KELLER 21C Ratiometric Pressure Transmitter

Name: KELLER 21C Ratiometric Pressure Transmitter | Seven Star LLC
Brand: KELLER
SKU: 21C

KELLER Series 21C is a compact piezoresistive pressure transmitter with a ratiometric 0.5…4.5 V output, powered from 5 VDC (+/- 0.5 V). The pressure range runs from 0…2.5 to 0…160 bar, in a stainless steel body 16.6 mm in diameter and about 57 mm long, weighing about 40 g.

Ratiometric output for 5 V controller inputs

A ratiometric output scales with the supply voltage, so a 5 V controller that uses the same rail as its ADC reference sees the pressure reading without a separate reference. The datasheet gives a 0.5…4.5 V signal on a 5 VDC supply, consumption below 8 mA, load above 5 kOhm and a limiting frequency of at least 800 Hz.

KELLER builds the 21C as a chip-in-oil (CiO) design: an insulated, encapsulated piezoresistive chip, mathematical modelling of the characteristic and hermetically sealed electronics. It is fully welded with no internal seals; the only elastomer is the external connection seal.

Key features as printed in the datasheet

Ratiometric 0.5…4.5 V, 3-wire, 5 VDC +/- 0.5 V supply.
Accuracy max. 0.25 %FS; total error band 1.5 %FS over -20…85 degC.
Fully welded, no internal seals, AISI 316L wetted parts.
Start-up under 5 ms (0…99 %); GND-case insulation above 100 MOhm at 500 VDC.
Typical uses: cooling and air-conditioning, pneumatics, hydraulics, automotive, pump technology.

Technical specification

Performance and electrical data

Parameter	Specification
Pressure ranges	0…2.5 to 0…160 bar (PR, PAA, PA)
Accuracy (BFSL, hysteresis, repeatability)	max. 0.25 %FS
Total error band	max. 1.5 %FS at -20…85 degC
Compensated temperature range	-20…85 degC
Long-term stability	max. 0.3 %FS per year
Position dependency	max. 1.5 mbar
Output	0.5…4.5 V ratiometric, 3-wire
Supply / consumption	5 VDC +/- 0.5 V / below 8 mA
Load / limiting frequency	above 5 kOhm / 800 Hz or more
Insulation GND-case	above 100 MOhm at 500 VDC
Start-up time	below 5 ms (0…99 %)

Mechanical and environmental data

Parameter	Specification
Wetted materials	AISI 316L connection and diaphragm; no internal seal; external seal FKM 75 Shore
Oil filling	Silicone oil
Pressure connections	G1/4 male, G1/8 male, M10x1, 7/16-20UNF male or female
Dimensions / weight	dia 16.6 mm x approx. 57 mm; approx. 40 g
Electrical connection	M12 x 1 round plug (4-pole), valve plug Form CI 9.4 mm, Packard Metri-Pack 150 (3-pin), PUR cable dia 3.3 mm (1 m or 2 m)
Medium temperature	-20…125 degC, optional -40…125 degC
Ambient temperature	-20…85 degC, optional -40…85 degC; no icing
Ingress protection	IP67 (M12, Packard, cable); relative pressure with plug IP54
Vibration / shock	10 g, 10…2000 Hz / 50 g, 6 ms
Pressure cycles	more than 10 million, 0…100 %FS

Selection and ordering guide

Standard ranges listed are 0…40, 0…60 and 0…100 bar with proof pressures of 120, 180 and 300 bar, alongside the lower ranges from 2.5 bar; the maximum is 160 bar. Order PR for vented gauge, PAA for absolute from vacuum or PA for absolute referenced to 1 bar.

The scope of delivery includes the mating plug for the valve plug version Form CI; M12 mating plugs and angled sockets or female connectors with 2 m or 5 m cable are listed as accessories.

Installation and operation notes

As the output is ratiometric, power the transmitter from the same regulated 5 V rail that feeds the converter, and note that a supply outside 4.5…5.5 V falls outside the specification. The ingress ratings are valid only with the mating plug connected.

For relative pressure with the Form CI valve plug KELLER specifies a cable with integrated capillary so the reference remains ventilated; with plug versions the datasheet gives IP54 for relative pressure.

Applications in Oman and the GCC

Ratiometric 5 V transmitters are common on OEM controllers for chillers, compressor packs, hydraulic power units and pump skids. In Gulf installations check the 125 degC medium and 85 degC ambient limits against rooftop plant and outdoor skids, and consider the optional -40 degC versions only if the application needs them.

Compatible accessories and related items

KELLER names Series 21Y (4…20 mA), 23SY (ranges from 0.1 bar) and OEM pressure transmitter modules such as 7LC, 9LC and 20PC as alternatives. The 21Y and 23SY are also listed on this site. Before ordering, confirm the pressure type (PR, PAA or PA), the ratiometric controller input range and the plug style against the datasheet, because the device is configured to the order.

Frequently asked questions

What does ratiometric mean for the 21C?

The 0.5…4.5 V output scales with the 5 V supply, so a controller using the same supply as ADC reference reads pressure without extra scaling.

What supply does the 21C need?

5 VDC +/- 0.5 V with consumption below 8 mA.

Which pressure ranges are available?

0…2.5 to 0…160 bar, as relative, absolute from vacuum or absolute from 1 bar.

What is the accuracy?

Max. 0.25 %FS, with a total error band of 1.5 %FS across -20…85 degC.

Does Seven Star LLC act as a KELLER distributor?

No, we are an independent supplier in Muscat and supply KELLER items on request rather than acting as a KELLER channel partner.

Documents and quotation

Download the official KELLER datasheet for 21C from this page. As an independent supplier based in Muscat, Seven Star LLC supplies KELLER items on request: use the quote form with the quantity you need. Check every value in the document against your own specification before you order. Browse similar items in the category.

Pressure transmitters and level probes

The KELLER range covers standard and front-flush pressure transmitters, differential and low-pressure transmitters, submersible level probes, digital pressure gauges and data loggers. Many X-line devices combine RS485 Modbus RTU with an analog output, and intrinsically safe or increased-safety versions are offered for hazardous areas.

Frequently Asked Questions