Keeping our kHz laser at the right temperature

[Matt Wilkinson]

Hi all

From about the beginning of 2017, the team at the SGF noticed some increased instability in our terrestrial range calibration values at the level of 2-3 mm.  It was particularly clear on the first calibration of the day being a low value and so we learnt to take at least two calibrations before starting an observing duty.

2 weeks ago we also saw an increase in the RMS of the calibrations and some periodic behaviour in the calibration range values.  We consequently took the kHz laser offline and began using our older 10Hz laser, which had very stable calibration values and RMS.

After some investigation and discussion with High-Q, the team found that the flow rate of the cooling water was much lower than it should have been.  The water used is a reservoir of distilled water, but after 10 years of use something had built up to block the waterways.

Temperatures in the laser bed were not being kept at a stabilised constant, causing energy fluctuations and range errors from the start diode. The temperature would change when the laser began firing after being off for a period (20+ minutes). It would stabilise after approximately 10 minutes, during which time calibration range differences were seen.

Running the flow backwards and using a descaler (Durgol was recommended) brought the flowrate up to specification levels.  Setting the best operating temperature removed the signals in the range values and now the calibration RMS and repeatability are now within acceptable levels.

Needless to say, we will be checking the flowrate more often from now on.

Matt

[Yarragadee]

Hi Matt,

We hardly ever have our laser not firing and very very rarely stop the water flow through the table but our start diode is possibly subject to temperature fluctuations since it is on the side near the air conditioning vent. Although we monitor table temperatures closely, I don't monitor anywhere near the start diode. I will install a thermo-couple on the diode housing today! Now that we are on our event timer, we can see much more subtle changes in system delay, so I am really on the hunt  for these little things - thanks.

Randall.

[Sven Bauer POT3]

We have a laser system similiar to HERL station. Even though we use purified water we have to clean the cooling circuit of our laser every few month. We will start to monitor the water flow in the tubes to see if too much deposits are reducing the flow and with that the cooling as well as the laser power.
We were talking to a technician from the company who said that the inside of the laser the pipes are made of Aluminium which probably reacts with the purified water ... generating white deposits that start to collect and block the pipes. Not much we can do about it except monitoring and cleaning. Descaler helps to remove the deposits quite well.

[delpino@riga]

We are installing in Riga a network of SLR temperature sensors using RapsberryPI's and the software in Python.
One RapsberryPI with 2 sensors is monitoring the laser room (where the 25 meters calibration path single mode optical fiber is stored) and another sensor is on the PMT thermal box attached to the Telescope receiving Coudé path (on open air when tracking) but connected to the Laser Room sharing the heating/cooling.

The second RapsberryPI with 2 sensors is monitoring the Control/Electronics room and the CFD/Event Timer electronics

Here is a plot of the daily max/min temperatures at the PMT box

[Matt Wilkinson]

Recovered post:

Hi,
I have a question especially for HighQ laser users (Matt, Sven, Franz et al.), what chiller are you using? Do you have some model from Lauda, with what specifications (cooling power, temperature stability; @Matt: how big deviations in temperature you had when you noticed the calibration changes?) We need to get a new one to replace our >10 year old Lauda in Metsähovi with a new one, and I'm trying to find the "best" option suited for our needs.

The backround is: we run into problems with the water cooling unit as we are not using our laser that much... even though the chiller has been on most of the idle time (and we have changed the water every once in a while), there was a major blockage inside the post amp. I had to use quite a lot of brute force, nasty chemicals and sharp tools to get the blockage open. Finally after this, everything went fine for some time until suddenly the water connector on the backside of the HighQ post amp analog modules broke down spraying the water all over the laser controller rack. Fortunately pretty much all of the electronics were shut down and everything seems to still work. Now finally the fan inside the cooling unit has stopped working (probably the motor is broken, pretty much impossible to replace) and the unit burnt two fuses while I was trying to operate the laser.

Lesson learned: be cautious and suspicious if you have water hoses connected on top of your electronics rack. Apparently it is the initial design from HighQ, seems that the connector was tightened too hard which had cracked the connector.

BR,
Arttu

[Matt Wilkinson]

Recovered Post:

Hi Arttu,
in Graz we are using (since delivery of the laser) a Lauda chiller. LAUDA ecoline RE110
Cooling power @20° = 500W; Heating power = 1500W;
We have a filter of 100µm at the output (renewing once a year). And we changed the position of the temperature sensor from internal (chiller reservoir) to the output tube of the laser for having shorter warm up times. As a “descaler” we are adding OptiShield to the distilled water. This mixture we are renewing around every 5 to 10 years but we have to add around half a litre of distilled water every week. (losing a lot ??) By the way, beside some very short service periods, we never switched the HQ-Laser system off.
Greetings, Franz

[Matt Wilkinson]

Recover Post:

Hi Arttu

Water spraying over the laser electronics doesn't sound like fun! 

We run a Lauda E100 at the SGF. When we had temperature variation problems it wasn't  the water cooler temperature that was unstable it was the temperature control of the amp. That temperature control system became unstable as the water flow around it had become very low, sounds similar to your problem but not so bad.

The fix was to add a bottle of Durgol Universal Descaler, as recommended by Michael Schmidt, to the water and run it for 2+ hours. This restored the water flow rate meaning the amp temperature control could work as intended again so no more temperature variation to cause the range variation we'd seen.

We have run the same Lauda water cooler continuously since the arrival of our kHz laser so 10+ years. I have replaced the fan motor 2 or 3 times though, it's not impossible to get to but access is tricky.

The water cooler has always been set at 20°. We don't run a filter on the outlet as Michael told us to remove it, we also stopped using OptiShield on his advice. We have always changed the water at least once a year. Micheal now recommends changing the water and flushing the system with Durgol every 6 months.

The last time I changed the water I added a bottle of Durgol before draining the old water and noticed an obvious improvement in water flow, despite having used Durgol at previous changes. It appears it doesn't take long for the limescale build up inside the laser waterways to have a noticeable effect on flow rate.

Toby