Dewar Monitor System Electronics




The UCO/Lick Observatory EL-5009 Dewar Monitoring System is used to monitor CCD dewars that have been removed from their mount and placed in the 120" dome basement. The system is designed to autonomously monitor the temperature of up to eight dewars and turn on an alarm if any experience a rise of more than about 5°C. In normal operation, the user can add or remove a dewar by connecting or disconnecting a DB25 connector to the dewar. The unit watches a signal line that is pulled down when a dewar is attached. When the signal goes low that channel is then monitored.


Controls and Indicators:


Front View
Click image for larger view


On the front of the box, from left to right in the image above you can find:

The "I'm Alive" LED flashes at about 1.4 hertz to indicate that the RIO is running its program. The serial and network connectors connect directly to the RIO. If the box is connected to the network it will send out a simple email whenever a

Each of the dewar connectors is wired to one of the two EL-5010 Quad Current Source/Amplifier boards. 0 through 3 connect to board #1 and 4 through 7 to board #2.

The Channel Connected panel indicates when a dewar is connected to a the system. The LEDs are illuminated when a connection is made via the GND pins of the dewar. That is, the DMS GND is connected to pin-9 of the DB9F connector, routed to pin-16 of the DB25 on the dewar electronics box via the interconnect cable, then out of the dewar on pin-20 of the DB25 and finally back to pin-5 of the DB9. This signal then is routed to both the channel connected LED and to a digital input to the RIO.

The Temperature readout consists of multiple elements, the digital meter, the green LEDs, the select paddle switch, and the "Push to Hold" pushbutton switch. In normal operation the control program cycles through any connected dewar channel. If only one dewar is connected, the corresponding green LED will stay lit and that dewars temperature will be continuously displayed on the meter. As more dewars are connected the program will automatically start cycling through each of the dewars. When a cycle begins, the lowest numbered channel will be active. The LED for that channel will be turned on and the average temperature for that dewar will be displayed. A timer will be set to 1.5 seconds and then be count down. When it reaches zero the program will move to the next active channel and repeat the procedure for that channel and so on. When the last active channel has completed its cycle the program will start the process again. If for some reason the operator would like to monitor a specific channel that can be done using the "Select" paddle switch to choose the desired channel and then hold down the "Press to Hold" button. In this mode the selected channel temperature will remained displayed.

The alarm panel consists of four elements. The most obvious in the photo is the sonalert. The sonalert is triggered when a channel has seen a rise of 5°C from its lowest temperature. When an alarm is triggered, the sonalert is activated and the green channel indicator LED flashes at a much faster rate then it does when it is the active channel. The reset button can be used in two ways. By pressing the button for less than three seconds, the program will reset the underlying array of temperatures for any channel that has warmed up. If, on the other hand, you hold the button for more than three seconds the program will reset all eight channels. The alarm in connector is provided for possible upgrades. Not shown in the photo are three LEDs that are, again, available for possible upgrades. Each LED is controlled via software and not dependent directly by the three alarm input bits.

The green LED above the AC power switch is connected to the -12V power supply.


The Dewar Monitor System has been built and programmed to require very little user interaction. Basic system operation is to take a reading of the dewar temperature, average it with the previous four readings and compare that average with the coldest temperature so far recorded. If the averaged temperature is colder than the coldest recorded temperature, then make that temperature the coldest temperature for that channel. If, howerver, the averaged temperature is found to be 5°C warmer than the coldest temperature that channel is flagged as being too warm. In this case, the sonalert alarm is triggered, the green LED for that channel is set to flash at a faster rate and if the network is connected an email is sent by the RIO.

A user can add a dewar at any time with one proviso; if any channels are already warm(ing) and have tripped the alarm they may not have a valid 'coldest' temperature saved and may not trip the alarm again even though they have warmed up.

For the most part, the user need only attach a dewar via one of the interconnect cables provided with the system and press the reset button for about three seconds. The system will then take a series of five readings of the dewar's temperature diode and average them and display that temperature. When more than one dewar is attached the software will cycle through each active channel. As it does, the green LED for that channel will light up and the averaged temperature for that channel is displayed on the meter.

Note, if you use this proceedure when a different channel already has and alarm the