How to Use Water Cooled Ophir Sensors

¹ Proportionally lower flow rate at lower power subject to absolute minimum

What flow rate do we need if the sensor is not running at full power? In principle, the water flow rate would then be proportionally less, i.e. if the power is ½, then the flow rate is ½ of the value in the table above. However, the sensor will not operate properly if the flow rate is too low so the flow rate should be within the limits of the table above.

What about the water inlet temperature? We usually specify that the water inlet temperature should be between 15degC and 30degC. The reason for not using a lower temperature is that this may result in water condensation on the absorber surface. If this is not a problem, then the minimum water temperature can be even colder than 15degC. However, note that there may be an offset of the zero where the cold sensor seeing the warmer room will register a positive reading even without any laser input. This can be offset in the user software.

What about the water temperature stability? The sensor measures the heat flow across the thermopile disc inside. If the water temperature changes suddenly, this will affect the heat flow and therefore also the power reading. Therefore, we specify that the water temperature should not change by more than 1degC/min. The reading of the power sensor may also be affected by sudden fluctuations in the water flow rate so those should also be eliminated. See below.

Requirements on the water cooling system: Ophir does not supply cooling and water flow equipment for the sensor but the chiller used should be able to maintain the specified water temperature and flow rate at maximum heat load, e.g. if you plan to use the sensor at 10kW, then the chiller should be able to maintain a temperature in the permitted temperature range at that power. In addition, the chiller should be able to maintain a constant temperature without large sudden fluctuations in temperature as stated above. Sometimes, the cooling water is daisy chained where it goes through several devices including the power meter in the same loop. In that case, care should be taken that the power meter is not immediately downstream from a device that is alternately putting heat in and not thus causing fluctuations in the water temperature. If the cooling system nonetheless is configured so that there are sudden changes in temperature from the chiller turning on and off or previous devices and this affects the reading, this may be lessened by putting a mixing water tank (>30 liter) between the chiller and the sensor to even out the temperature.

Does the water flow need to be on at all times, even at low powers? First of all, the water flow should be on for some time before starting measurement. The sensor takes time, up to 1 min, to stabilize after the water flow is started. As for use at low powers, water cooled sensors will not work properly at all unless the sensor is filled with water to make thermal contact between the disc and sensor. If the sensor is filled with water and the input and output connectors are stopped up, then the sensor can be used for a short time without water flow or at much reduced power continuously. Note, however, that when used this way, the response time of the sensor may not be optimal and it may be slow or overshoot.

Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with their environment. Metals are converted to a more stable form such as their oxides, hydroxides, or sulfides (Wikipedia).

Corrosion is caused by interactions between the metallic components of the sensor and the cooling water, which may contain a variety of dissolved ions. Many factors affect the risk of corrosion forming, but the most important are the pH and the mixture of ions in the water.

Most Ophir water cooled sensors have a hard anodized housing and in rare cases depending on the water composition, the housing can corrode. (see fig. 1) This usually begins at a crack in the anodize coating of the housing exposing the bare aluminum. Cracks can develop a local chemistry, resulting in localized acidity and concentrations of non-volatile impurities, that greatly accelerates corrosion.

How does the type of water affect corrosion? Many factors affect the risk of corrosion forming, but the most important are the pH and the mixture of ions in the water. In most cases there is no problem with corrosion when using tap water, deionized water or distilled water. However, there are a number of factors that can contribute to pitting corrosion including dissolved oxygen, chloride ions, more "noble"/cathodic metallic ions in solution, and excessive pH values (less than 4 or higher than 9).

What type of water is best for Ophir sensors? As stated above, some types of water promote corrosion. Therefore we recommend using deionized or distilled water in a closed circulating system. Note that deionized or distilled water is usually slightly acidic (pH 5.65) due to absorption of CO2 from the atmosphere. For best results, the cooling water can be neutralized by adding 5 ml of a 10 mM solution of NaOH for each liter of water in the cooling system. Note that deionized or distilled water is sometimes considered to be "hungry" for ions and therefore corrosive but this is not significant for closed circulating systems.

What about tap water? Tap water is usually okay but sometimes tap water contains various amounts of dissolved ions, some of which may degrade sensor performance or integrity. Chlorides, sulfates and heavy metal ions can greatly accelerate corrosion. We have found that the most problematic for Ophir power sensors is tap water containing chlorides and this should be avoided. Divalent metals and carbonate – their concentrations determine the "hardness" of the water - can precipitate out as scale which impedes heat transfer from the disk to the cooling water. Because water quality varies geographically, there is no way of knowing whether locally available water would encourage damage to the sensor. In light of this uncertainty, we recommend use of DI or distilled water which gives uniform operating conditions for all customers. Also tap water may have biological fouling from some combination of bacteria, fungi, and algae. To flourish they need nutrients from the water in the circulator. DI and certainly distilled water is likely to contain fewer of these nutrients than untreated tap water. If biofouling is a problem, various anti-fouling compounds can be used to try and control growth. Known antimicrobials include organotins, bis(trichloromethyl)sulfone, methylenebis(thiocyanate) (MBT), Beta-bromo-Beta-nitrostyrene (BNS), dodecylguanidine salts, and bromonitropropanediol (BNPD). If necessary, please consult a water treatment specialist.

The optimum values of dissolved or suspended material in DI water is given in the table below:

1. Recommended DI water
2. Typical conductivity < 300 µS/cm (30 mS/m)
3. Particle size: < 150 um
4. TDS: < 200 ppm
5. Total hardness: < 100 ppm
6. Total suspended solids: < 25 ppm
7. Alkalinity: < 100 ppm
8. Chlorides: < 25 ppm (50 mg/L)
9. Sulfates: < 25 ppm (130 mg/L)
10. pH: 6.5-8.2

What about Using Propylene or Ethylene glycols or other additives? It is not clear why one would want to use glycol additives as they degrade the cooling efficiency of the sensor and entail various degrees of toxicity. Normally these additives are used in systems to prevent freezing or boiling, which should not be an issue when operating Ophir/Newport sensors properly. While commercial glycol preparations include corrosion inhibiting additives, such additives can be added to water without using ethylene or propylene glycols. If glycol preparations are used, the pH of the mixture should be kept between 8 and 9. For closed circuit water cooling systems with copper and aluminum in them such as our power meters, there are commercial products that can help such as Optishield Plus.

What about when the sensor is not in use? To prevent corrosion it is important not to allow standing water to evaporate inside the sensor, thus concentrating contaminants as the water disappears. When disconnecting a sensor from the cooling system, the water channel should be cleared by blowing compressed air through it.

Is there an Ophir sensor immune to corrosion? For those customers still experiencing problems with corrosion, we recommend the new thermal thermal sensors 1000WP-BB-34 and 5000WP-LP2-50 that have a special design in which all materials that come into contact with the cooling water are either copper or nonmetallic. This sensor will not corrode even with water solutions having undesirable content.