employed to eliminate 'hooting' which to most people would look like noise but when properly analysed was oscillation at very high frequency. Remove these capacitors at your peril!

It is interesting to see that there are some of these still around.

Paul