Transcription

Warren StoneHalifax, Nova ScotiaMarch 2010Relative Humidity Measurement and Dehydrator OperationMonitoring of transmission line pressure is standard at most sites. Unfortunately pressure alonedoes not ensure dehydrators are operating properly. Keeping the transmission lines dry isperhaps the most important antenna related preventive maintenance item we can perform on theground. Sending wet air up the tower is a guarantee of trouble, especially at high power multiplecarrier sites. Transmission line relative humidity is easily monitored, making moisture damagevery easy to prevent. If the dehydrator is sending wet air to the antenna it should be shut downand replaced immediately. We can either use an inexpensive sensor to detect an early problemor wait for the antenna to fail.The relative humidity of air inside a transmission line must be low enough to ensure water vapourdoes not condense during the coldest possible weather conditions. Moisture can create corrosionon metal surfaces and cause contamination on the surface of insulators. Line attenuation willincrease as the surface corrodes. Contaminants could flow over the spiral insulator with thewater. When the water evaporates it will leave behind a trace of material that will reduce the highvoltage breakdown of the cable. At high power it could create arcing and result in catastrophiccable failure. Just one event in which water is allowed to accumulate on the copper surface oftransmission line can cause serious damage.Even on a summer day with the air temperature well above the dew point I’ve measured highvoltage DC leakage in splitter cables due to the water vapour present. RF leakage current candestroy cables or antenna panels rendering the antenna system useless. The failure can causeextended outages resulting in extensive and expensive repairs.Relative humidityRelative humidity refers to the ratio of the moisture content of air compared to the saturatedmoisture level of air at the same temperature and pressure. It is usually stated as a percentageor %RH.This site has an interesting explanation of relative humidity.Relative Humidity - Relative to What? (Included at end of y/humidity.htmlRelative humidity is largely dependent upon three items; temperature, pressure, and water vaporconcentration.If temperature increases while the other two remain constant, the %RH will decrease.If pressure increases while the other two remain constant the %RH will increase.If water vapor concentration increases while the other two remain constant, the %RH willincrease.Dew pointThe amount of water vapour in the air at any given time is usually less than that required tosaturate the air. If air is gradually cooled while maintaining constant pressure and moisturecontent its relative humidity will rise until it reaches 100%. The temperature at which the moisturecontent will saturate the air is called the dew point. If the air is cooled further, some of themoisture will condense. %RH will remain at 100% unless the temperature increases.Page 1 of 46

Warren StoneHalifax, Nova ScotiaMarch 2010Pressure dew pointBecause %RH will vary according to pressure the dew point will also vary. By measuring %RH athigh pressure, the corresponding dew point at low pressure can be calculated. A measurementof 0.7%RH at 21C results in dew point of about -40C if the pressure remains constant. If themeasurement was taken at 100PSI and the pressure reduced to 3PSI while maintaining aconstant temperature of 21C the resulting %RH will be 0.14%. The dew point will be about -55C.Bottled nitrogen (N50) is specified to have a maximum water vapour content of 10ppm. At2200psi the dew point inside the bottle is only about -16C. Once the air is released and regulatedto 3psi the dew point will drop to about -59C. Always ensure nitrogen tanks are chained orstrapped securely to prevent tipping and that the top caps are in place when not in use.Dew point measurementDew point is difficult to measure directly. A chilled mirror hygrometer provides a direct readingbut is expensive and impractical. In order to provide an inexpensive and reasonably accurateindication, dew point is calculated based on %RH and sample air temperature at a constantpressure. A typical dew point monitor is actually a %RH meter calibrated to display an equivalentcalculated dew point. We do a similar type conversion with RF wattmeters by measuring the RFvoltage and indicating an equivalent power level on the meter scale.Transmission lines are pressurized at room temperature and the air is sent outdoors to the towerwhere the temperature can drop considerably. By measuring the %RH at the dehydrator output,the expected dew point outside on the tower can be calculated.Be careful when allowing air from transmission lines to purge back into the building and over thesensor. During winter the air could be very cold and thus the measured %RH may beconsiderably higher than expected. This is entirely normal. For reliable interpretation ormeasurement of dew point, both air temperature and %RH are needed.Page 2 of 46

