Instruction Manual Model Constant Temperature Anemometer

Transcription

1 Instruction Manual Model 1750 Constant Temperature Anemometer

2 Manual History The following is a manual history of the Model 1750 Constant Temperature Anemometer (part number ). Revision Date First Printed March 1988 A October 1988 B January 1989 C April 1989 D May 1989 E March 1990 F December 1991 G February 1992 H July 1992 I August 1993 J March 1994 K April 1996 L January 1999 M January 2001 The February 1992 revision reflects a revision to Figure 2. Revision H reflects revisions to Figure 1 and the section How to Operate the Anemometer. Revision I reflects a correction made in the How to Operate the Anemometer section. A change bar indicates the altered text. Figure 1 was also changed. In revision J, TSI s customer service number was changed. In revision K, TSI s Limitation of Warranty and Liability on page iii was updated. In revision L, TSI s area code was changed from 612 to 651. In revision M, TSI s limitation of warranty and liability was updated. ii

3 Part Number / Revision M / January 2001 Copyright TSI Incorporated / April / All rights reserved. Address TSI Incorporated / 500 Cardigan Road / P.O. Box / St. Paul, MN / USA Telephone No (USA) or (651) Fax No. (651) Address fluid@tsi.com Limitation of Warranty and Liability (effective July 2000) Service Policy Seller warrants the goods sold hereunder, under normal use and service as described in the operator's manual, shall be free from defects in workmanship and material for (12) months, or the length of time specified in the operator's manual, from the date of shipment to the customer. This warranty period is inclusive of any statutory warranty. This limited warranty is subject to the following exclusions: a. Hot-wire or hot-film sensors used with research anemometers, and certain other components when indicated in specifications, are warranted for 90 days from the date of shipment. b. Parts repaired or replaced as a result of repair services are warranted to be free from defects in workmanship and material, under normal use, for 90 days from the date of shipment. c. Seller does not provide any warranty on finished goods manufactured by others or on any fuses, batteries or other consumable materials. Only the original manufacturer's warranty applies. d. Unless specifically authorized in a separate writing by Seller, Seller makes no warranty with respect to, and shall have no liability in connection with, goods which are incorporated into other products or equipment, or which are modified by any person other than Seller. The foregoing is IN LIEU OF all other warranties and is subject to the LIMITATIONS stated herein. NO OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR PARTICULAR PURPOSE OR MERCHANTABILITY IS MADE. TO THE EXTENT PERMITTED BY LAW, THE EXCLUSIVE REMEDY OF THE USER OR BUYER, AND THE LIMIT OF SELLER'S LIABILITY FOR ANY AND ALL LOSSES, INJURIES, OR DAMAGES CONCERNING THE GOODS (INCLUDING CLAIMS BASED ON CONTRACT, NEGLIGENCE, TORT, STRICT LIABILITY OR OTHERWISE) SHALL BE THE RETURN OF GOODS TO SELLER AND THE REFUND OF THE PURCHASE PRICE, OR, AT THE OPTION OF SELLER, THE REPAIR OR REPLACEMENT OF THE GOODS. IN NO EVENT SHALL SELLER BE LIABLE FOR ANY SPECIAL, CONSEQUENTIAL OR INCIDENTAL DAMAGES. SELLER SHALL NOT BE RESPONSIBLE FOR INSTALLATION, DISMANTLING OR REINSTALLATION COSTS OR CHARGES. No Action, regardless of form, may be brought against Seller more than 12 months after a cause of action has accrued. The goods returned under warranty to Seller's factory shall be at Buyer's risk of loss, and will be returned, if at all, at Seller's risk of loss. Buyer and all users are deemed to have accepted this LIMITATION OF WARRANTY AND LIABILITY, which contains the complete and exclusive limited warranty of Seller. This LIMITATION OF WARRANTY AND LIABILITY may not be amended, modified or its terms waived, except by writing signed by an Officer of Seller. Knowing that inoperative or defective instruments are as detrimental to TSI as they are to our customers, our service policy is designed to give prompt attention to any problems. If any malfunction is discovered, please contact your nearest sales office or representative, or call TSI s Laser Diagnostics Division at (USA) or (651) iii

