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Pudibei NR-950 radioactivity meter
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introduction
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 Since I'm interested in radioactivity an Geiger counters, I collected a couple of radiation meters. Some do it yourself kits and also a couple of 'off the shelf' meters. The Gamma Scout is a well known one and recently I bought this Pudibei NR-950 radiation monitoring devices. There are three Pudibei models available, the orange NR-750, the blue NR-850 and the red NR-950. Based on marketing information the NR-750 is the basic model, the NR-850 is equipped with a better quality Geiger tube and the NR-950 has reduced power consumption and has better product performance (whatever that may be). Since the price difference is not that big I bought the NR-950 model.
The device is rather straight forward. It has a built in Geiger tube for detecting Beta and Gamma radiation. The dose rate is indicated and the collected dose over time. There's a date/time set and there's an alarm level that can be set. If the dose rate or the accumulated dose exceeds the threshold limit, the display starts flashing, an alarm is sound and a vibration motor is activated. Also the 'ticker' can be activated for indicating the detection of a radioactive particle in the Geiger tube. There's no logging possibility nor the detection possibility for Alpha radiation. But the device is rather good quality for it's price. For (beginner) hobby purpose it's a fine device. The Gamma Scout (with logging and Alpha detection) is more advanced, but also approximately five times more expensive. So the NR-950 is a nice alternative. I'm happy with the device!
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 The rear of the device is rather straight forward. It's just a plain back with a battery cover. Behind the battery cover can two regular AA batteries be installed. The plastic cover feels good and fits great. Also the battery door 'snaps' rather good in place. The look and feel is just fine.
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inside
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The inside is alwauys interesting to see. Inside is a mainboard for all the electronics. There are two flatcables for the display unit and for the keypad. There's a rather large Geiger tube installed. Two wires are soldered to the glass tube and the tube is fixed with juse a piece of double sided tape. Well, this is not the best solution I can imagine, but it works and this is a compromise from the price point of view. The mainboard has a central microcontroller and some additional electronics. There are some components for the keypad signal handling but most of the components are used to generate the high voltage for the Geiger tube. It looks like there's a switching semiconductir and Jacobsladder installed that consists of diodes and capacitors to sweep up the voltage. The only input the device needs is the keypad signals and the pulse input of the Geiger tube. So the electronics is rather simple.
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inside details
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electronics
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The Geiger tube used is a J321 glass Beta/Gamma tube. The diameter is approximately 10 millimetres in diameter and the length is approximately 86 millimetres. The starting voltage is 350 VDC and the recommended operating voltage is 380 VDC. (600 VDC maximum.) The background count rate is approximately 25 counts per minute. The service live is more than 1 * 10^9 (1.000.000.000) pulses. That's approximately 76 years of permanent use with background radiation. Higher levels of radiation will shorten the lifespan of course.
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menu: default
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 The display is rather straight forward. The middle number is de dose rate like 0,08 uSv/h in this case. At the bottom right the accumulated dose is shown, this number will rise in time until the counter is reset. At the bottomleft is the date shown when the accumilated dose measurement started, so the current date is not shown! At the top right is de current time shown. At the left top is the alarm limit (20.00 mSv) visible for the accumulated dose and at the top right (0.50 uSv/h) is de dose rate limit visible. The limits can be set in the menu.
 The operation is rather simple. There are five buttons on the device that can he used. The [STANDBY] power button is for powering on/off and switching on/off the display backlighting. There's a [MENU/OK] button for entering the menu and for confirming a setting. There's an [UP] and [DOWN] button for navigating trough the menu and for increasing and decreasing some value. The [PLAY/PAUSE] button is used to go back to the previous/upper menu level. By pressing the [MENU/OK] key, the menu at the left is shown. With the up and down arrows can be scrolled to the menu items and by pressing [MENU/OK], the desired menu item is shown next.
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menu: units
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 In the [Setting unit] menu item it's possible to select the desired dose rate unit. The most common value is [uSv/h], but [CPM] and [CPS] (not shown on the image) are also posible. Micro Sieverts per hour (uSv/h) are the most logic choice here. Counts per minute (CPM) and counts per second (CPS) is also used quite often. By selecting one item and press [MENU/OK] the desires unit is selected.
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menu: alarm
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 In the [Setting Alarm] menu item it's possible to set the (accumulated) dose threshold level and the rate threshold level. When the alarm limit is reacthed, the device will give an alarm by a backlight blinking display, an audible alarm and a vibrating warning. The (accumulated) dose level can be reset by selecting [Delete Dose] in this menu.
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menu: alarm mode
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 The alarm type can be changed. As shown on the image left there are three options selected in this case. The first symbol is for the vibration warning, the second symbol is to indicate the audible warning and the last symbol is to indicate the flashing backlight of the display. There are a couple of variations of alarm combinations possible.
 An 'odd' item in the menu is indicated with three loudspeaker symbols. This is the alarm mode that activates the 'ticker'. For each discharge in the Geiger tube a clicking sound is made. This gives a nice audible indication of the dose rate.
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menu: display settings
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 In the [Setting disp] menu it's possible to switch between the Chinese and English language. Also the lcd contrast can be set here. As show the menu items and possible settings are rather straight forward. Not all the menu items and values are shown, but that's just a variation on a theme. I guess this gives a nice overview of the menu structure and options.
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