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788 Series Differential Pressure Purge Control Level Transmitter

  • Generates a constant flow of compressed air to purge a downpipe
  • Integral differential pressure transmitter provides 4 mAdc to 20 mAdc output proportional to liquid level
  • All-pneumatic design does not include integral transmitter
  • Provides 1:1 pressure signal to a compatible indicator or external transducer through 1/4 in. airline tubing
  • Holds a fixed pressure differential across a precision orifice to ensure constant flow rate of 1 cfh (cubic foot per hour), making it possible to maintain a linear 1:1 response of the sensing system
  • Backpressure sensed is equivalent to the hydrostatic head pressure due to actual liquid depth

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SERIES 788  
FITINGS 198 None, 1/4” NPT 398 Polyflo; (1) 3/8" tube, (1) 1/4" tube) 498 Swagelok; (1) 3/8" tube, (1) 1/4" tube
(compatible with copper or poly tubing)
RANGE 148 0 psid to 2 psid 168 0 psid to 10 psid 188 0 psid to 30 psid
158 0 psid to 5 psid 178 0 psid to 15 psid 198 0 psid to 50 psid

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Output signal 4 mAdc to 20 mAdc (milliamperes)
Accuracy 0-5 psid trans./control
0-10 psid trans./control
0-15 psid trans./control
0-30 psid trans./control
0-50 psid trans./control
± 0.34% full scale
± 0.27% full scale
± 0.24% full scale
± 0.22% full scale
± 0.21% full scale
Repeatability ±0.20% of calibrated span
Temperature range 0 °F to 120 °F/-16 °C to 54 °C
Maximum pressure 300% over-range
Burst pressure 200 psig
Power requirement* 14 Vdc to 40 Vdc
Compressed air requirement 35 psig to 150 psig supply pressure at control inlet. (Minimum supply pressure should be at least 20 psig greater than anticipated maximum liquid head pressure.)
Maximum depth measurement 1,937 inH2O(49.0 m)
DP Purge control Unit incorporates differential air flow regulator with flow control orifice. Fixed differential set to nominal 10 psid. Output is 1:1 pneumatic pressure (hydrostatic force equivalent). Includes an integral differential pressure (D/P) transmitter converting pneumatic pressure into 4 mAdc to 20 mAdc electronic output. Ranges available: 0 psid to 5 psid, 0 psid to 10 psid to 30 psid, 0 psid to 50 psid,
Flow rate (air purge) Nominal 1 cfh (cubic feet per hour) at specified differential. Maximum air consumption under 5 cfh.
Materials of construction Machined brass body with acrylic semi-gloss enamel finish; integral filter element is aluminum with acrylic semi-gloss enamel finish.

* Unregulated

Options & Accessories

TL-190    Differential Calibration Kit

Downpipe Check ValveDownpipe Check Valve

6363 Downpipe float check valve, 3/4” NPT connections
6563-10 Downpipe float check valve, 3/4” NPT outlet, 3/8” tube inlet


  • Prevents siphoning or backflow of liquid through a downpipe installed in tank level gauging applications
  • Utilizes a specially designed float pin that rises up to seal the valve shut when liquid enters from the downpipe through the bottom of the unit
  • The advantage of a float check is that it doesn’t create a pressure drop that would affect the level gauge reading (Spring-loaded check valves can impact the accuracy of a downpipe or bubbler system if installed at the tank entry point)

NOTE: Not recommended for use with heavy diesel fuel oil or other high viscosity liquids. Installation in conjunction with a gate valve will permit removal of check valve for inspection and cleaning. (We recommend cleaning the check valve once per year to ensure reliability.)

Material: 304 Stainless Steel, Nylon (float), FKM (seals)
Max. Pressure: 150 psig
Inlet/Outlet: 3/4" NPT-female
Weight: 1.2 lb (0.5 kg) 


Download 6563 Downpipe Check Valve Data Sheet_1200-30

9316-4 O-ring for D/P top plate 
7334-2 Safegard Service Kit
7334-19 Air Control Service Kit
7333-10 Replacement Filter Element Kit


A precision pneumatic control valve which will instantly close bubble-tight when subjected to a pre-set pressure.  It is used to prevent overpressuring an indicator.

To order, see Ordering Information chart and Part Number Example above.

1-1/2" NPT Tank Top Fitting (for 3/4” Pipe) single purge line connection with tube fitting
7746-13    Carbon Steel, polyflo 3/8" tube
7746-15    Carbon Steel, Swagelok 3/8" tube
7746-18    316 Stainless Steel, Swagelok 3/8" tube

1-1/2" NPT Tank Top Fitting (for 3/4" Pipe) with single 1/4" NPT purge port connection
2000FA    Carbon Steel
2000PA    PVC    
2000SG    316 Stainless Steel

1-1/2" NPT Tank Top Fitting (for 3/4" Pipe) with two 1/4" NPT purge/equalizer connection
2000FA-2   Carbon Steel
2000PA-2   PVC
2000SG-2  316 Stainless Steel

WARNING: This product can expose you to chemicals including Lead and Nickel, which are known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov