McNab Model AP-III Aqua Purometer Recording Monitor

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  • Meets Regulatory Requirements
  • Built-in Recorder, Various Chart Speeds
  • All Water Ranges
  • Automatic Solid State Operation
  • Rugged, Modular Operation
  • NEMA 4 Enclosure
  • Install, Operate, and Maintain with Non-Technical Personnel
  • Advanced IC Solid State Electronics


There is an emerging trend by governmental regulatory agencies in the pollution and health fields to require continuous monitoring and recording of water purity. This monitoring requirement often includes indicating, alarming, and controlling functions as well as recording. Even where not explicitly required, continuous and automatic monitoring is often more practical and economical than periodic manual laboratory checks. The Aqua Purometer Ill Recording Monitors were specially developed to meet this growing requirement.


The Aqua Purometer Ill Recording Monitors for water conductivity, resistivity and pH are designed to provide all the regulatory agency required monitoring functions in a unitized standard production unit. McNab has combined a rugged enclosure, modular construction, and integrated circuit, solid-state electronics to produce a compact and reliable instrument. Considerable design effort has been expended to make the unit easy to install, operate, and maintain even by non-technical plant personnel. Use of a standardized design with a minimum of variables gives an unmatched performance/economy ratio as well as an assurance of on-going support not possible with custom designs.

Regulatory Agency Requirements

The Aqua Purometer Ill Recording Monitors meet or exceed all existing agency requirements. Neither currently prohibited materials nor materials likely to be prohibited have been used at the water stream. When required the cell may be temperature sterilized without damage. The instrument automatically corrects for broad temperature variations of the water. Because the recorder is driven directly by the automatic operator, interpretive errors are eliminated. The recorder provides the user with a printed, "hard copy" record of the water purity. The tapes are easy to store and can be retained for a minimum of 7 years without significant deterioration. Total system performance for conductivity and resistivity systems can be externally validated by use of the optional McNab Cell Test Unit.


The integrated recorder-monitor package makes installation easy. The enclosure may be either wall mounted or flush mounted with only four bolts. The only electrical connections required are the AC line cord and either three color-coded cell leads for conductivity or a keyed plug-in connector for pH. The installer no longer needs to worry about electronic compatibility among separate components. Mounting of the cell is equally simple, merely screw it into a standard NPT piping tee appropriate for the cell size chosen (either 3/4" or 1" NPT).


Set up of the instrument is easily accomplished by non-technical personnel. The only operator controls are the external power On/Off switch and the tamperproof, interior alarm/controller Level Set controls. To enter the minimum acceptable purity level (i.e. the point on the meter at which the alarm light and controller relay are activated), the operator merely depresses a spring-loaded switch and rotates the level control until the desired set point is displayed on the large 41/2" meter, then releases the switch. This simple system eliminates errors associated with translating dial calibrations to meter calibrations, as well as those from loose pointer knobs and smudged or worn away dial markings. Use of the large 4 meter insures easy readability and higher accuracy in setting of the alarm and controller threshold than is possible in non-metered systems.

Once the set point is entered, operation is automatic.  The instrument will simultaneously indicate the water quality on the meter, record it on the chart recorder, and activate the alarm light and controller relay should the water quality change to the set point level. Terminals are provided inside the cabinet for control or remote alarms or solenoid valves, if desired.


Only minimal maintenance of the systems is required. The solid-state electronics are extremely reliable. Because of the modular design, there are only eleven replacement parts in the entire panel. Conductivity cell deterioration is minimized by the use of noble metals at the electrode face. PH probe electronics utilize the patented "Precision Driven Shield" principle. Standard product configuration requires chart paper replacement every sixty days.

The Cell

ap-III_cell.JPG (38369 bytes)For conductivity and resistivity, various threaded cells (3/4 or 1" NPT) are available, temperature compensated in all ranges. Materials at cell/fluid interface are extremely corrosion resistant - special materials of construction also available.

For pH, cell is a combination, gel-filled, one-piece probe with patented "precision driven shield" design.

Electrical Installation

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  • Conductivity-6 ranges from 0-2 through Q-200,QOQ microohm/cm
  • Resistivity 0.5 - 20 megohm cm
  • pH (Q-1Q) or (5-9) pH units
Cells Contact factory
Accuracy .2% of scale as calibrated
Indicating Meter Large 41/2" white on black. (Recommended option)
Set Internal tamperproof control. Set point can be displayed on meter
Controller Sealed, plug in relay provides line voltage at 5 amps energized normally open and normally closed contacts. Separate non-energized normally open and normally closed contacts also provided.
Recorder Built-in recorder has 2 3/16" usable chart width. Front access to chart paper permits handwritten notations. Standard recorder has 60 day duration, other durations to 9 months optionally available.
Chart Paper Standard 20 division paper
Max. Cell Pressure Varies with type of cell; typical, 150 PSI. 28" vacuum.
Max. Cell Temperature Varies with type of cell contact factory.
Power Requirements 115 VAC 50/60 Hz
Mounting Style Wall or flush
Size Basic unit with mounting brackets: 8 3/4" X 1313 9/16 X 6 1/2"
Weight 14 1/2 lb (panel), 1 lb (cell)
Options Alarm bell, conductivity cell test unit, Replacement chart paper (6 rolls per box), cells for sanitary fittings. Semi-custom special purpose AP Ill systems can also be made.

383-2 East 29th Street | Buena Vista, VA | USA 24416-1293

540.261.1045 voice | 540.261.1268 fax |

Copyright 1999 - 2010  McNab, Inc. All Rights Reserved.  Last modified: 08.12.2010

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