Warren StoneHalifax, Nova ScotiaMarch 2010Dew point calculationThe US National Weather Service, Hydrometeorological Prediction Center, uses the followingequations. Check the html source code to extract the rh.shtmlUUUEs 6.11* 10 (7.5*T/(237.7 T))E 6.11* 10 (7.5*DP/(237.7 DP))RH (E/Es)*100Es Saturation vapour pressureRH % Relative humidityT Air temperature CE Actual vapour pressureDP Dew point CPlace the equations for E and Es in the RH equation and the only variables that remain are T andDP. Solve the equation for DP and this formula can be used for calculating dew point based on%RH and air temperature. It works well in Excel.DP 237.7*LOG10(0.01*RH*10 (7.5*T/(237.7 T)))/(7.5-LOG10(0.01*RH*10 (7.5*T/(237.7 T))))It can be simplified by using an intermediate calculation. I use this in my TI83 calculator.A 0.01*R*10 (7.5*T/(237.7 T))(Single letters replace RH and DP for the TI83 )D 237.7*LOG10(A)/(7.5-LOG10(A))Assume the line pressure remains constant. This table converts %RH to expected dew point atvarious sample air temperatures using this formula. Knowledge of the sample air temperature iscritical for an accurate dew point calculation.Here is another set of equations based upon another ity.html* Spreadsheet-ready equations for each unknown in terms of the two knowns:RH: 100*(EXP((17.625*TD)/(243.04 TD))/EXP((17.625*T)/(243.04 T)))TD: 243.04*(LN(RH/100) ((17.625*T)/(243.04 T)))/(17.625-LN(RH/100)-((17.625*T)/(243.04 T)))T: 243.04*(((17.625*TD)/(243.04 TD))-LN(RH/100))/(17.625 LN(RH/100)-((17.625*TD)/(243.04 TD)))( replace "T", "TD", and "RH" with your actual cell references)( T and TD inputs/outputs to the equations are in 0C-40.8C-43.5C-47.2CDew point at sample air .7C-40.4CPage 3 of 5C-23.1C-30.6C-33.6C-37.6C

Warren StoneHalifax, Nova ScotiaMarch 2010Moisture calculatorsI use PhyCalc for most moisture related calculations. It can determine change in pressure dewpoints, moisture concentration, % relative humidity, and many other items. It is by far the mostversatile moisture related software I’ve found. If you plan to get a humidity calculator this oneshould be at the top of the list. It can be downloaded for free from PhyMetrix web site.http://www.phymetrix.com/Software.htmDownload moisture calculatorHHHUUUUUUEnvironment Canada suggested the Vaisala calculator. Here is the lHHHUUUUUUThe results will vary slightly due to the use of different humidity models and formulas, particularlyat very low levels of %RH. Here are some other useful online calculators that are available.Pressure dew point conversionhttp://www.howelllabs.com/dew point conversion.xlsHHHUUUUUUWeather conversion er Products/wxconversions.htmHHHUUUUUURelative Humidity MeasurementMany dehydrators use an indicating silica gel to display proper operation. It is a bead or granulethat has been washed with a concentration of cobalt chloride (a heavy metal salt). The cobaltchloride is a deep blue color when dry and turns from blue to violet to pink as it becomessaturated with moisture. The desiccant beads are an inexpensive but unfortunately unreliableindicator of dehydrator performance. The beads will gradually start to change color from blue toviolet at about 10%RH and then to pink and eventually white at about 50%RH. The dew point for10%RH is about -10C. 50%RH dew point is about 10C. By the time a gradual color change isnoticed the lines will be filled with wet air. It cannot be monitored remotely.A more reliable approach is to use the remote control system at each transmitter site tocontinuously monitor %RH of air supplied by the dehydrators and to use a sensitive dew pointmonitor as part of routine maintenance or following drying system repair.I have experimented with Honeywell HIH4000 and Ohmic Instruments HS-00-1 sensors. TheHIH4000 easily connects to the remote control system. The HS-00-1 could be used forconfirmation during preventive maintenance or following dehydrator rebuild. Spec sheets areincluded near the end of this document.Other sensorsThe Davicom RHS-1 relative humidity sensor uses a HM1500LF sensor. Its measurement rangeis 10-95%RH. It is meant for monitoring room air or from a mechanical cooling system, not airfrom a dehydrator. Its price is 130. Notice the coefficients are similar to those for the HIH4000.http://www.davicom.com/ catalogue/cat/17/id/80The HM1520LF sensor is optimized for 0-20%RH measurement. It may be another useful sensorfor monitoring dehydrator systems. It is available at Digi-Key for about com/downloads/HM1520LF.pdfOther sensors are available to provide simultaneous temperature and %RH readings but they usea serial data output. This is more difficult to connect to a remote monitoring system analog input.Page 4 of 46

Warren StoneHalifax, Nova ScotiaMarch 20101. Honeywell HIH4000This is a wide range sensor 0-99%RH that uses a 5VDC supply. It is used in the 300 humidityoption in several Andrew dehydrators. It is the active element (relabeled HC610) in the OhmicInstruments ( 1000) AMM-15 Airline Moisture -610man.pdfRemove the case and microprocessor from the AMM-15 dew point monitor and mount theHIH4000 sensor in a brass Tee fitting. The site remote control system can supply the smarts toreplace the microprocessor for continuous monitoring. The total cost is less than 25. In thepicture above, the black and orange wires connect directly to the sensor. The red wire has aninline 5V regulator consisting of a 390 ohm resistor and a Zener diode sealed under heat shrinktubing. It can then be powered by the site 12VDC power supply or a 9V battery. Operatingcurrent is less than 20mA. A 100kohm terminating resistor across the RTU analog inputterminals is suggested.Parts list, %RH sensorHoneywell HIH4000 sensor¼” Tee connector, 2203P-042x ¼” NPT male to 3/8” compression fitting 68C-06-04¼” NPT dome connector RD07AA1N4733 5.1V zener390 ohm resistorHookup wireHeat shrink tubingTotalUU1B1BUU2x 1.94 15.62 3.65 3.88 1.40 24.55I’ve used the HIH4000-001 version. It has the pins spread for 0.1 inch spacing. It does not comewith a calibration sheet. Sensor calibration will be described below.There are also slight variances in each HIH4000 sensor. It is also sensitive to supply voltagechanges. Each 5V zener diode will provide a slightly different output voltage. For these reasons,each assembled sensor will supply a slightly different 0%RH output voltage. This makescalibration more critical when operating at very low %RH levels. For optimal measurements the0%RH reference must be determined experimentally.This problem is not exclusive to the HIH4000. It also applies to the MPX5100 pressure sensor,temperature sensors, and most other sensors being operated near their absolute limits.The Ohmic Instruments AMM-15 dew point meter is calibrated to the specific sensor installed andmust be returned to the factory for calibration if the sensor is replaced.Page 5 of 46

Warren StoneHalifax, Nova ScotiaMarch 2010Similarly, each RTU will have a slightly different ADC reference voltage. Using the RTU voltagesduring calibration will factor its ADC errors into the VTS coefficients. This will provide bettermeasurement accuracy compared to an external voltmeter for determining the zero reference.Liquid Air N50 grade nitrogen ( 10ppm water vapour) is about 0.05%RH at 3PSI and 21C. Thisis a reasonable and readily available 0%RH reference. Connect the sensor to the RTU and allownitrogen to flow past the sensor to pressurize the lines. Leave it for about 30 minutes to ensureall moisture has purged from the surface. Set the VTS coefficients to A 0 B 1 C 0 D 0. Displaytwo decimal places in order to measure the actual voltage reading. Monitor for the minimumvoltage. This will be the 0%RH reference voltage. It should be between about 780mV and850mV.The slope of the response curve is needed for measurements above 0%RH. Unfortunately I haveonly a single calibration point. At least one more point is needed to determine slope precisely.As a compromise, 29mV/%RH seems to be a reasonable value. %RH can be calculated.%RH (V - 780mV) / 29mVwhere V is the measured sensor output voltage and 780mV isthe measured 0%RH voltage from calibration above.2Using quadratic formula, %RH Ax Bx C, the following VTS coefficients are used.A 0B 1 / 0.029 34.48 C -0.780 / 0.029 -26.89Alternately, C - B x (zero value), -34.48 x 0.78 -26.89Where 0.029V / %RH is the response slope, and 0.780V is the 0%RH calibration.Install the sensor, select B 34.48 and tweak the C coefficient to show 0%RH with nitrogen.Monitor for a few minutes to ensure the %RH value does not become negative.You can also set C 0 and monitor the minimum VTS reading with nitrogen or a known dry airsource. The highest %RH reading will be the -C coefficient.Most broadcast sites have relatively high levels of RF inside the building. Bypass capacitorsshould be installed across the power and analog sensing lines to help stabilize readings. Two0.1uF capacitors on each end seem to work well. Shielded wire with the shield connected at theremote control system end assists as well.Don’t be scared by the nitrogen calibration. A simplified mode of operation could be to monitorfor an increase of %RH. Calibrate with a known good dehydrator and monitor the output voltagefor a rise of 150mVDC. This will indicate an increase of about 5%RH and can be used to triggeran alarm. The absolute value is less important than the measured increase. Andrew dehydratorsset the sample trip point at 1.0VDC to alarm at about 7.5%RH. Using the MARC system tomonitor for a sensor output voltage above 1.0VDC would provide an early warning that thetransmission lines are wet. This is considerably better (and easier to fix) than detecting aproblem when transmitters shut off due to VSWR caused by arcing or water filled lines.The sensor should be placed inline immediately following the dehydrator. It should be the firstitem that moisture, oil, or other contamination touches. A contaminated sensor can be cleaned orreplaced. A new part isn’t expensive and is much easier to replace than antenna or transmissionline components.A valid question is whether we really need accuracy to within 1%RH or the ability measure dewpoint precisely to within 1C. A properly operating dehydrator will produce air with less than1%RH. The dew point will be lower than -40C. A failure will cause the %RH to rise, even ifadequate output pressure is maintained. The rise in %RH is what we need to detect. A faileddehydrator will often produce air with about 20%RH. The sensor voltage will be about 1.35VDC.Page 6 of 46