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5 Contents Manual History... ii About This Manual... vii Getting Help... vii Submitting Comments... vii Sections Introduction...1 Power-supply Requirements...1 Probe and Control Resistor...2 How to Measure the Sensor Resistance...4 How to Operate the Anemometer...4 How to Check the Frequency Response...5 Figures 1 Electrical Block Diagram of the Anemometer Frequency-response Test Connections Signal Outputs for a Square-wave Test...6 Tables 1 Appropriate Temperature Coefficients of a Typical Sensor...3 v

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7 About This Manual This manual describes the operation of the Model 1750 Constant Temperature Anemometer. Getting Help To obtain assistance with this hardware, or to submit suggestions, please contact: TSI Incorporated Laser Diagnostics Division P.O. Box St. Paul, MN USA Fax: (651) Telephone: (USA) or (651) Address: Submitting Comments TSI values your comments and suggestions on this manual. Please use the comment sheet, on the last page of this manual, to send us your opinion on the manual s usability, to suggest specific improvements, or to report any technical errors. If the comment sheet has already been used, mail, fax or your comments on another sheet of paper to: TSI Incorporated Laser Diagnostics Division P.O. Box St. Paul, MN Fax: (651) Address: fluid@tsi.com vii

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9 Introduction The Model 1750 Constant Temperature Anemometer is designed for lowcost, multichannel applications. The anemometer provides a Trim control that allows you to optimize the frequency response for a given cable length. It also provides a Gain control that is set at the factory according to the type of sensor used (film or wire). The Gain and Trim controls may require slight adjustment to obtain the best frequency response. The circuitry of the anemometer protects the sensor against the removal of the control resistor and loss of power. The instrument has noise and temperature-drift specifications that are comparable to TSI s bench-type anemometers. Power-supply Requirements The anemometer needs only one power supply with an output of +12 volts to +25 volts DC. A typical supply will have an output of +15 volts and 0.7 ampere, with noise of less than 0.05 volt RMS (120 hertz ripple). Such a supply will provide a maximum bridge output of about 12 volts and about 0.5 ampere through a sensor operating at 10 ohms. If more voltage or current are needed, a supply with +25 volts and 1.0 ampere will provide a maximum bridge output of about 22 volts and about 0.75 ampere through a sensor operating at 13 ohms. Also, if only a 12-volt supply is available, the maximum bridge output will be about 9 volts with about 0.4 ampere to a sensor operating at 8 ohms. * Probe and Control Resistor The bridge resistor in series with the sensor is 20 ohms ±0.2 percent and the bridge ratio is 5:1 ±0.1 percent. The probe and control resistor represent two legs of the anemometer bridge (Figure 1) in which the probe is the active sensor and the control * If your power supply has a variable voltage selector, set the output to 15 volts before switching the current to the anemometer. Do not operate below 12 volts. 1

10 resistor sets the operating temperature of the probe. Matched probes and control resistors are often furnished by TSI; they need only be connected to the proper anemometer terminals. Similarly, if the control resistor is another temperature-sensitive element for temperature compensation, the leads for temperature compensation are connected to the control resistor terminals (C and B). E F R1 20 Ohms R2 Gain Control 100 Ohms A R s Control Resistor Trim B C D G H Figure 1 Electrical Block Diagram of the Anemometer Probes purchased from TSI specify the recommended operating resistance on the cover of the box. To determine the value of the control resistance, multiply this number plus the lead resistance by 5. For example, if Op Res equals 8.3 ohms, then Control Res equals (8.3 ohms + lead resistance) 5. For optimum stability, the control resistor you use should have a low temperature coefficient and a rating of 1 watt or greater. For probes that you provide, you must first establish the correct operating resistance, as described by the following equation: 2 Model 1750 Constant Temperature Anemometer

