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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
NE Technology Farmer Dosemeter model 2570/1
This is my new NE Technology Farmer Dosemeter model 2570/1 which can automatically compute dosage v. time calculations for betas, X and Gamma, my most advanced instrument yet. I bought it on ebay for £39.99 - about $63 - because no one else bid for it.
It is completely portable, and runs off two standard 1.5V D cells inserted into a holder in the back panel.
The ionization chamber polarising voltage is nominally 250V (251V shown), while the sign of the polarising voltage may be reversed by a switch on the back panel, which also offers switchable voltage division options of V/2, V/4, and V/8.
The great joy of the instrument is that when standard Famer-type chamber sizes are selected - 0.03 cc, 0.6 cc, or 600 cc, the instrument will function as a direct reading dosimeter.
Using the keypad, one can select a dose - say 1 Gy - and press the start button, and the instrument will trip the X-ray source via a back panel port when that dose has been delivered.
Use of free-air ionisation chambers is made possible by the correction keys - T for temperature, P for pressure in mBars, and CF for the chamber correction factor, the most troublesome of the corrections. Happily, parts of the chamber correction aggregate are available pre-computed in tabular form for 10 keV, 25 keV, 50 keV and above, thanks to the BNIP.
The instrument can produce results for both Absorbed Dose in Gy, and Exposure in C/kg.
The back panel is home to the battery locker, which houses 2 x 1.5V D Cells. A serial port can directly feed results to a printer, as well as tripping a relay to start and stop an X-ray or other source at a given dose or time.
The dessicator cartridge may be inspected for colour change, and replaced with a fresh one if necessary, via the perspex plate at right.
The sign of the polarising voltage for the ionisation chamber can be set by S1, while S101 allows the voltage to be progressively reduced by halves. Both switches are locked into position by a safety cover plate, which must first be unscrewed before a change of setting can be made.
The only practical problem I have now to solve is with the triaxial TNC connector for the front panel chamber input. BNC triaxial plugs and sockets are much cheaper and easier to get than the threaded type, though "cheaper" still means about £20 - $31- for a single BNC triaxial plug from Farnell, so I will look into the feasibility of changing the existing front panel socket for a BNC triaxial type. Everything in the triaxial world looks like an invitation to insolvency.
The first braid layer in the triaxial cable is used to guard the inner conductor and the guard islands in the ionisation chamber against insulation leakage, which can become a significant part of the apparent chamber current at ultra high impedance.
I am looking forward to comparing the results produced by this instrument with my existing Precitron Janus Dual Channel Electrometer, which once I thought to be the absolute bees knees!
And no, I don't know why NE Technology decided to call it a 'dosemeter' rather than a dosimeter, and yes, I have finally discovered how to link to hi res images without being sent to bed without any supper
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NE Technology Farmer Dosemeter 2570/1