Warren StoneHalifax, Nova ScotiaMarch 2010As a wide range sensor the HIH4000 isn’t specifically designed for our requirements of monitoringnear 0%RH. However, if its shortcomings are known and understood the results can be used.These sensors have been implemented in expensive dew point monitors using the calibrationtechniques described. There is no reason we can not use them in a similar manner. Constantmonitoring for an increase in %RH will provide an immediate indication that something has failed.You can then visit the site to determine what has happened before condensation causes damage.It is an inexpensive and reliable sensor that can detect early failures within dehydrating systems.A 25 investment can easily save many thousands of dollars by providing an early alert ofmoisture. Compare to the expense of replacing a transmission line and an extended outage. Afailed antenna is 100% reliable at eventually detecting a failed drying system, but considerablymore work and effort to put back together.The sensor also needs a slight flow of air. Still air, even if perfectly dry may cause the outputvoltage to increase. I’ve seen this even after purging with pure nitrogen and closing the valves.I’m uncertain of the reason for this, but it has been suggested that plastic air line could havesome effect on the air. Some dew point monitor manufacturers recommend against using plasticair lines and others suggest opening a valve to allow a small amount air to escape over thesensor. The use of copper lines may improve the problem but is a less than practical solution inmany cases.Look closely at the plastic fitting that holds the HIH4000 sensor. It is meant for sealing wiresentering an electrical enclosure. Normal operating pressure would be 0psi. It is used in theAndrew dehydrators up to about 7PSI. It is perfectly acceptable at 3PSI for normal transmissionline pressure. I have one monitoring air at 30-50PSI. If you plan to install the sensor at higherpressures another approach may be required.Install a sensor and monitor the transmission line humidity anywhere you have a dehydrator. I’veused crimp connectors to attach terminals at each wire end. A 9V battery terminal allows thesensor to be battery powered, and a multimeter displays sensor output voltage. This creates asuper simple and effective dehydrator test tool. Measure the voltage on the orange wire duringroutine maintenance visits. It should always be less than 1.0V.If there is money to burn there are many other dew point monitors using the HIH4000 as theactive sensing element that claim to measure down to -40C. At 1000 per system you will needvery deep pockets. The remote control system is usually capable of monitoring the sensor outputvoltage and triggering an alarm at a predetermined level. That is all we need.Page 7 of 46

Warren StoneHalifax, Nova ScotiaPage 8 of 46March 2010

Warren StoneHalifax, Nova ScotiaMarch 2010Honeywell HIH4000 Relative Humidity SensorRemote control setupWiringOrangeRedBlack20 January 2010 - Warren Stone18B15 B13 BUUU%RH analog output 12VDC supply with 5V regulator to sensorGroundNAC SetupDigital type selectAnalog inputSystem type selectINFRNAC DescriptionTX LINE HUMIDITYRTU DescriptionTX LINE HUMIDITYAlarm NAC ViewYesAlarm PriorityMajorAlarm Schedule Select NoneRTN PriorityRTNInformation%Relative Humidity. Dew point conversion 3%RH -25C2%RH -30C 1.2%RH -35C 0.7%RH -40C2B2B2BUUUDavicom MACElectralert Site SentryMARC ScalingBar Scale LowBar Scale HighNAC Normal ValueNAC Scaling RequiredCoefficient ACoefficient BCoefficient CCoefficient DNAC UnitsNAC Decimal places051No0100%RH1