11 R H = R o [1 + (t h t o )] where R H = resistance at operating temperature t h R o = resistance at ice point temperature t o = temperature coefficient of resistance. To use this equation, the sensor resistance R o at the ice point t o must be known, as well as the temperature coefficient of resistance. Some typical values are given in table 1. For an accurate value of t h, and R o must both be measured. It is even more accurate if you can determine R H directly by exposing the sensor to t h. Often, the exact temperature is not necessary. Then a simple overheat ratio can be established and set, where the overheat ratio (OH) equals R H /R C and R C is the sensor resistance at the temperature of the fluid. In air, typical OH ratios are 1.5 for TSI film sensors and 1.8 for tungsten wires. These result in a wire temperature of about 250 C if the Fluid temperature is around 20 C. Since, as a rule, the sensors must be calibrated anyway, the precise temperature of the sensor is not important for many applications. Table 1 Appropriate Temperature Coefficients of a Typical Sensor Type of Sensor a [ohms/(ohm C)] Platinum film sensors Tungsten wire Platinum wire Platinum-iridium wire How to Measure the Sensor Resistance Thermal anemometer sensors (and fine hot-wires in particular) often overheat when measuring their resistance with a standard ohmmeter. A bench-type anemometer is recommended (that is, TSI s Model 1050) to measure the sensor resistance. Otherwise, carefully check the specifications of the ohmmeter you are using. A good, 5-place DVM with resistance-measuring capability is often satisfactory particularly if the sensor is placed in water to increase the rate of heat transfer to the environment. Model 1750 Constant Temperature Anemometer 3

12 If excess current is used, two problems may develop: 1. Since the sensor heats up, the measured resistance may be significantly greater than the ambient temperature. 2. With a fine wire, burnout can occur with some ohmmeters. To perform a rough check on the amount of heating, move the sensor in the fluid and look for a change in measured resistance. How to Operate the Anemometer Connect the probe s cable to terminals A and B of the terminal block. If you are using a coaxial cable, connect the center conductor to A and connect the shield to B. * The type of terminal block used provides for simple connection of the wires; slip in the wire and tighten. Connect the control resistor across terminals B and C. If you are using a coaxial cable, connect C to the center conductor and B to the outer shield. Connect the output cable to E and F. (If you use a coaxial cable, connect the cable shield to F.) Connect power supply terminals G and H, and switch on the power supply. The bridge voltage should now vary with the velocity of the flow past the sensor. Observe the bridge output with the oscilloscope to ensure that the operation of the unit is stable at maximum velocity. How to Check the Frequency Response To check the frequency response of a thermal anemometer, one accepted technique especially for hot wires is to send current into one side of the bridge. (Figure 2 shows the electrical connections using a standard signal generator.) A square-wave at about 1 kilohertz gives a suitable stepchange that repeats itself for easy viewing on an oscilloscope. The output is shown on the simulated trace of figure 3. The pulse amplitude for a wire should be less than 150 millivolts; for a film, it should be approximately 300 millivolts. The effective frequency for wires is f = 1/1.5t where t is the recovery time within 3 percent of the initial value. * If your power supply has a variable voltage selector, set the output to 15 volts before switching the current to the anemometer. Do not operate below 12 volts. 4 Model 1750 Constant Temperature Anemometer

13 Probe Control Resistor 50k To Oscilloscope +15V GRD A B C D E F G H PROBE BRIDGE OUT +15V GRD CONTROL RES SQUARE WAVE CTA TRIM GAIN Constant Temperature Anemometer Model 1750 Serial Figure 2 Frequency-response Test Connections For hot films the proper adjustment is less clear, since the pulse does not immediately or completely return to the initial value. Even so, recent evidence suggests that if t is taken as the pulse width, f = 1/t gives reasonable results. Model 1750 Constant Temperature Anemometer 5

14 Figure 3 Signal Outputs for a Square-wave Test; a for a wire sensor; b for a film sensor 6 Model 1750 Constant Temperature Anemometer

15 DOC: 1750.DOC (p/n ) FILE: Market Groups/MG1/1-MG Manuals DISK: MG1 CE: Sue Alfini CHG: DATE: 30 March 1988 (print w/fractional widths) 30 March October October 1988 (2x) 12 January January 1989 (lucky Friday!!) (Trial with Optima font) 14 January January April April May March 1990 (reformatted) 20 March 1990 (reset with Utopia font) 3 April December 1991 (per ECO # , rev upped to F) 4 February 1992 (per ECO # , rev upped to G per Sue Alfini) 6 July 1992 (changes to pages 2 and 4 per Reader s Comments sent to Preeti and confirmed with engineer) 2 August 1993 (made changes to How to Operate the Anemometer and reformatted whole manual, upped to rev. I per ECO # by Sue Alfini) 16 December 1998 (changed TSI s area code from 612 to 651) 12 January 2001 changed TSI s limitation of warranty and liability and changed Fluid Mechanics to Laser Diagnostics division (ECO # )